LIGHT ADAPTATIONS OF PLANTS:
A MODEL BASED ON THE SEAGRASS
ZOSTERA MARINA L.
RECOMMENDED:
Chairman, Advisory Committee
Program Head
APPROVED:
PL
J
L
Director, Institute of Marine Science
D #<Lt inr\
i n
Date
JGL
Dean of the College of Environmental Sciences
Date
k c ^ i T h .T B
Vice Chanc
ancellor for R^searcl/ and Advanced Study
Date
r SCIENCE LIBRART
UNiVEKSiTY OF ALASKA
LIGHT ADAPTATIONS OF PLANTS:
A MODEL BASED ON THE SEAGRASS
ZOSTERA I-'ARIilA L.
A
THESIS
Presented to the Faculty of the
University of Alaska in partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE
/ 'W
^
By
-
William Dennison, B.A.
Fairbanks, Alaska
December, 1979
ABSTRACT
Adaptations to light by a temperate seagrass, Zostaro: marina L.
(eelgrass), were investigated along a depth transect representing a gra­
dient of plant development.
Various light adaptive strategies are pro­
posed in a conceptual model and tested along the natural gradient a;id
under in situ light manipulation experiments.
The major light capturing strategy which Zostera employs is that
of changing leaf area.
Chlorophyll a to b ratios and amounts, measures
of adaptation to light quality and quantity, demonstrated little or no
adaptive trends when integrative samples were used.
The altered light
experiments did not affect chlorophyll content but did affect leaf pro­
duction rates.
Although the relative vertical distribution of leaf
area is constant along the transect, the absolute leaf area varies, as
measured by leaf area index (LAI = area of leaves/area of bottom),
A
measured maximum LAI of 17 is higher than other aquatic and most terres­
trial ecosystems.
PREFACE
This study investigates the acaptations of plants to light gra­
dients.
The adaptation to gradients of light quality and quantity is
both a physiological and biological plant response.
The different scale
of response mechanisms has led to the broad approach of this study, from
molecular pigment ratios to total light absorbing surface area.
The
various parameters analyzed are approximations with varying degrees of
relevance to the adaptation that they address.
For example, the measure
of leaf area corresponds to the proposed leaf area adaptation differentl
and perhaps more closely, than tha measure of chlorophyll a to chloro­
phyll b ratio corresponds to the proposed chromatic adaptation.
As the
choice of parameters reflects the relevance to the adaptation, the choic
of sampling schemes reflects the scale of the problem being addressed.
The assessment of the plant community and the assessment of single shoot
require a different approach.
This is saliently demonstrated by a com­
parison of the distribution of chlorophyll in single shoots and in inte­
grated canopy samples.
This study is on an ecosystem level, and the san
pling scheme must necessarily be one of appropriate scale.
Methods of
aquatic and marine plant biology are largely oriented toward microscopic
organisms, an inappropriate scale for the macroscopic seagrasses.
Conse
quently, methods of terrestrial biology were applied to this particular
marine ecosystem.
The vertical sectioning of the leaf canopy into hori­
zontal strata (stratified clip technique) used in this study is an
adaptation of terrestrial methodology.
The integrated sampling of old,
young, large and small leaves and shoots serves to group the plants and
iv
plant parts into strata:
the strata are what the quanta or light par­
ticles encounter as they enter the ecosystem.
The distribution of life
in horizontal layers, or strata, combined with vertical gradients of
environmental factors, especially light, serve as selective pressures
for physiological and genetic adaptations to conditions the organism
encounters.
This research delves into the manner that plants go about
reconciling themselves with their environment.
This restless world
Is full of chances, which by habit's power
To learn to bear is easier than to shun.
Armstrong,
Act of Preserving
Health, Book 2
v
ACKNOWLEDGEMENTS
I am grateful for the many unique learning opportunities provided
by my graduate advisor, Dr. C. Peter McRoy.
In addition to learning how
to swim underwater through an eelgrass jungle, walk over mud flats with
snowshoes and live in WWII quonset huts, I have gained some perspectives
on what it is to be a scientist.
He has provided insights on how to
address a particular question; additionally how to find the question to
address.
My graduate committee, Drs. V. Alexander, J. M. Colonell, J. J.
Goering and K. van Cleve, helped guide me through the academic maze and
gave me their time and support.
I was fortunate to have access to Dr.
M. J. Klug on sabbatical leave from Michigan State University.
Participation in a team research effort, the Seagrass Ecosystem
Study (SES), has provided me with the opportunity to meet and exchange
ideas with other seagrass researchers, and more importantly, provided the
framework for this particular investigation.
SES technicians, principal
investigators and graduate students assisted in the collection and
processing of samples.
Discussions of ideas and problems with Drs. R. J. Barsdate, E. J.
Brown, F. S. Chapin III and T. C. Royer provided fresh perspectives.
Fred Short assisted with the sampling design and made helpful sugges­
tions.
The formidable task of maintaining a field camp at Izembek Lagoon
was made simpler by the Izembek National Wildlife Refuge staff.
The
U.S. Department of the Air Force 7 1 4 ^ Aircraft Control and Warning
vi
Squadron, Cold Bay and associated R.C.A. personnel also provided lo­
gistical support and saved me from the SES culinary disasters.
The staff at the Seward Marine Station helped construct and the
crew of the research vessel Aoona helped transport equipment used in
this study.
This research is a contribution of the Seagrass Ecosystem Study
supported by the Office of International Decade of Ocean Exploration
of the National Science Foundation (Grant No. OCE-77-27050).
vii
TABLE OF CONTENTS
ABSTRACT................................................................. iii
P R E F A C E .................................................................. iv
ACKNOWLEDGEMENTS......................................................... vi
LIST OF F I G U R E S ...........................
ix
LIST OF TABLES........................................................... xi
INTRODUCTION...........................................................
1
CONCEPTUAL MODEL.......................................................
3
MATERIALS AND METHODS ................................................
8
Study S i t e .......................................................
8
Plant Pigment A n a l y s i s .............................................11
Leaf Canopy Structure............................................... 12
Light Manipulation Experiments ..................................
13
Leaf Production..................................................... 15
R E S U L T S .................................................................. 15
Plant Community Parameters ......................................
15
Pigment Ratio....................................................... 17
Pigment Concentration............................................... 20
Leaf A r e a . ......................................................... 23
Light Manipulation Experiments ..................................
27
DISCUSSION................................................................ 27
Testing the Conceptual Model ....................................
27
Leaf Canopy Structure............................................... 39
Leaf Area Index..................................................... 44
SUMMARY AND CONCLUSIONS ..............................................
48
REFERENCES................................................................51
APPENDIX.................................................................. 57
viii
LIST OF FIGURES
Figure
1. A conceptual model of light adaptations by plants
represented by a four way adaptive scheme with pre­
dicted plant responses under different light regimes
and time scale for adaptive response.....................
4
Figure
2. Location of Izembek Lagoon, Alaska.......................
9
Figure
3. Grant Point transect map showing locations of the six
sampling stations. Diagrammatic cross section of the
transect at low tide with a projected high tide ........
10
4. Top and side view of sketches of sun reflector and
light shading screen used in the light manipulation
experiments ........................................
14
5. Plant community parameters along the transect (JuneAugust, 1978).
(a) Total shoot density; (b) leaf dry
weight per shoot; (c) leaf biomass; and (d) maximum
percent of reproductive shoots (% of total shoots). . . .
16
Figure
Figure
Figure
6. Ratio of chlorophyll a to chlorophyll b of Zostera
marina leaves (June-August, 1978).
Chi a!chi b relative
to distance above the bottom at all stations.
Chi a!
chi b relative to s t a t i o n .................................. 18
Figure
7. Frequency histogram of chlorophyll a to chlorophyll b
ratios...................................
19
Figure
8. Chlorophyll a content within a single shoot of Zostera
marina compared to chlorophyll a of integrated leaf
canopy samples............................................... 21
Figure
9. Total chlorophyll content of Zostsra marina, leaves
(June-August, 1978).
Chi a+b per leaf area relative to
distance above the bottom at all stations.
Chi a+b per
leaf area relative to s t a t i o n .............................. 22
Figure
Figure
10. Leaf canopy structure of Zostera marina. Vertical dis­
tribution of leaf area and leaf biomass along the tran­
sect (Stations A-F) at the seasonal maximum biomass (20
July 1978)................................................
24
11. Relative leaf area (% of maximum) and relative canopy
height (% of total) distributions for the six sampling
stations (June-August, 1978).....................
25
ix
LIST OF FIGURES (continued)
Figure 12.
Figure 13.
Mean and seasonal maximum LAI along the transect
(June-August, 1978) ......................................
26
Range of chlorophyll a to chlorophyll b ratios found
in plants (from Bjorn, 1976) with the Zostera chloro­
phyll ratio from this s t u d y .............................
32
Figure 14.
Diagram of tidal influence on leaf canopy structure of
Zostera plants.
At high tide the circles represent
space occupied by a shoot with overlap indicating self­
shading; at low tide the lines represent leaves floating
at the water surface.........................................37
Figure 15.
Influence of leaf chlorophyll content on maximum energy
yield for terrestrial plants (from Gabrielsen, 1948)
with the Zostera chlorophyll content from this study. . .
41
Leaf canopy structure of Zostera compared with the two
major terrestrial and aquatic plant groupings (from
Monsi et a'L. , 1973; Ikusima, 1965).......................
43
Hypothesized major limiting factors along the transect.
50
Figure 16.
Figure 17.
x
.
LIST OF TABLES
Table 1.
Chlorophyll ratios and amounts and standing stock turn­
over times in the light manipulation experiments............. 28
Table 2.
Comparison
Table 3.
Comparison of the maximum LAI of aquatic and terrestrial
ecosystems..................................................... 45
of plant chlorophyll ratios and amounts........... 33
INTRODUCTION
The acquisition of radiant energy and subsequent conversion to
chemical energy is the major concern of primary producers.
Radiant
energy, or more specifically, light energy between the wavelengths ca.
400-700 nm, is not uniformly distributed on the earth's surface.
Varia­
tions of light intensity and changes of the light spectrum occur and
plants correspondingly adapt to these nonuniformities.
In this study
various light adaptive mechanisms available to plants are proposed in
a conceptual model.
The model groups the adaptations into four cate­
gories and predicts the plant responses under various light regimes.
The proportional amounts of the different light adaptations are assessed
in a seagrass ecosystem in order to test plant responses to shifts in
light quality and quantity.
From the prediction and testing of light
adaptations an understanding of the plant's interaction with the envi­
ronment can be gained.
Seagrasses are a functional grouping of approximately 50 species
of marine flowering plants.
Shoots, or leaf turions, are bundles of
leaves that arise from a horizontal stem, the rhizome.
Roots serve to
anchor the plants and absorb and transport nutrients from the sedi­
ments.
There is little structural material; seagrasses are largely
supported by the water itself.
Reproduction is both sexual (hydro­
philous pollen) and vegetative (lateral branching of the rhizome).
Seagrasses are distributed along the coastlines of the world's oceans
except for the most polar seas (den Hartog, 1970).
1
2
Nearshore marine ecosystems that are dominated by marine flowering
plants are regions of high biological productivity (McRoy and McMillan,
1977).
Seagrass carbon is largely channeled into a detritus food web
with little direct grazing (McConnaughey and McRoy, 1979):
large
exports of carbon from seagrass ecosystems contribute to the food webs
of coastal waters (Barsdate et aZ. , 1974).
Seagrasses affect silt and
sand deposition, organic matter accumulation, nutrient cycling and water
movement.
These marine vascular plants contribute to the surface area
available for colonization, serve as a nursery for fishes and inverte­
brates, effectively pump nutrients from the sediments into the water
column and stabilize sediments (McRoy and Helfferich, 1977; Phillips
and McRoy, 1979).
Seagrass ecosystems, though recognized for many years, have only
recently been intensively investigated.
The proliferation of research
efforts has in part led to the Seagrass Ecosystem Study (International
Decade of Ocean Exploration/National Science Foundation).
The major
theme of this study has been the investigation of various components of
the seagrass ecosystems along a gradient of successional maturity.
The
testing of the hypothesis of the succession of ecosystem processes (pro­
cess succession) has been employed in various tropical and temperate
seagrass ecosystems.
This research investigates the responses of a
temperate seagrass ecosystem to changes in light quality and quantity.
Seagrass ecosystems of temperate and high latitudes of the Northern
Hemisphere are typically dominated by Zostera marina L.
(eelgrass).
ec.olugical role that Zostera plays in the ecosystem is variable, in
The
3
classical ecology terminology, ranging from a colonizer to a climax
species (den Hartog, 1970).
CONCEPTUAL MODEL
The light adaptive mechanisms available to plants are conceptual­
ized as being components of the total plant response (Figure 1).
The
model includes predictions of plant responses under different light
regimes.
Each of the adaptive mechanisms will be considered in detail.
Chromatic adaptation refers to the change in relative amounts of
various photosynthetic pigments as a response to the changes in light
quality.
Chromatic adaptation has been long recognized (Engelmann,
1883; Gaudokov, 1903), usually as a factor in depth zonation of various
algal types (Steemann-Nielsen, 1975).
The alternative explanation for
algal depth zonation (Oltmanns, 1892) emphasizes the effect of light
quantity (Dutton and Juday, 1944).
The effects of light quality and
quantity are distinguished in this conceptual model as chromatic and
quantum capture adaptation, respectively.
The selective filtering of
the light passing through water (Jerlov, 1976) or leaf canopies (Federer
and Tanner, 1976) leads to a spectral shift in the light available for
photosynthesis.
The absorbance peaks for accessory pigments (pigments
other than chlorophyll a) are at shorter wavelengths than that of chlo­
rophyll a (chi a) (cf. Smith and French, 1963).
as light harvestors,
Accessory pigments act
transferring the light energy with varying effi­
ciencies to specialized chi a molecules to be ultimately converted, to
chemical energy.
The relative amounts of chi a molecules and the various
h/
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C H R O M A TIC
A D A P T A T IO N
Q UANTUM
CA PTURE
A D A P T A T IO N
• • •
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• « •
0*0
Low Light
Blue Light
•
°°0°000
0°n0 • • • 0 0
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° ° 000 0 0 ^ „
• • • •
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High Light
Red Light
hv
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*
•
• • •
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w
hours
Quantum
Photochemical Reaction
Center (PCRC)
•
Chlorophyll a
o
Accessory Pigments
Pigment Content
Leaf Area
T O T A L P L A N T RESPONSE
Figure L.
ptive scheme
A conceptual model of light adaptations by plants represented by a _four way ada
adapt ive
f
o
r
with predicted plant responses under different Light regimes and time scale
response.
5
accessory pigments determine the relationship between photosynthetic
efficiency and radiant energy wavelength, often expressed as an action
spectrum (Haxo and Blinks, 1950).
This model proposes that plants can most efficiently absorb light
relatively enriched in the red wavelengths (unfiltered light) with a
pigment complement rich in chi a (high ratio of chi a to accessory pig­
ments) .
Light relatively enriched in the blue and green wavelengths
(light filtered by water or leaf canopies) is most efficiently absorbed
by plants with relatively more accessory pigments (low ratio of chi a to
accessory pigments).
The chi a to chi b ratio is used in this study as
an estimate of the chi a to accessory pigment ratio.
This measure of
the chromatic adaptation ignores the carotenoid accessory pigments in
Zostera but does include the major accessory pigment, chlorophyll b.
The quantum capture adaptation is related to the functional state
of the photosynthetic pigments.
This model recognizes the pigment func­
tional state as a major adaptive mechanism although few studies have
explored the ecological implications of this recently recognized adap­
tation (Clough et ah., 1979).
The capability of determining the amounts
cf photochemical reaction centers (PCRC = chi a molecules capable of
exciting electrons) and light harvesting pigments (LPH = molecules that
transfer light energy to the PCRC) has provided a measure of the effi­
ciency of quantum capture by pigments (cf. Sestak and Demeter, 1976).
Hi is model predicts relatively lesser amounts of LHP (lower LHP/
PCRC) in high light intensities and relatively greater amounts of LHP
(higher LHP/PCRC) in reduced light intensities.
Terrestrial data from
Berry (1978) supports this hypothesis; a "shade" plant had a higher
LHP/PCRC (approximated by chi a + o/photosystem 1 PCRC) than a "sun"
plant and the "sun" plant increased the LHP/PCRC ratio with reduction
of light.
Kawamura et al.
(1979) measured the entire pigment comple-
nents of both photosystens (I and II) in several species of algae and
found higher LPH/PCRC values in reduced light and lower LPH/PCRC values
in higher light.
In this study the ratio of chi a to chi b is used as
a crude indicator of the LHP/PCRC inverse.
The chi a molecules are the
only pigment containing PCRCs while chi b molecules are entirely LHPs,
Using the chi a to chi o ratio as an estimator of quantum capture adap­
tation assumes the chi a molecules contain a constant proportion of
PCRCs.
Pigment adaptation refers to the change in the amount of photo­
synthetic pigments as a response to the changes in light quantity.
The
concept of changes in pigment amount in relation to the amount of light
has been recognized for some time (Boysen-Jensen, 1933).
A more refined
approach expresses this adaptive mechanism in terms of leaf chlorophyll
concentration as a function of light intensity (Gabrielsen, 1940, 1948).
In the aquatic realm, changes in pigment content of phytoplankton as
function of irradiance have been noted (cf. Steemann-Nielsen and
Jorgensen, 1968).
This model predicts increasing amounts of chlorophyll per leaf area
with decreasing light intensity.
The two major pigments in Zostera,
chlorophylls a and c, are analyzed as a function of leaf area to assess
pigment adaptation.
7
Leaf area adaptation is the relationship between the plant’s light
absorbing surface and light intensity.
The leaf area distribution of
plant canopies has a large influence on the interception of photosynthetically active radiation (PAR = 400-700 nm light energy)
et ai. , 1973).
(cf. Monsi
In an early study, Monsi and Saeki (1953) related the
leaf inclination of plant canopies to the interception of light.
Since
then, vertical and horizontal leaf distribution, leaf arrangement and
inclination, and light regimes within plant canopies have been investi­
gated (Monsi et al., 1973).
The various leaf canopy structure adapta­
tions to light have been simplified for this conceptualization.
Erect
leaf canopies (vertical leaf orientation) generally absorb and utilize
light more efficiently than more planar leaf canopies (horizontal leaf
orientation)
I
j
(Tanaka, 1972).
Also, higher irradiances can support
higher leaf areas than can
low irradiances
The scale of the leaf
area adaptation
(Mooney, 1972).
as expressed in this studyis
on the plant community level, while the preceding adaptations are phys­
iological adaptations within a single plant.
The model predicts a
dense, erect leaf canopy at high light intensities and a sparse, nonerect leaf canopy at low light intensities.
This adaptation is assessed
by determining the leaf area distribution within the leaf canopy and by
integrating the leaf area of the canopy over a unit of ground surface
(LAI = one-sided leaf area index).
LAIs are useful for comparison of
light absorbing surface within and between ecosystems.
The objective of this
of a
t
I
i
!
I
research was to
assess the light adaptations
seagrass ecosystem as conceptualized in the four way adaptive
scheme.
Gradients of light quality and quantity are found within leaf
canoni;5 a;id ..'it., varying water d<=pch.
Consequently, this study inves­
tigates light adaptations within vertical profiles of the leaf canopy
and on a transect of variable water depths.
In sizu light perturbation
experiments were used in conjunction with the study of the natural
system.
The Zostera plants were analyzed for chi a/dal b (chromatic
and "'.’"ntuci capture adaptations), chi a+i/leaf area (pigment adaptation)
and leaf area distribution and production (leaf area adaptation).
MATERIALS AND METHODS
Study Site
2
One of the largest stands (- 115 km ) of Zostera marina in the
world is in Izembek Lagoon, Alaska (55°15'N, 163°05'W).
This large
2
lagoon (- 218 km ) is a shallow embayment of the Bering Sea located on
the distal end of the Alaska peninsula (Figure 2).
Izembek Lagoon has
a mean tide range of 1 meter; 67 percent of the lagoon water exchanges
daily with the Bering Sea (Barsdate et at., 1974).
Little fresh water
dilution occurs, however there are significant temperature changes in
the lagoon (Biebl and McRoy, 1971).
Sediment accumulation and binding
by eelgrass plants form raised benches or mud flats.
In Izembek Lagoon
the raised benches cover most of the lagoon (78%) with a network of tidal
channels accounting for the remaining 22 percent (McRoy, 1966).
A 0.7 km transect from intertidal to subtidal was used as a repre­
sentation of areas throughout the lagoon (Figure 3).
were selected along the transect (A-F).
Six sampling sites
The transect is located in a
Figure 2.
Location of Izembek Lagoon, Alaska.
10
Figure 3.
Grant Point transect map showing locations of the six sampling
stations (above).
Diagrammatic cross section of the transect
at low tide with a projected high tide (below).
11
small inlet in the lagoon; the configuration of the basin and the tide
range produce extended ebb tides and rapid flood tides in the inlet'
The basin has the characteristics of a tide pool (Biebl and McRoy> 1971),
the shallow water stations (A-C) remain submerged at the lowest nea?
tides.
The stations with deeper water at high tide (D-F) are actually
relatively more exposed at low tide.
More than a spatial sampling scheme, the transect
represents
an
annually reoccurring gradient of eelgrass system development that lS
reflected
data).
in
several
plant
community parameters (McRoy,
unpublished
The trends exhibited in plant growth form, stratification, re~
productive strategies, life cycles, and production/biomass relati°n;3^ P s
reflect a gradient of ecosystem development as delineated by Odum (1969).
Plant Pigment Analysis
Leaf samples were collected weekly for a three month period (June_
August) at low tide from the six sampling sites, stations A-F (Appenc^ x^
Vegetative (non-reproductive) shoots were clipped at the sediment"'-73Ler
interface and placed in water tight bags out of direct sunlight.
Prlor
to analysis, the Zostera shoots were stored overnight in a seawatef tu^
in reduced light.
Three to six shoots with undamaged leaves roughly
equal to the average leaf canopy height were selected for pigment
sis.
Leaf epiphytes were removed by scraping the leaf surface with tis
sue paper and a razor blade.
Four samples were taken at 5-10 cm it>ter
2 W
vals from the base of the shoot; two samples for leaf area (± 0.01 cm ''
dry weight (± 0.001 g) determinations and two samples for chlorophyll
extraction.
A manually operated tissue grinder or mortar and pestle was used
in reduced light to macerate the leaf segments rmtil the ^lar.t tissue
was visibly destroyed.
Pigment extraction was performed with 90 percent
acetone and a small amount of MgCO .
dark refrigerator for 20-24 hours.
Extraction was carried out in a
Natural settling of the plant tissue
during extraction resulted in blank corrections less than 0.010 absorb­
ance units.
Absorbance values were read at 665 and 645 nm with a 750 nm
turbidity correction on a Baush & Loumb Spectrophotometer 70 with 1 cm
disposable cuvets.
The chlorophyll extraction procedure was largely
that of Vollenweider (1969) while calculations were made with SCOR/
UNESCO (1966) trichromatic equations modified for just chlorophylls a
and b :
mg chi a/dm' or g = (11.64 x 0D665 + -2.16 x OD645) x CF
mg c'nl b/dm~ or g = (-3.94 x 0D665 + 20.97 x OD645) x CF
0D665 = Corrected optical density at 665 nm
0D645 = Corrected optical density at 645 nm
2
vol acetone (ml) x leaf area (dm ) or dry wt (g)
cell length (cm) x 1000
Leaf Canopy Structure
Vegetative shoots were collected weekly throughout the three month
sampling period.
Of the shoots collected at each of the six stations
(A-F), 20 intact shoots of full canopy height were chosen for leaf area
and leaf dry weight determinations.
The leaf canopy was sectioned into
5 or 10 cm segments by the stratified clip technique (Monsi and Saeki,
1953).
Leaf area was determined with a Lambda LI-3100 area meter
13
2
(± 0,01 cm ), samples were then dried at 90°C for 24 hours,
cooled to
rooia temperature and weighed (± 0.001 g) .
Estimates of shoot density and plant biomass were made by first
clipping shoots at the sediment-water interface, and then counting and
2
weighing the shoots within the sampling area (0.02-0.25 m ).
Twenty
shoot density/plant biomass samples were taken at each station through­
out the three month period.
Leaf area index was calculated by the pro­
duct of the vegetative shoot density and the average one-sided leaf
area/shoot.
A correction term was used to adjust the LAI values to the
corresponding leaf biomass values.
Light Manipulation Experiments
A sun reflector was placed in the eelgrass bed at two locations;
Station A (high shoot density) and Station D (high LAI).
The reflec­
tors were 1.2 x 2.4 m pieces of plywood with several coats of high gloss
white paint (Figure 4).
Supported by fence posts and guylines,
reflectors were oriented vertically lacing south.
the
Incident PAR was mea­
sured by a hand held quantum sensor (LI-135A) placed 1 cm below the
water surface.
The maximum PAR increase was - 50 percent 20 cm in front
(south) of the reflectors at low tide, with decreasing additions of
light at increasing distances away from the reflectors.
Light shading screens were also placed in the eelgrass bed at Sta2
tions B and D (Figure 4).
The 2.S m
hexagon screens were made of three
layers of fine mesh fiberglass screening supported by a plastic pipe
frame.
They were buoyed by styrofoam floats and remained - 10 cm above
S U N R E F L E C T O R (Increased Light)
Painted Plywood (High gfoss white)
SHADE SCREEN (Decreased Light)
Plastic Pipe Frame
Screening
^Sampling A rea*
Center P ost^
SIDE VIEW
Figure 4.
^^ww W ww wPW M w ^^Styrofoam Floats
TOP VIEW
Top and side view of sketches of sun reflector (above) and 1i ^ t
shading screen (below) used in the light manipulation exper i
15
the water surface as they moved up and down the tides, affixed from
lateral movement by a central rod.
PAR under the shading screen was
= 20 percent of incident PAR.
Leaf Production
Leaves were marked by punching small holes in ca. 30 shoots at each
treatment, a variation of the staple technique of Zieman (1968).
The
holes were punched with a syringe needle a few millimeters above the
bundle sheath.
The shoots were harvested 14 days after the holes were
punched and the scars located.
two categories:
Leaves on a shoot were sectioned into
"new leaf growth" and "initial leaf."
"New leaf growth"
was that which had been produced since the holes were punched, between
the sheath and scars.
"Initial leaf" was that which had been present at
the time of marking; the stem portion of the plant and between the scars
and the leaf tips.
sured for leaf area.
The "new leaf growth" and "initial leaf" were mea­
A ratio of leaf production was calculated; new
leaf area/initial leaf area.
This was divided by the number of days
(14) and the inverse taken as the standing stock turnover time (in days).
RESULTS
Plant Community Parameters
Several spatial trends are evident in the plant community para­
meters (Figure 5).
tide pool stations.
The total shoot density generally increases toward
The anomaly of the general tendency of density,
Station A, is likely due tc an edge effect.
The inshore areas of
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0.1
4,000
I x
i-
2,000
i
i
A
B
i
C
i
D
i
E
i
F
IO
0
1
(c)
o
3
Q
O
DC
*
<
2
20
A
B
I
C
J ______ |______ |
D
E
F
S TA TIO N
S T A T IO N
- T ID E P O O L
It - I —
J
L
-T ID E P O O L
S U B T ID A L -
S U B T ID A L
-
(d)
10
n
0
J
I
L
E
F
S TA TIO N
'igure 5
Plant; community parameters along the transect (June-August, 1978).
(a) Total shoot density
(X i 95% C.I.); (b) leaf dry weight per shoot (X i 95% C.I.); (c) leaf biomass (X t 95^ C.I.),
and (d) maximum percent of reproductive shoots (% of total shoots).
17
Izembek Lagoon are periodically scoured by winter ice.
These localized
perturbations lead to reduced plant growth; the plants are maintained in
a colonizing state.
While the tide pool plants are very dense, they
also are relatively small as indicated by lower leaf dry weight per
shoot values.
The small, dense plants of the tide pool are contrasted
with the large, sparse plants of the subtidal.
The leaf biomass (per
2
m ) has a maximum at Station D, decreasing both inshore and offshore.
The reproductive strategy of the Zostera plants is variable.
In general,
the subtidal plants largely or entirely produce new shoots by lateral
branching of the rhizome, a form of vegetative reproduction.
The small
tide pool plants produce sexually reproductive shoots; up to 20 percent
of the shoots bear pollen and flowers.
Pigment Ratio
The ratio of chi a to chi 3 was found to be significantly higher
(a = 0.05) with increasing distance above the base of the shoots
(Figure 6).
Chi c/chi o also showed significant variation (a = 0.05)
between stations, with the higher ratios found in the more subtidal
stations, D-F (Figure 6).
The overall trend is one of increasing chi a /
chi b in the upper portions of the leaf canopy and in the more subtidal
stations.
The statistical determination of confidence intervals and
comparison of means was done with the assumption that the ratio was
normally distributed.
This assumption was endorsed by a frequency
histogram distribution of chi a/chl b (Figure 7).
(cm)
BOTTOM
ABOVE
DISTANCE
STA TIO N
* -
Figure 6.
Ratio of chlorophyll a to chlorophyll b of Zostera mainna
Chi a /Chi b (X ± 95% C.I.) relative to distance above the
Chi a !Chi b (X ± 95% C.I.) relative to station (right).
TID EP O OL
SUB T ID A L
leaves (June-August, 1978).
bottom at all stations (left).
I
—"
00
FREQUENCY
(NUMBER
OF O C C U R R E N C E S )
80 r
60
-
40
-
20
-
0 -- ‘
1.0
3
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
C H L O R O P H Y L L R A T IO ( C H L a /C H L b )
Figure 7
Frequency histogram of chlorophyll a to chlorophyll b
20
Pigment Concentration
The concentration of chi a. varied within individual shoots (Figure 8).
The distribution of plant pigments appears to be related to the growth
characteristics of Zosis?a.
The youngest leaves on a shoot grow from the
basal meristem between the next youngest leaves.
The oldest leaf on a
shoot (furthest from the center) contained the greatest concentration of
pigments with lesser concentrations evident in the younger leaves.
The
variation of chlorophyll content within single shoots leads to variation
between shoots.
The variable number of leaves per shoot (mode = three
leaves/shoot) results in shoots with younger leaves (fewer leaves/shoot)
and shoots with older leaves (more leaves/shoot).
The shoots with young­
er leaves have relatively less chlorophyll than shoots with older leaves.
A large sample containing both young and old leaves, and small and
large shoots integrates the variations within and betxveen shoots in one
measurement.
Employing this sampling scheme, the chi a+b concentration
per unit leaf area did not vary significantly (a = 0.05) throughout the
leaf canopy (Figure 9).
Significant changes (a = 0.05) in the mean
chi a+b content per leaf area did occur between all six stations (Figure 9), largely due to high chi a+b!dm
2
values at Station C.
No signifi2
cant differences were evident when comparing the mean chi a+b/dm
without Station C.
The significance of the high chi a+b/dm
2
values
values at
Station C is questionable; the arbitrary fractioning of the leaf canopy
can lead to a relative sample weighting of the stem versus leafy portions
of the plants.
For example, a small sampling of the leaf canopy would
not likely contain any of the stem portion of the plant
(less chloro­
phyll), while more intense sampling probably would contain stem portions.
S IN G L E S H O O T
IN T E G R A T E D L E A F C A N O P Y
□
□
□
□
□
E3
□
H
□
1.5
□
0-
□
Figure
Chlorophyll a content within a single shoot of Zostera marina (left)
compared to chlorophyll a of integrated leaf canopy samples (right).
(cm)
BOTTOM
ABOVE
DISTANCE
S T A T IO N
T ID E P O O L
Figure 9.
S U B T ID A L
Total chlorophyll content of Zostera marina leaves (June-August, 1978).
Chi a+b pur
leaf area (X ± 95% C.I.) relative to distance above the bottom at a}! stations (let f) .
Chi a+b per leaf area (X ± 95% C.I.) relative to station (right).
23
Leaf Area
The distribution of leaf surface area represents a measure of the
light absorbing area.
Unlike chlorophyll ratio or concentration, the
leaf area and leaf biomass are not uniformly distributed throughout the
leaf canopy (Figure 10).
The highest leaf area is found at roughly-one
fifth of the total canopy height, with a gradual diminution of leaf
area above and below this point.
Leaf biomass, on the other hand, is
greatest near the bottom, decreasing toward the top of the leaf canopy
(Figure 10).
The ratio of leaf area to leaf biomass (specific leaf
area) or the inverse ratio (specific leaf weight) is constant throughout
the leaf canopy except for the stem region.
The overall mean specific
2
leaf area is 2.5 ± 0.1 dm /dry g (X ± 95% C.I.) and the corresponding
specific leaf weight is 0.39 ± 0.02 dry g/dm^ (X ± 95% C.I.).
The vertical distribution of leaf area follows the same general
pattern at the six stations.
This is evident when the canopy heights
and leaf area maximums of the different stations are normalized.
The
plots of relative canopy height and relative leaf area of the six sta­
tions are markedly similar (Figure 11).
These normalized graphs reveal
that the small, dense leaf canopies of the tide pool have the same
relative distribution of leaf area as the large, well developed canopies
of the subtidal stations.
While the relative distribution of leaf area in the plant canopy
was essentially the same at all stations, the absolute leaf areas
changed.
The LAI, a measure of the absolute leaf area, does vary
between stations (Figure 12).
The mean LAIs are significantly different
120
□
110
Leaf Bio mass f - ^ - )
Leaf Area
(£)
100
(cm)
ABOVE
70
60
DISTANCE
80
BOTTOM
90
50
fa
I
I
m
-m
U.
W/v
II
%
/f'
p/0/Js\
40
I f l
10
_L
6 4
2
2 3
6 4
2
1 2 3
6 4
2
j_I
1 2
3
TIDEPOOL
F i g u r e 10.
w
y
W
20
mm
'/
'i4
30
i
i
6 4
2
J L
1 2 3
■I - I L L
6 4
2
w
1
1
'M
J
1 2
L
J__L
3
6 4
j i i
2
1 2 3
SUBTIDAL
Lea f c a n o p y s t r u c t u r e of Z o s U m m r i m .
V.rtic.1
area and leaf biomass along the transect (Stations A-T) at Lhe sea.
maximum biomass (20 July 1978).
4>
(%)
Station A
Station B
Station C
I _
L
1
50
R E L A T IV E
CANOPY
HEIGHT
100-it
R E L A T IV E
CANOPY
HEIGHT
(%)
h
R E L A T I V E L E A F A R E A (%)
Figure 11.
R E L A T I V E L E A F A R E A (%)
R E L A T I V E L E A F A R E A (%)
Relative leaf area (% of maximum) and relative canopy height (% of total) distributions
for the six sampling stations (June-August, 1978).
1-0
Ln
26
12.0
10.0
(LA I)
8.0
LEAF AREA
INDEX
6.0
I
4.0
I
2.0
I
*
Mean ± 95% Confidence Interval
Maximum Seasonal LAI
J____________1____________I____________ L
C
D
STATIO N
TID EPO O L
Figure 12.
S U B TID A L
Mean and seasonal maximum LAI along the transect
(June-August, 1978).
r
27
from each other (a = 0.05) with a mid-transect maximum (Station D ) .
The
1973 maximum LAI was 12, in 1979 the highest observed LAI was 17.
Light Manipulation Experiments
The different in situ experimental light regimes were analyzed, for
plant response to the altered light conditions.
At both stations, B and
D, the chlorophyll ratios and concentrations were not significantly
altered (a = 0.05) during the 14 day experiments (Table 1).
On the
other hand, the standing stock turnover times, determined from the leaf
production measurements, were substantially different at the two statis.
T1"
turnover times at Station B were relatively short, with
longer turnover times at Station D.
At Station B there was little va­
riation in turnover time under the different light regimes.
However,
at Station D a faster turnover was indicated for the increased light
condition.
DISCUSSION
Testing the Conceptual Model
Changes in light quality in nature are inevitably accompanied by
changes in light quantity.
Consequently, the separation of the light
quality and light quantity adaptations in plants is generally difficult.
Bjorn (1976) predicts a 12 nm absorbance peak decrease with every ten­
fold decrease in light quantity.
The accessory pigments, especially
chi b with 100 percent conversion efficiency of light energy to chi a,
act entirely as light harvesting pigments.
The accessory pigments also
r
28
Table 1.
Chlorophyll ratios and amounts (X ± 95% C.I.) and standing
stock turnover times in the light manipulation experiments.
Station
Light
Regime
Chlorophyll
Ratio
(chi a/chi b)
Chlorophyll
Amount
(chi a+b/dm )
Turnover
Time
(days)
B
Decreased
2.3
±0.0
2.4
±0.4
44
B
Control
2.4
±0.1
2.4
±0.1
37
B
Increased
2.3
±0 . 2
2.4
±0 . 8
34
Decreased
2.4
2.2
172
o
± 0.1
Control
Increased
2.4
±0.2
2.4
± 0.0
*Estimation
2.4
±0.3
2.2
± 0.1
160"
65
29
absorb light at shorter wavelengths than chi a.
Thus a plant could
adapt to both a spectral shift and concomitant decrease in irradiance
simply by increasing the relative amounts of accessory pigments.
The
ratio of chi a to accessory pigments can, therefore, be a response to
light quality and/or light quantity.
ratio as an approximation of the chi
This study uses the chi a to chi b
to accessory pigment ratio.
Chi a!chi b is used to assess both chromatic and quantum capture adapta­
tions.
The conceptual mode], predicts a decrease in the chi a/chi b ratio
in the lower portions of the leaf canopy with relatively more accessory
pigments to absorb the shorter wavelength light and act as light harvest­
ing pigments.
Light becomes enriched in blue and green wavelengths and
the intensity is diminished as it passes through the leaf canopy.
A
decrease of chi a/chi b in the lover canopy does occur in the Zostera
leaves (Figure 6).
The trend is similar in form but not in magnitude
to that found in single eelgrass shoots by Stirban (1968).
The range
of chi a /chi b for single shoots of Zosters: Karir,a was 0.8-2.8 (base to
tip), and in this study the range of integrated sample means is 2.1-3.0
(bottom to top).
Not only does the chi a/chl :: decrease at the bottom of the leaf
canopy (relatively more chi b ) , the relative carotenoid pigment concen­
tration probably increases.
Stirbaa (1968) has shown a decrease in the
chi a to carotenoid pigment ratio toward the base of single Zostera
shoots.
The photosynthetic contribution of these pigments is not well
defined; however, they have been found to absorb shorter wavelength
light than chi a (Steemann-Nielsen, 1975).
30
The conceptual model predicts a
decrease of chi a/chi b with an
increase in water depth resulting inmore light harvesting
absorbing diminished light at shorter wavelengths.
in Zostera indicates the reverse; an
creasing high tide water depth.
it first appears.
pigments
The trend observed
increase of chi a!chi b with in­
This may not
be non-light adaptive as
The long shoots and shallow low tide water depths at
Stations D-F produce a temporary floating mat of eelgrass leaves in the
top few centimeters of water.
The ratios could be adaptive to only the
low tide condition (possibly higher rates of photosynthesis).
It also
may be simply a morphological trait of Zostera to increase the chi aj
chi b with time.
The longer leaf turnover times, and consequently older
leaves of Stations D-F and older leaf portions in the top of the leaf ■'
canopy, may account for the trends observed in chi cz/chl b.
The
reported
effect of lightquality on seagrass photosynthesis has
(Buesa, 1975). However,
been
the changes in light quantity appear
to be the overriding controlling factor in the depth distribution of
tropical
seagrasses (Buesa, 1975; Wiginton and McMillan, 1979).
suggests
the chi a/chi b ratio of
This
seagrasses can be regarded as a measure
of the quantum capture adaptation rather than chromatic adaptation.
A
chi a/chi b trend with Xv’ater depth was observed with some tropical sea­
grasses (Wiginton and McMillan, 1979); however, other temperate sea­
grasses did not demonstrate a depth gradient of chi a/chi b (Drew, 1978).
Seaweeds on the other hand appear to adapt to both light quality and
light quantity with changes in the chi o. to accessory pigment ratio
(Ramus e-b aZ. , 1976).
31
The physiological responses to light quality and quantity, both
appraised by chi a/chi b, seem to be quite limited.
The chi a/chi b of
a plant is likely a characteristic of functional plant groupings (Keast
and Grant, 1976).
A general trend of increasing ratio with altitude is
found in plants with chlorophylls a and b (Figure 13).
The overall
average chi a/chi b ratio of Zostera (2.5) falls on the lower end of the
observed range (Table 2 and Figure 13).
A hereditary or genetic adaptation reflected in the plant genotype
occurs on a different time scale than the aforementioned physiological
type of adaptations.
Chromatic adaptation is expressed as a phylogenetic
trait; the pigment complement of algal types and higher plants is fixed
(Steemann-Nielsen, 1975).
The relative amounts of various accessory
pigments can vary only between the major plant groupings.
The success­
ful invasion of the marine environment by land plants was probably
delayed until the evolution of certain flowering plants (monocotyledons)
(den Hartog, 1970).
Higher plants have a complex leaf structure with
only chlorophylls a and b plus an assortment of the ubiquitous carotenoids.
As a result, they generally do not utilize blue light (common in
seawater) very efficiently (Gabrielsen, 1940).
The monocotyledonous
plants have a relatively simple leaf structure leading to less light
sc.ittering (Inada, 1976).
The graminoid growth form results in less
light absorbtion by structural material (Mooney, 1972).
The monocots
also have the plasticity to develop structural modifications for life
in the aquatic environment, e.g. epidermal chloroplasts (Hutchinson,
1975, p. 139).
These morphological characteristics of monocots were
MAXIMUM
ALPINE
Chi a/Chi b = 2 . 5
SUBMERGED
WATER PLANTS
Figure 13.
Range of chlorophyll a to chlorophyll b ratios found in
plants (from Bjorn, 1976) with the Zostera chlorophyll
ratio (X = 2.5) from this study.
Table 2.
Comparison of plant chlorophy.il ratios and amounts.
Plant
Zosbera
mg chi a /
g wet
mg chi at
g dry
0.48
1.3
0. 38
2.0
mg ch1 a+b/
g wet
0.68
mg chi a+bj
dm2
1.81
chi a!
chi b
2.48
Reference
This study
McRoy, 1966
0.30-1.20
McRoy, 1970
0.81-2.81
llalophila
0.97
1.64
Syvingodium
0.84
1.95
Haloclule
1.95
2.15
Thaiassia
1.03
2.41
Stirban, 1968
Wiginton and McMillan, .1979
1.7
Pomeroy, 1960
1.6
Buesa, 1974
2.1-3.0
Burkholder and Burkholder,
1959
Margalef, 1961
1.4
Posidonia
2.4-5.0
1.6-3.2
Cymodocea
2.2-3.9
2.2-4.0
Drew, 1978
M
2.0-4.0
Bjorn, 1976
Terrestrial
. -
0 2 8.7
Gabrielsen, 1948
2.5-7.5
Bjorkman, 1978
34
possibly necessary to overcome the lack of various accessory pigments
and have at least in part made life in the sea possible.
In this
respect, the pigment complement or chromatic adaptation of seagrasses
has probably affected their evolution from land to the sea.
Other than measurements of the chi a/chi b ratio, the amount of
quantum capture adaptation has not been assessed in seagrasses.
The
increase of the relative amount of light harvesting pigments with plant
age has been reported (Sestak and Demeter, 1976).
If this relationship
holds true for Zostera, a possible light adaptative mechanism exists due
to variable leaf turnover rates.
There is little evidence of light adaptation in this Zostera eco­
system by pigment concentration.
The lack of variation of the chloro­
phyll content of leaves as a function of depth in the leaf canopy indi­
cates that the Zostera leaf canopy does not respond to light variations
by changing chlorophyll amounts.
The profile of chlorophyll is basi­
cally a reflection of the profile of leaf area.
The small variations
of chlorophyll along the transect also indicate uniformity of pigment
content.
The lack of measurable chlorophyll response in the experimen­
tal light conditions lend support to this.
Although the experiment '/as
of relatively short duration (14 days), the rapid turnover of chloro­
phyll molecules (Riper et al. , 1979) and rapid response of chlorophyll
content of tropical seagrasses in reduced light experiments (Wiginton
and McMillan, 1979) lend support to the conclusion that the pigment
content does not vary.
The physiological capability of pigment adaptation does not seem
to be reflected in the ecological distribution of pigments in this study.
35
The one month aquaria experiments of Wiginton and McMillan (1979) used
irradiances generally less than those found in natural conditions, but
the experiments demonstrated the ability of the tropical seagrasses to
alter their chlorophyll content.
The seasonal chlorophyll content
varies in Zostera (McRoy, 1966) indicating a similar capability of pig­
ment adjustments.
The seagrasses adapted to more pronounced depth gradients (1 to
42 m) do demonstrate higher chlorophyll contents in the plants living
in light conditions approaching the compensation point (Drew, 1978;
Wiginton and McMillan, 1979).
Ramus et al.
(1976) found higher chloro­
phyll contents of marine red and green algae in reduced light.
However,
the large brown algae do not appear to vary the chlorophyll content of
the fronds (McFarland and Prescott, 1959).
The studies of Drew (1978) and Wiginton and McMillan (1979) demons­
trate the effect of a pronounced light gradient on the degree of pigment
adaptation.
While large light gradients do occur within the leaf canopy
of Zostera, they are compacted to within the few meters of water depth
in Izembek Lagoon.
The gradients of light quality and quantity in this
seagrass ecosystem can therefore be snanned by single shoots.
To suc­
cessfully adapt to light by change in pigment ratio, the pigment func­
tional state or the pigment amount, the adaptation would necessarily be
within a single leaf.
This appears less likely an occurrence than if a
leaf were able to adapt to a constant light regime throughout its devel­
opment.
This provides a partial explanation for the lack of appreciable
adaptive response by pigment ratio, functional state, or amount.
36
For the remaining adaptive mechanism, the conceptual model indi­
cates an erect leaf canopy under high light, a more planar one under
low light.
The leaf canopy of seagrasses in general is largely sup­
ported by the water column, and changes in the water level or the buoy­
ancy of the shoots affect the leaf inclination.
The bulk of the eel-
grass leaf canopy is made up of long strap-like leaves.
The leaves are
gas filled sacs, up to 50-60 percent by volume (McRoy and McMillan,
1977).
The amount of gas in the lacunal system varies, it appears to
increase during periods of intense photosynthesis (0^)•
Zostera shoots
were observed to approach a vertical inclination during high light and
a more horizontal inclination during low light.
Hence the leaf canopy
has an active response mechanism to variable light.
This is rather
unique, in that most plants have fixed leaf inclinations.
The above general response mechanism is important in subtidal eel­
grass meadows but other factors largely control leaf inclinations in
Izembek Lagoou,
Izembek Lagoon and other Alaskan eelgrass beds contain
high subtidal and even intertidal eelgrass growth (McRoy, 1970).
leads to the dominating factor in leaf inclination,
This
the tidal state.
In
Izembek Lagoon the water currents generated by the tides and the height
of the tide largely control the leaf inclination (Figure 14).
tide the leaves are inclined more vertically than at low tide.
At high
The more
erect leaves at high tide indicate more efficient light absorption and
higher canopy photosynthesis (Duncan, 1971; Sheeny and Peacock,
1977);
however, there are other factors which favor photosynthesis at low tide.
There
is less water absorption of light at low tide and self-shading is
37
H IG H T ID E
LOW T ID E
SIDE
VIE W
TOP
VIE W
S P A D C C
DEN SE
SPARSE
DENSE
G RO W T H
GROWTH
GROWTH
G RO W T H
; l o w LAI)
( H IG H LAI)
(LOW LAI)
(H IG H LAI)
Figure 14.
L
Diagram of tidal influence on leaf canopy structure of
Zostera plants.
At high tide (left) the circles repre­
sent space occupied by a shoot with overlap indicating
self-shading; at low tide (right) the lines represent
leaves floating at the water surface.
38
probably not as prevalent as with most higher plants.
The thin Zosteva
leaves with their simple structure allow light to reach the lower canopy
and the currents and waves change the leaf positions allowing a more
equitable light distribution.
Also the summer water temperatures at lo'.:
tide are generally higher, more closely approaching the optimum for eel­
grass photosynthesis (Biebl and McRoy, 1971).
These factors preclude an
understanding of the influence of tide level on light absorption and
canopy photosynthesis.
The conceptual model predicts an increase of light absorbing sur­
face at high light intensities.
The leaf area has been used as an indi­
cator of light absorbing surface.
While there are other ways of
changing the light absorbing surface, e.g. chromatophore position
(Nultsch and Pfau, 1979) or size (Zurzycki and Metzner, 1977), the leaf
area is the major mechanism for changing the photosynthetic light surface.
The integrated leaf areas, LAIs, are substantially different from
each other along the transect.
There is no simple relationship between
LAI and light regime, the mid-transect LAI maximum at Station D suggests
a combination of factors affecting LAI.
The light manipulation experiments indicate that the effect of light
on leaf production in the tide pool (Station B) was not substantial.
This is contrasted to the results obtained at Station D, where increased
leaf production was indicated at the increased light experiment.
The
decreased light experiment did not appear to affect the leaf production
at Station D, however Backman and Barilotti (1976) did observe a signi­
ficant decrease of Zostera shoot density under in situ decreased light
intensities.
39
The results of the experiments along with the study of the natural
system serve to delineate the major light adaptive mechanism.
Of the
various adaptations conceptualized in this study, the change in leaf
area is the major mechanism which the Zostera ecosystem utilizes to
efficiently capture light.
Leaf Canopy Structure
The components of the leaf canopy structure; the stem region, re­
productive shoots, and theleafy portion will be
to light absorbtion.
This
discussed in relation
will be followed, witha comparison
of the
Zostera leaf canopy structure with that of other ecosystems.
The stem region of Zostera. appears to be functionally different
from the leafy portion.
The shoot
tially a bundle of all the
is thicker in the stem; it is essen­
leaves.The chlorophyll amount
is reduced in
the stem, and in addition Stirban (1968) reported a drastically reduced
chi a /chi b.
From carbon uptake profiles of Zostera arnerioo.:aa (Harrison,
1978) and freshwater plants (Ikusims, 1965) it appears that the stem is
not a site of appreciable photosynthesis.
This portion of the plant is
probably a net respiratory loss for Zostera mariria especially in the
well developed canopies with little light reaching the lower levels.
Consequently, the remaining leafy portion of the canopy is the major
photosynthesizing leaf material.
The reproductive shoots of Zoszera are longer than the more abun­
dant vegetative shoots (den Hartog, 1970).
These shoots are very buoy­
ant and they extend above the rest of the leaf canopy.
This probably
40
aids in pollen and seed dispersal.
Like the stem region of the plant,
the reproductive shoots are not likely sites of appreciable photosyn­
thesis (Harrison, 1978), especially considering the short term phenology
of reproduction.
2
The average chlorophyll content of the leaves is 1.8 mg chi a+b/dm ,
slightly less than the 2.0 mg chi a+b/dm
2
that Gabrielsen (1948) indi­
cates as the upper limit for a first order relationship between chloro­
phyll content and energy yield (Figure 15).
He defines energy yield as
the percent of incident radiation that is converted to chemical energy.
His study demonstrates that leaf chlorophyll content above 2.0 increases
energy yield at proportionally smaller increments to a maximum at 4-5 mg
2
chi a+b/dm , beyond which point additional chlorophyll does not affect
the rate of photosynthesis.
It is apparent from Gabrielsen's classic
study that Zostera has a leaf chlorophyll content that very efficiently
utilizes light energy.
A plant that can respond to light with varying
leaf areas can most efficiently utilize its chlorophyll if low pigment
concentrations are maintained.
Most terrestrial plants probably cannot
maintain high leaf areas due to water loss and nutrient limitation.
The
higher leaf chlorophyll contents of land plants account for a loss of
efficiency of light capture probably in order to provide sufficient
quantity of capture.
The high tide vertical distribution of Zostera leaf area demons­
trates a maximum in the lower portion of the canopy.
The leaf area
distributions of terrestrial plants fall into two major categories; the
grass-type with a mid canopy maximum of leaf area and the [orb- or herbtype with an upper canopy maximum (Monsi et at., 1973).
The eelgrass
MAXIMUM
ENERGY
YIELD
(mg C 0 2 /cal.)
41
CHLO RO PHYLL CO NTENT
Figure 15.
mg Chl(a + 6 )/d m 2
Influence of leaf chlorophyll content on maximum
energy yield for terrestrial plants (from Gabrielsen,
1948) with the Zostera chlorophyll content (X = 1.8)
from this study.
leaf canopy with a -maximum leaf area in the lower canopy appears to fall
into a separate grouping (Figure 16).
The general scheme of Zostera
leaf canopy structure is complicated by the influence of tides in
Izembek Lagoon.
The leaf area distribution of Zostera is dynamic, going
from an upper canopy maximum (herb-type) at low tide to a lower canopy
maximum at high tide.
Lower canopy maxima plants probably include other
seagrass species (unpublished data) but not freshwater plants (Ikusima,
1965).
The stratification of leaf area insures optimal rates of canopy
photosynthesis under variable light conditions (Odum et at. , 1958).
Light saturation of eelgrass occurs at an irradiance ca. 40 percent of
surface irradiance (McRoy, 1974; Williams, 1977).
The depth of the
light saturation in the leaf canopy will be dependent on the total irra­
diance such that relatively high light will force the light saturation
zone to a lower level in the canopy.
At high light intensities, the
leaf canopy is light saturated in the upper portions which reduces
the photosynthetic efficiency.
However, the greater amounts of light
reaching the greater leaf areas in the lower canopy compensates for
the upper canopy light saturation, yielding a larger net amount of
total canopy photosynthesis.
At low light, the leaf canopy receives
quanta at rates less than that of light saturation.
The high tide
leaf canopy distribution for Zostera and perhaps seagrasses in general,
leads to efficient net canopy photosynthesis under the variable light
conditions prevalent in the submerged habitat.
RELATIVE
CANOPY
HEI GHT
(% of T ot al )
43
R ELA TIVE LEAF A R E A
Figure 16.
(% o f M a x i m u m )
Leaf canopy structure of Zosteva compared with the
two major terrestrial and aquatic plant groupings
(from Monsi et at. , 1973; Ikusima, 1965).
Leaf Area Index
The maximum leaf area index (LAI) observed in this Zostsra stand
is higher than that of other aquatic and most terrestrial ecosystems
(Table 3).
The maximum LAI of this seagrass ecosystem is most similar
t” the terrestrial forest ecosystems.
The production of the high LAI
is certainly an important feature of this ecosystem, but why such a' high
LAI?
A high LAI combined with a relatively low leaf chlorophyll content
can still absorb a large portion of the available light.
The production
of leaf area in seagrasses is not restrained by water budgets or to a
large degree by nutrient limitation as is leaf production of terrestrial
plants (Grier and Running, 1977).
Also, the seagrasses are periodically
flushed with waves and tides, cleaning leaf blades of particulate matter
and expunging concentration gradients.
Life in the shallow sea elimi­
nates the problem of water loss and allows the exploitation of nutrient­
laden sediments (McRoy et at., 1972), which reduces the limitation of
leaf area production.
A high surface area can be advantageous, provid­
ing a large exchange surface for absorption of the essential compounds
and elements.
The growth form of seagrasses is suited to the production of high
LAIs.
Submerged aquatic plants, in general, have little or no aboveground
structural material with the possible exception of the stem region and
aboveground rhizome sections.
This provides for the absorption of light
largely or entirely by photosynthetic tissue.
Presumably the transloca­
tion of carbon within and between shoots dampens inequities in the light
45
Table 3.
Comparison of the maximum LAI of aquatic and terrestrial eco­
systems.
Ecosystem
Maximum LAI
Reference
Seagrass:
Posidonia
II
16.8
This study
\D
O
i
—1
Zostera
II
Brown, 1962
14.2
Bay, 1978
8.3
Drew, 1971
7.0
Drew, 1978
9. 3*
Gessner, 1971
2.3
McRoy, unpublished
Cymodoeea/Caulerpa
5.5*
Gessner and Hamner, 1960
Cymodocea
1.4
Drew, 1978
11
-i/ICC LCLSS'hCL
Temperate Evergreen Forest
20
Waring and Franklin, 1979
Temperate Deciduous Forest
13.7
Arunga and Monsi, 1963
Tropical Rain Forest
12
Mooney, 1972
Grasses and Herbs
11.1
Arunga and Monsi, 1963
Agriculture
10
Marzola and Bartholomew, 1979
Marine Algae
9.3
McFarland and Prescott, 1959
Freshwater Plants
8.1
van der Valk and Bliss, 1971
Alpine Tundra
3.6
Caldwell et
Arctic Tundra
1.4
Dennis et al., 1979
0. 7
Arunga and Mon^l, 19 oj
0.6
Conner et a l ., 1974
Prairie
dl.3
1974
-■'indicates reported LAI was divided by 2 to obtain one-sided LAI
46
profile.
The lateral branching of the rhizome with new shoot production
allows for rapid response to increasing seasonal or annual irradiances.
Correspondingly, the sloughing of shoots can rapidly adjust for decreas­
ing light.
The rapid response to changing light regimes leads to more
efficient adaptation.
There are few naturally maintained monocultures:
have many plant species.
most ecosystems
Yet the temperate and some tropical seagrasses
form single species stands of the major plants (den Hartog, 1970).
Fields of plants dominated by a single species typically have LAI values
greater than corresponding mixed species plant stands (Arunga and v onsi,
1963; Dennis et at., 1979),
In addition to these possible factors contributing to the high LAI
observed in this study, the temperate seagrasses have unique character­
istics that may account for the LAIs that are higher than other aquatic
angiosperms.
The high latitude seagrass stands tend to be in shallower
water than lev latitude seagrasses.
The shallow water accounts for less
light filtering by water especially at low tide.
Also the epiphytes, a
major component of tropical seagrass ecosystems (McRoy and McMillan,
1977) and of aquatic macrophyte stands (Hutchinson, 1975, p. 515), do
not appear to be a major component of the Zostera. stand at Izembek Lagoon.
The relatively low epiphyte standing stock is likely due to factors
other than those contributing to the eelgrass standing stock.
Some in­
vestigators feel that epiphytes operate independently of the macrophyte;
they suggest different controlling factors (Capone et at. , 1979).
47
The high LAI that the eelgrass meadow produces has important
effects on the ecosystem.
The high LAI provides and intensifies feed­
back to the physical environment.
An effect of the leaf area is to dramatically increase the habitat
complexity.
An eelgrass bed with a high LAI can effectively increase
the sea floor by 30 times (one-sided LAI x 2).
An obvious effect is
the increased substrate for attachment by epiphytic organisms (McRoy
and McMillan, 1977).
Other than epiphytes, an associated microfauna
and other animals utilize the eelgrass leaves in various ways (Kikuchi
and Peres, 1977).
The functioning of this seagrass ecosystem as a nurs­
ery area for juvenile fishes and invertebrates (Kikuchi and Peres, 1977
is extremely enhanced with high LAIs.
During an ebbing tide at Izembek Lagoon, water f].ows from the
raised benches that contain eelgrass into the tide channels.
The leaf
blades serve as baffles reducing the water flow (Ginsburg and
Lowenstam, 1958; Burrell and Schubel, 1977).
Some water is usually
retained until the flood tide, protecting both the eelgrass and benthic
fauna from dessication.
The baffling effect of a high LAI also leads
to extraction and entrapment of fine waterborne particles.
The binding
and stabilization of the sediments by the root system along with the
increased production of sedimentary material can bring about dramatic
alteration of the sedimentation processes (Burrell and Schubel, 1977).
48
SUMMARY AND CONCLUSIONS
The variety of physiological and biological plant responses to gra­
dients of light quality and quantity can be grouped into four categories
that make up a predictive model of light adaptation in plants.
Testing
this model was done in a temperate seagrass ecosystem dominated by
Zostera marina.
The light adaptations were investigated along a depth
transect in Izembek Lagoon.
The transect ran across a natural gradient
of plant development which reflected a gradient of ecosystem processes.
Changes in the light regime were induced in situ at two points along the
transect, with an increased and decreased light experiment at both
points.
The eelgrass canopy was sampled by collecting vertical sections
of leaves (stratified clip technique) for measurements of chlorophyll a
to b ratios (chromatic and quantum capture adaptations), chlorophyll
amounts (pigment adaptation) and leaf area distribution (leaf area adap­
tation).
Chlorophyll ratios and amounts and leaf production measure­
ments were made at the different light treatments.
The chlorophyll ratios and amounts did vary substantially within
and between shoots.
However, when large integrative samples were mea­
sured, there was little evidence of major adaptive trends.
The chloro­
phyll ratios and amounts did not vary substantially along the natural
gradient of seagrass development or under the in situ increased and de­
creased light regimes.
The relative vertical distribution of leaf area
was constant throughout the eelgrass meadow.
However, the absolute dis­
tribution of leaf area, LAI, changes significantly along the gradient
of plant development.
Leaf production was affected by the changes in
49
light.
The results of the light manipulation experiments and the dis­
tribution of LAI along the transect indicate a light limiting situation
in the subtidal stations.
Combining this with transect sediment nutrient
pools (McRoy, unpublished data) which indicate a nutrient limiting situa­
tion in the tide pool stations yields a hypothesized gradient of limiting
factors (Figure 17).
In conclusion, the change of leaf area is the major adaptive mecha­
nism to changing light regimes of the Zostera marina stand in Izembek
Lagoon.
The other physiological adaptive mechanisms, photosynthetic
pigments ratios, functional state or amounts, are less important.
The
chlorophyll a to b ratio is relatively low (2.5), but within the range
of other submerged aquatic plants.
The chlorophyll concentrations of
2
the leaves (1.8 mg chi a+b / d m ) is also relatively low compared to other
plants.
The maximum LAI (17) is one of the highest reported for any
aquatic or terrestrial ecosystem.
The low leaf chlorophyll content,
shallow water habitat, tidal flushing, growth form, and single species
stand are factors contributing to the high LAI.
The LAI interacts in
the seagrass ecosystem to increase habitat complexity, provide attach­
ment substrate, increase water retention, and stimulate sedimentation.
LIM ITA TIO N
OF
DEGRE E
STA TIO N
<- T I D E P O O L
Figure 17.
S U B T I D A L -»•
Hypothesized major limiting factors along the transect.
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APPENDIX
AF AREA/DRY W EIGHT AND CHLOROPHYLL D A T A FROM
IZEMBEK LAGOON, 1978
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4> LO hO I— 1 4> LO t-o h—* LO N3 h-1 1 3 I-* h—1 Ln
O O O o O O O O O O O i.) Ln o
O 1— * LO
• • •
LO
O Ln
o
•
•
H
-vj M
LO LO H
o
•
o
o
II
H
h-4 h-1 o
O n ON LO
00 CO
I
LO
O
o
M
O
1
o
•
o
H*
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o h-1O
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00 Ln VO
to '•j to 00 LO
LO O
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>
«
oo
o Oo o o
• • • • •
o o o o o
o
H o 1-*
ho O'1O
No
NO
o
•
o
o
o
•
o
o
o
• Dry Weight
o
o (g dry/shoot)
l 1
1 1
1
Lo
M
O
Ln
LO 4> Ln LO Lo ho M
« • » • • • * 1
VvJ LO LO H-* )— 1 CO VO
1
M
h-» ho ro H* M I— 1 h-*
• • • • ■ •
1 1 1• •
ho
ho O
VO o 4> VO ON
i
4> hO o
1 ’ •
hO O O N
1
1 •
h-*
o
1
O o
•
LO LO
1
1
Ui
1
Distance Above
B o ttom (cm)
o H
M H* M
h-*
H*
H* N3 H4 M I—4
mg c h i a/dir,
* 1 1 • ^ i 1 • 1 •
11 •
Ln
vO O
LO
Ln ho Ln 4>
vO N> ON Ln 4>
H*
ON
K> O •^J 00 00
o^ o 00
O b-^ I'O Ln
O
O O
o
o O O
o o o O
O o o o O
o o O o
1 • • •
1 • * • •
1 1•
1 l • i • • •
1 1• • •
Ln
O
N
O
N
VO
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N
O
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a
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0
o
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vO NO
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o LO
ho ho ho
1 • •
ro ho
Leaf Area
(cm^/ s h o o t )
—1
M H o o o M M M o o O !
O o ho H* H-* Lo M O o o o
•
• • • • • • • • • • • • • • • • • • • • • • • •
Ln
Ln vO ON LAI
4>* OD o LO H*
CO
ro hO O
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ho O hO 00 LO O Ln M bo -£> LO 00 -^i LO a> ho CO hO N3 ON
o h-1a> o
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o
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o o
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VO O N Ln
O
Ln Ln
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Ln O 00
I
—* 1
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o
1
Total Shoot
Density (No./m2 )
o
o OOo o
• • • • •
o o o o o
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CO 00 vO 1— 1 Ln U t h
|
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n
Station
■£> LO
Ln 4
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0 0
0 0 O 0
O 0 0 0
1 1 1 1 ? 1 1 ? 1 1 J 1 ?
0-1 ■FN LO t o M ON Ln 4> LO I'O M Ln
o o O O O O o O O o o O O
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• • •
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1
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a
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10
Ln
Ln
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Pr-i
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10
20
CO
1
w
o
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53
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10
20
30
40
t
h-» M M h-*
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—1
k • • • • • • I I I • • •
M 4> Ln Ln M O VO
fo H U
4> vO
CO
40
50
1
10
15
15
20
10
20
30
H*
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z z z z
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fD
CO
1
1
Ul
LO
LO
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mg chi blg dry
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z z z z
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to
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z
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Ln
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CO
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LO Ln OJ CO
4>> o o>
0.006
o
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M
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•
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M
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0.70
K* ho ro
• • •
LO
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ro VO -o
O'
o
•
o
o
•
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•
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o
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O
•
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h—* o
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M
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Ln
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to
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1
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1 0 0 0 0 0 0
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vo
1
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Ln 4> Ln O '
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1
1
LO LO LO LO ro
1 • • • « * 1
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1 * • • •
1
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LO
Dry Weight
M 0
• •
O LO LAI
h - 1 H*
• •
ro O
O ' ro
Ln Ln Ln LO
• • •
O ' O
O O
O
•
Ln -P' LO
O -P"
Leaf Area
o, .
ro
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M
l • •
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0
l_n-P'U)hoHJ>-
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LO LO
• • • •
1 •
VO M VO vO VO
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CTn
0 O 0
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0 O O
ro ro H 1 0
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• • • • • • •
VO H-* 4> 00 CTN LO O
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VO LO Ln M
VO 00 ^4
Ln VO Ln
1 •
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LO VO ro oo
CO
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M
0
0
•
0
ro
4> ro O
•
CO LO Ln
VO LO ro
vo
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•
Total Shoot
.
, o.
Density (No./m )
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LO
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O '
0 0 O O
• • •
1
CO -o "J O n
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CO
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O
0
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00
LO
z
O
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•
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1
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0
O
w W H
o o o i
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0 O 0 O
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LO ho VO vO O ' ro 00
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00
CO
VO -C> CT>
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O O 0
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LO -P- Ln Ln
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o o p o oO| oO| c01 oO1 o01 oO| o01 oOI oO| oO| Canopy Segment:
I I I T I I I I I I I I I I .
6
H-1 00 ***4 O ' Ln -P- LO to H ■o- (cm)
Ln
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LO -P- o> Ln Ln
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Lo
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-> M
Distance Above
Bo ttom
(cm)
mg chi a/dmz
mg chi b/g dry
W' • ‘
ho
ho
ro
c_<
c
ho
C_
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p
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CO
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to
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1-0
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to
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1
O O Q O O Q Q O O O p Q O Q O O Q O L n O L n O L n O
i
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i i T T i ‘pT Ti T Ti Tr 'i i Ti Ti iT i1 iT •
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h - * O Oh -*H -*h O}— ‘ M O O O O O O O O f —l b - * O O O I —J H-l O
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LO M H
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• • • • « • • • • • • • •
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z
z
f—1 M M H M H o
• • ■ • • • •
Ln oo NO O o
CO
CO M Ln
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m to
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f_u ON Density
Ln -P> oo NO I-1
ON
Canopy
0
? ? 1 ? ? ? ?
(cm)
ON Ui -O oo NO f—*
o o o o O o o
OO Ln
# • •
CTx
ON
Ln Ln NO
00
•
Ln
WO
ON
•
NO
ON
00
•
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CO
o
• Leaf
4>
4> (cm2
1 o
o> Ln
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ro NO 1—* M
1 • • • •
WO M Oo
Ln NO
r
O
•
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o
O
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Ui Ui Ln NO
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On
1
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•
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Distance Above
Bottom (cm)
-O
'
oo
1h-*
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1« « • • •
1
LAI
o
• • • 1
-C
>o o
M
Area
/shoot)
• • • I
Oo OO -J
ro Ln O n Ln 4> 00 O
WO CO Ln
1 • • • • • • • 1 • • • •
wo I—
o -!>
• M M 00 00 NO
1wo
•
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Segment
o o O O o O o o o
o o h-* ro NO 00 Oo ro o
'-j
4> WO Ln ro WO NO
o O
o OOo Oo o
• • 1 • •
1 « • • •
••J Ln Ui 4> £ > Oo Ln
wo Ln
00 O
o wo 00 ■--J Ln wO Ln
•
( N o ./, r )
o o O O o Oo o O
? D r y Weight
• • « • • • • • •
o o o O H-1 f—* NO
• • • « • • •
o> NO ON WO
WO NO
Ul M oo ro Ln O
O n Ln -O 00 NO H
O O o O O o
z
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4>
C_
u0 cP Date
ro ro
'■'J
oooo
[-r]t3 )Station
to -C'Total Shoot
•
4> ro
•
1
mg chi b / g dry
CTN
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ro
LO
c-<
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p
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CO
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a
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LO
C-s
c
p
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Ln
Ln
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o
Ln
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co
LO
•
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h-»
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Total Shoot
density (No./m2)
Canopy Segment
(cm)
1eaf Area
(cm2/shoot)
o o O O O o o
• • • • • • •
o o o o o o o
to ro IO to
M
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h-1 o (g dry/shoot)
Mo O O
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to 4> CO H* Ln sl OJ sl M •P*
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Ln oo Ln
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Ln to Ln o ui
o o O o o o
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Ln
4> Ln 0J ro
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I
1M
Distance Above
lottom (cm)
mg chi a /dm2
mg chi Z)/dm2
NO ro ro to ro OJ to h *
0J OJ OJ 4> OJ IO
• • • • •
1 • • • • • • • • •
P
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o o
H o
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ON 00 00 o b-4 VO -P- <D 0J
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20July78-A 4782
It
tt
11
11
20July78-B 5680
tt
11
ft
11
11
tl
11
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11
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20July78-C 4006
II
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tt
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11
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11
11
11
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tt
tl
0-10
10-20
20-30
0-10
10-20
20-30
30-40
40-50
50-60
0-10
10-20
20-30
30-40
40-50
50-60
60-70
4.15
5.07
2.36
2.50
4.14
4.16
3.28
1. 92
0.32
2.05
3.66
3.95
3.39
2.83
1.39
0.42
70
80
90
100
10
20
30
40
50
60
70
80
90
-10
20
30
40
50
60
70
80
0.019
0.014
0.006
0.008
0.010
0.010
0.008
0.005
0.001
0.008
0.011
0.011
0.009
0.007
0.003
0.001
1.98
2.42
1.13
1.42
2. 35
2. 36
1. 86
1.09
0.18
0.82
1.47
1.58
1.36
1.13
0. 56
0.17
10
20
10
20
30
40
50
"H
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G
CO
e
0.49
0.55
0.46
0.42
0. 30
0.37
0.49
0.63
0.57
0.51
0.53
0.46
0,47
0. ~n
0. 79
1,19
1.49
1.44
2.12
2.24
2.51
2.17
0.34
0.49
0.61
0.58
0.84
0.89
0.97
0.87
-
1.6
1.9
1.7
1.5
0.4
0.8
1.4
1.7
1.6
1.8
1.7
1.3
1.5
1.4
0.7
1.0
1.4
1.7
1.5
2.1
3.2
2.4
-
-
-
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10
20
30
40
50
60
-
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1.12
1. 20
1.55
1.21
1.31
-
1.01
1.19
1.12
0.90
1.04
1.28
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0.45
0.42
0.47
0.52
0.68
0.56
0.57
-
0.41
0.51
0.48
1.40
0.46
0.63
—
dry
dry
a/g
4.0
4.5
4.0
3.7
1,2
2.1
3.4
4.3
4.1
4.7
4.5
3. 7
4.2
4.0
1.7
2.4
3.5
4.2
3. 8
5.2
8.4
5.9
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1.02
0.81
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s
1.23
1.31
1.09
1.00
0.83
0.94
1.23
1.60
1.46
1.34
1.38
1.29
1.33
x.-ti;
mg
1.19
0.88
0.23
0.16
0.81
1.28
1.98
2.08
1.94
1.72
1.40
1.04
0.71
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0.15
0.73
1.13
1. 79
1. 70
1.52
1.37
1.08
0.81
0.44
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0.003
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0.035
0.038
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Canopy
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9.15
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70-80
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Distance Above
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Leaf Area
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Segment
Total Siioot
Density (No./m2
64
2.3
1.8
-
2.4
3. 7
4.8
2.8
-
1.0
1.0
1.0
1.6
2.1
1.3
-
-
-
1.4
4.5
3.8
2.9
2.5
2.4
0.6
1.9
1,7
1.3
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Density (No./m2)
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o o o o O O O O o O O o o o ° Bottom (cm)
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