University of Montana
ScholarWorks at University of Montana
Theses, Dissertations, Professional Papers
Graduate School
1997
Spatial patterns of woody plant regeneration in two
California Central Valley floodplain forests
William Marquiss Jones
The University of Montana
Follow this and additional works at: http://scholarworks.umt.edu/etd
Recommended Citation
Jones, William Marquiss, "Spatial patterns of woody plant regeneration in two California Central Valley floodplain forests" (1997).
Theses, Dissertations, Professional Papers. Paper 6839.
This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for
inclusion in Theses, Dissertations, Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more
information, please contact [email protected].
Maureen and Mike
MANSFIELD LIBRARY
The University o f
IVIONTANA
Permission is granted by the author to reproduce this material in its entirety,
provided that this material is used for scholarly purposes and is properly cited in
published works and reports.
** Please check "Yes" or "No" and provide signature
Yes, I grant permission
No, I do not grant permission
**
X
___
Author's Signature
Date
6Æ / f f
Any copying for commercial purposes or financial gain may be undertaken only with
the author's explicit consent.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SPATIAL PATTERNS OF WOODY PLANT REGENERATION IN TWO
CALIFORNIA CENTRAL VALLEY FLOODPLAIN FORESTS
by
W illiam M arquiss Jones
B.A. U niversity o f C alifornia at Berkeley, 1990
presented in partial fulfillm ent o f the requirem ents
for the degree o f
M aster o f Science
U niversity o f M ontana
1997
Approved by:
Chanperson
Dean, Graduate School
Date
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Number; EP3764Û
All rights reserved
IN FO R M A TIO N TO ALL U SER S
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI
OisMTtation PuWishing
UMI EP37640
Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
All rights reserved. This work is protected against
unauthorized copying under Title 17, United States Code
Pro.Q^st'
ProQuest LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, Ml 4 8 1 0 6 - 1 3 4 6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Jones, William Marquiss, M.S., May 1997
Environmental Studies
Spatial patterns of woody plant regeneration in two California Central Valley floodplain
forests (32 pp.)
Committee Chair: Len Broberg
A bstract
Because flooding is a frequent and predictable disturbance in riparian plant
communities, many studies have focused on the influence o f abiotic factors on woody
plant regeneration in these communities. However, biotic factors may also affect
patterns o f plant regeneration in riparian communities, especially in portions o f the
floodplain subject to less frequent and intense flood disturbances. I examined the
correlation between woody seedlings and groundcover and overstory composition at two
sites with low flood recurrence intervals in the floodplain o f the Sacramento River,
California. Seedlings were distributed nonrandomly among both groundcover and
overstory classes at both study sites. Seedlings of all species examined were negatively
spatially associated with forb and liana cover and positively spatially associated with
litter-covered substrate. Additionally, seedling densities were lower in forb and liana
cover than in more open groundcover types for several species as well as all species
combined. Seedling distribution among overstory cover types was strongly species
specific. My results imply that plant-plant interactions may constrain the regeneration o f
woody species at these sites.
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acknowledgments
Many good and kind people helped me with this research: Peter Hujik and Steve Greco
took time out of busy schedules to think hard about how I could refine my research
methods; Tom Griggs always made time for my questions and allowed me to set up field
camp on Nature Conservancy land; Dawit Zeleke offered hearth and home; Stacy
Cepello, Julie Cunningham, and Joyce Lacey were unstintingly generous with their time
and expertise; and Jim Snowden allowed me access to California Department o f Fish and
Game lands along the Sacramento River. This project would have been infinitely more
difficult to implement and not nearly as much ftin without their help.
I also wish to thank my committee members, Drs. Len Broberg, Ray Callaway, and Vicki
Watson, for their thoughtful comments and encouragement throughout this project. 1 am
also grateful to Dr. Colin Henderson for help with statistical analyses.
For a late-season kick in the pants, I am grateful to Lyn Baldwin, editor extraordinaire.
And finally, I wish to thank my parents, who have long known that magic informs this
world and that wonderment is our natural inheritance.
Ill
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE OF CONTENTS
A b s t r a c t ...................................................................................................................................................................................................... ii
A c k n o w l e d g m e n t s .............................................................................................................................................................................. iii
U N T R O D U C n O N .................................................................................................................................................................................. 1
S T U D Y A R E A ..........................................................................................................................................................................................3
M E T H O D S ................................................................................................................................................................................................ 5
S p e c ie s ........................................................................................................................................................................................................... 5
S a m p l e D e s i g n ......................................................................................................................................................................................... 6
D a t a A n a l y s i s ......................................................................................................................................................................................... 7
R E S U L T S ............................................................
9
SriE C h a r a c t e r is t ic s a n d S e e d l in g s ..................................................
9
SEEDLING D ist r ib u t io n a n d d e n s i t y a m o n g g r o u n d c o v e r C l a s s e s .....................................................................9
S e e d l in g d is t r ib u t io n a n d D e n s it y A m o n g O v e r s t o r y C l a s s e s ...........................................................................14
S e e d l in g D is t r ib u t io n a n d D e n s it y U n d e r C a n o p y a n d in C a n o p y G a p s ........................................................ 16
D I S C U S S I O N ........................................................................................................................................................................................ 18
S e e d l in g -G r o u n d c o v e r A s s o c ia t io n s ...................................................................................................................................18
S e e d l in g -O v e r s t o r y A s s o c ia t io n s ......................................................................................................................................... 21
S e e d l in g -C a n o p y G a p A s s o c i a t i o n s ...................................................................................................................................... 23
C o n c l u s io n ............................................................................................................................................................................................. 2 4
R E F E R E N C E S ..................................................................................................................................................................................... 2 6
A P P E N D I X A ........................................................................................................................................................................................3 1
A P P E N D I X B ......................................................................................................................................................................................... 3 2
LIST OF TABLES
Table
Table
Table
Table
T a b le
1.
2.
3.
4.
5.
P r o p o r t io n o f G r o u n d c o v e r a n d O v e r s t o r y C l a s s e s a t E a c h S t u d y S i t e ............................ 9
S e e d l in g n u m b e r a n d d e n s i t y a t E a c h S t u d y S i t e ................................................................................ lo
D is 'ir ib u t io n o f S e e d l in g s with R espe c t t o G r o u n d c o v e r a t E a c h S t u d y S it e ................. 11
SEEDLING DENSITIES WITH RESPECT TO GROUNDCOVER AT EACH STUDY SITE ................................ 14
D i s t r i b u t i o n o f J. c a u f o r n i c a , P. f r e m o n t h , a n d ^ . n e g u n d o S e e d l i n g s w it h R e s p e c t
TO OVERSTORY AT THE KOPTA SLOUGH S t u d y S ite
................................................................................... 15
T a b l e 6. S e e d lin g D e n s i t i e s w it h R e s p e c t t o O v e r s t o r y f o r J. c .4 jjf o r n ic a , P. f r e x i o n w , a n d
A . NEGUNDO AT THE KOPTA SLOUGH STUDY SITE............................................................................................. 16
T a b l e 7. D i s t r i b u t i o n o f A l l S e e d lin g s , J. c a u f o r n ic a , a n d ^ I. n e g u n d o w i t h R e s p e c t t o
C a n o p y G a p s a t the K o p t a S l o u g h S t u d y S it e ............................................................................................. 17
T a b l e 8. s e e d l i n g d e n s i t i e s w i t h r e s p e c t t o C a n o p y G a p s f o r a l l S e e d lin g s , j . c a u f o r n i c a ,
P. FREMONTII, A N D /i. NEGUNDO AT ITIE KOPTA SLOUGH STUDY SITE........................................................ 17
IV
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
INTRODUCTION
Flood disturbance can be important in explaining the distribution o f riparian plant
species (Johnson et al. 1976, Nanson and Beach 1977, Robertson et al. 1978, Hupp and
Osterkamp 1985, Harris 1987). Flood disturbances do not occur uniformly across a
river’s floodplain, but vary along elevational and hydrological gradients. Flood events
are longer, more severe, and more frequent on younger fluvial landforms, such as point
bars, than on higher terrace deposits (Wissmar and Swanson 1990). On younger fluvial
landforms, abiotic factors, such as soil texture, soil moisture, and flow regime, have been
strongly correlated with successful regeneration of woody plants (McBride and Strahan
1984, Sacchi and Price 1992). It is possible, however, that the relative importance o f
factors influencing woody plant regeneration vary along this flood disturbance gradient.
On older landforms subject to less frequent and intense flood disturbances, such as
floodplain and terrace deposits, biotic interactions may become important for successful
seedling establishment and survival.
The Sacramento River in California’s Central Valley is an ideal system to study
regeneration patterns within floodplain forest. The Sacramento is a large, meandering
river with a well developed floodplain. Channel-floodplain dynamics have been
substantially altered by flood control levees and reservoirs, but the unconfined river reach
in the upper Sacramento Valley is subject to active lateral channel migration and
supports remnant patches o f floodplain forest. Plant assemblages along the Sacramento
River are distributed predictably along an elevational gradient (Conard et al. 1977,
Strahan 1984, Cepello 1991). Point bars are characterized by open substrate, and channel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
shelf deposits tend to be dominated by a dense cover o f Salix exigua Nutt, (sandbar
willow). Floodplain deposits are characterized by gallery forests o f Populus frem ontii S.
Watson ssp. frem ontii (Fremont cottonwood) and Salix gooddingii C. Ball (Goodding’s
black willow). Higher floodplain and terrace deposits are characterized by a structurally
and floristically diverse mixed riparian forest, with an overstory o f P. fremontii, Juglans
californica S. Watson var. hindsii Jepson (California black walnut), Quercus lobata Nee
(valley oak), and Platanus racemosa Nutt, (western sycamore). Finally, high terraces are
characterized by an open Q. lobata forest.
Conard et al. (1977) and Strahan (1984) hypothesized that differing hydrological
regimes caused these vegetation patterns. Cepello (1991) analyzed vegetation patterns
with respect to hydrology, soil texture, density o f overstory, and density of groundcover.
He found that the distribution o f woody vegetation had the greatest correlation with
hydrology, as defined by flood recurrence interval. He defined three distinct fluvial
landforms on the basis o f flood recurrence: channel shelf (recurrence interval less than
1.5 yr), floodplain (recurrence interval between 1.5 yr and 10 yr), and terrace (recurrence
interval greater than 10 yr). He found Salix spp. strongly associated with channel shelf;
Populus fremontii, adult S. gooddingii, A cer negundo L. var. californicum (Torrey & A.
Gray) Sarg. (box elder), and Sambucus mexicana C. Presl. (blue elderberry) strongly
associated with floodplain; Ficus carica L. (edible fig), an exotic, strongly associated
with terrace; and Juglans californica associated equally with floodplain and terrace.
The correlation between species distributions and flood recurrence interval
suggests that flood disturbance exerts an important influence in structuring vegetation
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3
assemblages among fluvial landforms. Flood disturbance may restrict certain species to
floodplain and terrace landforms; however, regeneration within these landforms might be
limited by other factors. Successful regeneration by seed occurs at three distinct phases;
seed dispersal, seed germination, and seedling survival (Jones et al. 1994). Successful
regeneration sites must satisfy a species’ life history requirements at all three phases. It
is possible that biotic interactions delimit the regeneration niche (sensu Grubb 1977) o f
species associated with floodplains and terraces at one or all o f these developmental
phases.
If biotic factors, such as plant-plant interactions, influence within-landform
regeneration success, it is likely that seedlings will be nonrandomly distributed with
respect to vegetative cover in either groundcover or the overstory. If certain plant species
or cover types facilitate seedling establishment or survival, then seedlings should be
positively spatially correlated with those species or cover types (McAuliffe 1988,
Callaway 1992). If certain plant species or cover types interfere with seedling
establishment or survival, then seedlings should be negatively spatially correlated with
those species or cover types (Maguire and Forman 1983, Clinton et al. 1994). I tested
this hypothesis by examining the spatial distributions o f woody seedlings with respect to
groundcover and overstory composition at two sites with relatively low flood recurrences
on the middle Sacramento River.
STUDY AREA
The study area was on the middle Sacramento River, California, between the
town o f Red Bluff and Chico Landing. This reach o f the river contains most o f the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4
largest and least disturbed remnant patches o f riparian vegetation remaining in the
Sacramento River watershed (California Resources Agency 1989). Though banks have
been locally stabilized with rock revetment, this portion o f the river is largely unlevied
(California Resources Agency 1989). Thus, the river channel and associated fluvial
landforms are still shaped by erosional-depositional processes.
Using vegetation atlases o f the Sacramento River (McGill 1987, California
Department o f Fish and Game 1988), I delineated remnant riparian vegetation patches on
the basis o f overstory composition and fluvial landform. McGill (1987) defined two
landforms for established riparian vegetation; high terrace, which is generally free from
inundation except during high river flows, and low terrace, which is generally inundated
at only moderately high river flows. Additionally, potential study sites were limited to
locations in public ownership accessible by land. I used these criteria to select one high
and one low terrace site.
The first site, Kopta Slough, was a high terrace site located at river mile 219.0 in
Tehama County (55 m elevation, 39°55' N, 122°05’ W). The second site. Pine Creek,
was a low terrace site located at river mile 197.0 in Butte County (40 m elevation, 39°44'
N, 121°58' W). The Kopta Slough site is on the Woodson Bridge Nature Preserve
managed by the California Department o f Parks and Recreation. The site is dominated
by Juglans californica and has a high proportion o f liana cover as well as dense pockets
o f Ficus carica (Appendix A). The Pine Creek site is on the Pine Creek Wildlife Area
managed by the California Department of Fish and Game. The site is dominated by
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5
Populus frem ontii and is characterized by a more open understory dominated by forbs
(Appendix B).
METHODS
Species
I evaluated seedling distributions o f eight native woody species common in
California Central Valley riparian forests. I defined seedlings as stems < 1.0 cm diameter
at ground level. Species examined were Acer negundo var. californicum, Juglans
californica var. hindsii, Quercus lobata, Sambucus mexicana, Populus frem ontii ssp.
frem ontii, Arisiolochia californica Torrey (California pipe vine), Platanus racemosa, and
Fraxinus latifolia Benth. (Oregon ash). Nomenclature follows Hickman (1993).
Because o f low sample size, F. latifolia (n = 3 seedlings) and P. racemosa (n = 1
seedling) were not included in the analysis.
Though the six species examined vary in their life history traits, all are associated
with floodplain or terrace landforms at one or more life stages. Populus fremontii
produces large amounts o f small wind dispersed seeds with short viability, is shade
intolerant, and usually colonizes open substrates (Holstein 1984, Ribiero 1991). Acer
negundo produces somewhat heavier seeds which are less well dispersed and is shade
tolerant (Holstein 1984, Ribiero 1991). Both J. californica and Q. lobata produce heavy
seeds that are dispersed by gravity or animals. Though it is unclear how important
animal-mediated dispersal is for J. californica, frugivorous birds are an important
dispersal agent for many oak species (Darley-Hill and Johnson 1981, Harrison and
W erner 1984). Sambucus mexicana produces large crops o f berries, and there is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
evidence that bird dispersal is important for other congeneric species (Debussche and
Isenmann 1994).
Sample Design
I defined the sampling area at each location as the portion of the site occupied by
either P. frem ontii or mixed riparian forest. Portions o f sites along active channel
margins or on landforms such as point bars were not included in the study. Using aerial
photographs (USAGE 1986), I measured the sampling area at the Kopta Slough and Pine
Creek sites at 76 ha and 32 ha, respectively. I divided each location into 50 x 50 m cells
and randomly selected individual cells until I had sampled approximately 4 percent o f
each site. I completed all field work in August and September, 1995.
I used 50-m line-intercepts to measure the proportion of groundcover and
overstory cover types (Brower et al. 1990). Line-intercepts were laid out along randomly
chosen compass bearings using the centers of selected cells as their midpoints. I
measured both groundcover and overstory composition along 0.1-m intervals. I recorded
the dominant groundcover type along the line-intercept interval and the dominant
overstory species or canopy gap vertically over the line-intercept interval. Groundcover
types included bare mineral soil, leaf litter, woody debris, and vegetative cover, which
was recorded to species or life form. Overstory was recorded to species. Where multiple
species occurred in the canopy, only the tallest was recorded.
I measured seedling-cover type associations in 20 x 50 m plots bisected by lineintercepts. I recorded the dominant groundcover immediately adjacent to and the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
7
presence and composition o f the overstory vertically above each seedling. I recorded the
presence o f all native woody seedlings.
Data Analysis
To determine patterns o f seedling distribution with respect to groundcover, I
collapsed similar groundcover types into five cover classes; (1) litter, which consisted
primarily o f vegetative litter (leaves, duff, organic debris), but also included bare mineral
soil and recent alluvial deposition (soil texture predominately sand or silt. Pine Creek site
only); (2) woody debris; (3) graminoid, which included both grasses and sedge species;
(4) forb, which was dominated by Artemisia douglasiana Besser (mugwort) and Solarium
americanum Miller (American black nightshade); and (5) liana, which was dominated by
Vitis californica Benth. (California wild grape) and the vines Ruhus discolor Weihe &
Nees (Himalayan blackberry) and R. ursinus Cham. & Schldl. (California blackberry),
but also included dense groundcover o f woody species such as Ficus carica.
To determine patterns of seedling distributions with respect to overstory, I also
collapsed overstory types into five classes: the three dominant overstory species [(1)
Juglans californica, (2) Populus frem ontii, and (3) Acer negundo], (4) all other overstory
species (predominantly Platanus racemosa and Salix gooddingii), and (5) canopy gaps.
For this analysis, I was primarily interested in intraspecific patterns between seedlings
and adults. I therefore limited my analysis to seedlings o f the three dominant overstory
species, J. californica, P. frem ontii, and A. negundo. Because seedlings at the Pine
Creek study site occurred exclusively under P. frem ontii overstory, 1 only examined
seedling-overstory patterns at the Kopta Slough study site. Finally, to test for spatial
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8
associations between seedling distributions and canopy gaps, I collapsed overstory data
into two cover classes: under canopy and canopy gap.
I used two approaches to quantify spatial associations between seedling
distributions and cover class proportions. To test if seedlings were positively or
negatively associated with cover classes, I used chi-square goodness-of-fit tests (Sokal
and R ohlf 1995). This analysis tests for departures from a random distribution over all
cover classes. To test if seedling distributions differed with respect to specific cover
types, I compared seedling dei^ities within cover classes. Seedling density within each
cover class was calculated for each plot. The extent o f each plot occupied by a given
cover type was estimated by extrapolating from line-intercept proportions. On a few
plots, seedlings occurred within cover types that were not represented along the lineintercept, and these seedlings were not considered in the analysis.
Data were tested for normality with Shapiro-Wilks tests and for homogeneity o f
variance with Levene tests (Sokal and Rohlf 1995). As these data were both non-normal
and heteroscedastic, I used Kruskal-Wallace tests (Sokal and Rohlf 1995) to compare
seedling densities among cover classes. Where seedling densities were significantly
different, I used simultaneous unplanned comparisons (Gibbons 1976) to determine
which cover classes differed significantly from one another. Mann-Whitney U tests
(Sokal and Rohlf 1995) were used to compare seedling densities m gaps with densities
under canopy.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
RESULTS
Site Characteristics and Seedlings
Table 1 presents the proportion o f sample plots occupied by each groundcover
and overstory class at the Kopta Slough and Pine Creek study sites. Seedlings o f eight
woody species were recorded at the Kopta Slough study site, and seedlings o f five woody
species were recorded at the Pine Creek study site (Table 2). The number o f seedling
species was greatest at Kopta Slough; however, overall seedling density was greatest at
Pine Creek (Table 2).
T a b le 1— ^Proportion of groundcover and overstory classes at the Kopta Slough and
Pine Creek study sites. Proportion was derived from proportionate coverage of 50-m lineintercepts {n = 34, Kopta Slough; n = 14, Pine Creek).
Groundcover Classes
litter
woody debris
graminoid
forb
liana
Proportion
Kopta
Pine
Slough
Creek
0.271
0.262
0.124
0.183
0.142
0.124
0.092
0.311
0.371
0.120
Overstory Classes
open
Juglans californica
Populus fremontii
Acer negundo
other species
Proportion
Kopta
Pine
Creek
Slough
0.028
0.072
0.490
0.025
0.272
0.764
0.100
0.087
0.083
0.079
Seedling Distribution and Density Among Groundcover Classes
Seedling distributions were significantly nonrandom with respect to groundcover
composition at both study sites (chi-square goodness-of-fit tests; Table 3). While
seedling distributions among groundcover types varied by species, some patterns were
consistently found across all species. For all species, seedlings were positively associated
with litter and negatively associated with forb and liana cover (Table 3). Associations
between seedlings and woody debris and seedlings and graminoid cover varied by species
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
T able 2— Seedling number and density per hectare of each species and all species
Location
Kopta Slough
Pine Creek
All seedlings
Juglans californica var. hindsii
Acer negundo var. californicum
Sambucus mexicana
Populus fremontii ssp. Jremontii
Quercus lobata
Arisiolochia californica
Fraxinus latifolia
Platanus racemosa
Seedling Number
695
272
270
43
44
39
24
2
1
Seedling Density
(# seedlings/ha)
204.41
80.00
79.41
12.65
12.94
11.47
7.06
0.59
0.29
All seedlings
Acer negundo var. californicum
Juglans californica var. hindsii
Populus fremontii ssp. fremontii
Quercus lobata
Fraxinus latifolia
339
307
28
2
1
1
242.14
219.29
20.00
1.43
0.71
0.71
Species
but were generally positive (Table 3). Because of low expected frequencies o f seedlings
o f some species in one or more cover types, I did not include Arisiolochia californica
seedlings or Juglans californica seedlings at the Pine Creek study site in this analysis.
More seedlings occurred in litter than expected if seedlings were randomly
distributed with respect to groundcover type (Table 3). Litter covered 27 percent o f the
Kopta Slough study site, but contained 45 percent o f all seedlings (Table 3). Though all
species occurred in this cover type at greater than expected frequencies, the proportion
ranged from 36 percent o f Q. lobata seedlings to 70 percent o f S. mexicana seedlings
(Table 3). At Pine Creek, litter covered 26 percent o f the study area but contained 44
percent o f all seedlings (Table 3).
Seedling distributions within woody debris varied with both species and location
(Table 3). At Kopta Slough, more seedlings occurred in woody debris than expected
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11
T a b le 3— Distribution of seedlings of each species and all species combined with
respect to groundcover class at the Kopta Slough and Pine Creek study sites. Chi-square
statistics refer to goodness-of-fit tests for number of seedlings associated with each cover type.
_____________________________________ Kopta Slough____________________________________
Cover Type
litter
woody debris
graminoid
forb
liana
Cover Type
Utter
woody debris
graminoid
forb
Uana
Proportion
0.271
0.124
0.142
0.092
0.371
Pro­
portion
0.271
0.124
0.142
0.092
0.371
Number of associated
seedlings (all species
Number of associated
Acer negundo
Number of associated
Juglans californica
Observed Expected
315
188
156
86
152
99
30
64
258
42
« - 695 seedlings
X^ = 370.04, df = 4
P<.0001
Observed Expected
118
73
78
33
44
38
10
25
20
100
n = 270 seedlings
f = 162.44, d f = 4
P < 0001
Observed Expected
117
74
46
34
76
38
18
25
15
101
n = 272 seedlings
X^ = 142.41, d f = 4
P< .0001
Number of associated
Quercus lobata
seedlings
Observed Expected
14
11
5
18
5
5
4
1
1
14
n = 39 seedlings
= 48.94, df= 4
Number of associated
Sambucus mexicana
seedlings
Observed Expected
30
12
5
5
5
6
0
4
3
16
« = 43 seedlings
= 40.93, df = 4
f <0001
Number of associated
Populus fremontii
seedlings
Observed Expected
25
12
2
6
16
6
1
4
0
16
« = 44 seedlings
X^ = 50.73, d f = 4
f <0001
f < .0001
Pine Creek
Cover Type
litter
woody debris
graminoid
forb
liana
Pro­
portion
0.262
0.183
0.124
0.311
0.120
Number of associated
seedlings (all species
combined)
Observed Expected
150
89
68
62
68
42
46
105
7
41
n = 339 seedlings
X^=119 .83, d f = 4
P< .0001
Number of associated
Acer negundo
seedlings
Observed Expected
147
81
56
48
66
38
43
95
37
3
« = 307 seedlings
%' = 135.26, df = 4
P< .0001
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
12
when all seedlings were combined: woody debris covered 12 percent o f the study area
but contained 22 percent o f all seedlings (Table 3). However, spatial associations
between seedlings and woody debris varied greatly by species: only 5 percent o f P.
jrem ontii seedlings occurred in woody debris, compared to 46 percent o f Q. lobata
seedlings (Table 3). At Pine Creek, spatial associations between seedlings and woody
debris were not pronounced (Table 3).
For most species, more seedlings occurred in graminoid cover than expected
(Table 3). At Kopta Slough, graminoid cover occupied 14 percent o f the area but
contained 22 percent o f all seedlings and 36 percent of P. frem ontii seedlings (Table 3).
At Pine Creek, graminoid cover occupied 12 percent o f the study area but contained 20
percent o f all seedlings (Table 3).
Fewer seedlings occurred in both forb and liana cover than expected if seedlings
were randomly distributed with respect to groundcover type (Table 3). Negative
associations were consistent for all species examined at both study sites. At Kopta
Slough, forb cover occupied 9 percent o f the study area but contained 4 percent o f all
seedlings, and liana cover occupied 37 percent o f the area but contained 6 percent o f all
seedlings (Table 3). At Pine Creek, forb cover occupied 31 percent o f the study area but
contained 14 percent of all seedlings, and liana cover occupied 12 percent o f the study
area but contained only 2 percent o f all seedlings (Table 3).
Seedling density was significantly different among cover types for several
species. At Kopta Slough, seedling density was significantly different among cover types
for all species combined (Kruskal-Wallace test:
= 36.63, d f = 4, P < .0001) as well as
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
for Acer negundo (Kruskal-Wallace test:
californica (Kruskal-Wallace test:
(Kruskal-Wallace test:
= 16.06, d f = 4, P = .0029), Juglans
= 23.93, d f = A , P ^ .0001), and Quercus lobata
= 12.08, d f = 4, P = .0168) (Table 4). Seedling density was
not significantly different among cover types for Sambucus mexicana (Kruskal-Wallace
test:
= 5.25, d f = 4, P = .2627), Populus frem ontii (Kruskal-Wallace test: x^ ^ 3.18,
d f = 4, P = .5276), znà. Arisiolochia californica (Kruskal-Wallace test: x^ = 9.24, d f = 4,
P = .0554) (Table 4). At Pine Creek, seedling density was significantly different among
cover types for all species combined (Kruskal-Wallace test: x^ " 21.13, d f = 4, P =
.0003) and for A. negundo (Kruskal-Wallace test: x^
16.09, d f = 4, P = .0029) but was
not significantly different f o r/, californica (Kruskal-Wallace test:
= 4.75, d f = 4, P
= 3135) (Table 4).
At Kopta Slough, seedling density in litter, woody debris, and graminoid cover
was significantly higher than in forb and liana cover for all species combined, and
seedling density in litter, woody debris, and graminoid cover was significantly higher
than in liana cover for / californica (Table 4). At Pine Creek, seedling density in litter,
woody debris, and graminoid cover was significantly higher than in liana cover for all
species combined, and seedling density in litter was significantly higher than in liana
cover for A. negundo (Table 4). The simultaneous test procedure I used to make
unplanned comparisons was very conservative (experimentwise error rate o f a = .0025)
and did not correct for ties (Gibbons 1976). Though Kruskal-Wallace tests found that
seedling densities were significantly different among groundcover classes for A. negundo
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
14
T a b le 4— Seedling densities per 0.1 ha with respect to groundcover class at the Kopta
Slough and Pine Creek study sites. Data are means + 1 SD. Different superscript letters within
a row indicate significantly different values (P < .05, Kruskal-Wallace test followed by
simultaneous multiple comparisons), n —number of plots occupied by a given cover type.
Species
all species combined
Acer negundo*
Juglans californica
Quercus lobata*
Sambucus mexicana
Populusfremontii
Arisiolochia californica
litter
«= 31
43.1 + 51.4*
16.3 ±30.2
12.4 ± 16.3*
2.6 ±5.8
5.5 ±21.4"
4.7 ± 19.7*
1.7 ±4.4*
woody debris
« = 31
58.3 ±92.2*
26.3 ±48.2
21.0±43.1*
6.5 ±15.2
2.1 ±6.7*
1.3 ±6.5*
1.0 ±2.5*
Cover Type
graminoid
« = 25
32.7 ±44.5*
10.2±21.5
14.8 ±17.3*
0.7 ±2.2
0.7 ± 1.9"
5.7 ±28.6*
0.5 ± 1.8*
forb
« = 20
8.5 ± 12.8”
3.5 ± 10.3
4.8 ±7.7*”
0.2 ±0.7
0.0 ± 0.0"
0.1 ±0.4*
0.0 ±0.0*
bana
« = 31
4.1 ±8.2”
1.9 ±5.4
1.7 ±3.7”
0.2± 1.1
0.2 ± 0.9"
0.0 ± 0.0*
0.1 ±0.2*
Pine Creek
Cover Type
litter
woody debris
forb
graminoid
liana
«=12
n- 9
« = 14
« = 14
n = 14
Species
all species combined
58.7 ± 125.9* 35.0 ±57.0*
48.8 ±70.4*
8.4 ± 12.1*”
1.4 ±4.8”
Acer negundo
57.2 ± 126.4* 27.3 ± 59.4*”
47.7 ±71.2*”
7.7 ± 12.3*”
0.5 ± 1.6”
Juglans californica
1.5 ±3.7*
5.5 ± 12.9*
0.8 ±3.0*
0.5 ± 1.6*
0.6 ±2.2*
* Though Kruskal-Wallace tests found seedling densities significantly different among groundcover classes,
simultaneous multiple comparisons found no significant pairwise comparisons for these species.
and Q. lobata at Kopta Slough, no pairwise comparisons were significant for these
species.
Results for F. frem ontii seedlings should be viewed with caution. Populus
frem ontii seedlings were not common at either study site; only two P. fremontii seedlings
were found at Pine Creek, and at Kopta Slough, 32 of the 44 seedlings found occurred on
one plot. Thus, results are based on very localized distribution patterns.
Seedling Distribution and Density Among Overstory Classes
Juglans californica seedlings were significantly nonrandomly distributed with
respect to overstory composition: more J. californica seedlings occurred under
conspecific overstory and fewer occurred under all other overstory types than expected
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15
(Table 5). Juglans californica covered 49 percent of the Kopta Slough study site but
contained 74 percent o f J. californica seedlings, whereas P. frem ontii covered 27 percent
o f the area but contained 11 percent o f seedlings, and canopy openings covered 7 percent
o f the area but contained 4 percent o f seedlings (Table 5). Additionally, J. californica
seedling density was significantly different among overstory classes (Kruskal-Wallace
test:
= 15.16, d f ==4, f = ,0044): seedling density under J. californica canopy was
significantly higher than in canopy openings and Mndtx F. frem ontii canopy (Table 6).
Populus frem ontii seedlings were also significantly nonrandomly distributed with
respect to overstory: more P. frem ontii seedlings occurred under canopy openings and
fewer occurred under conspecific a n d /, californica overstory than expected if seedlings
were randomly distributed (Table 5). Canopy openings covered 7 percent o f the study
area but contained 50 percent o f P, frem ontii seedlings, whereas P. fremontii overstory
covered 27 percent o f the area but contained 2 percent o f seedlings (Table 5). However,
P, jrem ontii seedling density was not significantly different among overstory classes
(Kruskal-Wallace test: y} = 1.97, d f = 4 , P = .7408) (Table 6).
T a b le 5— Distribution of J. californica, P. fremontii, and A. negundo seedlings with
respect to overstory class at the Kopta Slough study site. Chi-square statistics refer to
goodness-of-fit tests for number of seedlings associated with each cover type.
Cover Type
open
J. californica
P. fremontii
A. negundo
other species
Pro­
portion
0.072
0.490
0.272
0.087
0.079
Number of associated
Juglans californica
seedlings
Observed Expected
20
11
200
133
74
30
24
15
21
16
n ~ 272 seedlings
= 68.53, df = 4
P<.0001
Number of associated
Populus fremontii
seedlings
Observed Expected
22
3
14
22
12
1
4
4
3
3
« = 44 seedlings
X^= 133.33, d f= 4
f <0001
Number of associated
Acer negundo
seedlings
Observed Expected
29
19
135
132
74
91
24
1
14
21
« = 270 seedlings
%' = 32.93, d f = 4
P< .0001
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
16
T a b le 6— Seedlings densities per 0.1 ha with respect to overstory class at the Kopta
Slough study site. Data are means ± 1 SD. Different superscript letters within a row indicate
significantly different values {P< .05, Kruskal-Wallace test followed by simultaneous multiple
H - 10
Juglans
n 28
4.5 ±11.6®
7.4 + 23.3®
5.7 ±12.7®*’
10.2 ± 11.7”
0.8 ±2.9®
7,7 + 11.8®
open
Species
Juglans californica
Populusjremontii
Acer negundo
-
Cover Type
Populus
M= 18
3.8 ±8.3®
0.0 ± 0.0®
17.6 ±44.3®”
Acer
«=12
5.3 ± 10.6®”
2.9 ±9.3®
0.0 ± o.o”
O th e r spp.
« = 10
6.9± 11.1®”
2.1 ±7.2®
7.5 ±17,1®”
Finally, Acer negundo seedlings were also significantly nonrandomly distributed
with respect to overstory: A. negundo seedlings showed a strong negative spatial
association with conspecific overstory and were more common under J. californica and
P. frem ontii overstory (Table 5). Adult A. negundo occupied 8 percent o f the Kopta
Slough study area but contained less than 1 percent of A. negundo seedlings (Table 5).
Acer negundo seedling density was significantly different among overstory classes
(Kruskal-Wallace test:
= 13.01, d f = 4 ,P = .0112): seedling density under J.
californica was significantly higher than under A. negundo (Table 6).
Seedling Distribution and Density Under Canopy and in Canopy Gaps
Relatively few seedlings occurred in canopy gaps, and for some species, no
seedlings occurred in gaps. Because o f insufficient sample size, I limited the chi-square
goodness-of-fit analysis to seedlings o f J. californica, A. negundo, and all species
combined, and the Kruskal-Wallace analysis to seedlings o f J. californica, A. negundo, P.
frem ontii, and all species combined.
There was no general spatial association between all seedlings combined and
canopy gaps (Table 7). However, both A. negundo a n d /, californica seedlings were
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
17
T a b l e 7—^Distribution of seedlings of all species combined as well as J. californica and
A. negundo seedlings with respect to canopy gaps at the Kopta Slough study site. Chi-square
statistics refer to goodness-of-fit tests for the number of seedlings associated with each cover
__________________________________________________
Cover Type
canopy gap
under canopy
Proportion
0.072
0.928
Number of associated Number of associated Number of associated
seedlings (all species
Juglans californica
Acer negundo
combined)___________ seedlings____________ seedlings_____
Observed Expected Observed Expected Observed Expected
11
20
60
50
29
19
261
252
241
251
635
645
n = 270 seedlings
n = 272 seedlings
n = 695 seedlings
x ' = 2.16,df= 1
%^ = 4.37,df= 1
X' = 5.66, d f - 1
P=.0173
P=.1421
P=.0365
significantly nonrandomly distributed with respect to gaps: more A. negundo seedlings
occurred in canopy gaps than expected, but fewer J. californica seedlings occurred in
canopy gaps than expected (Table 7). Canopy gaps occupied 7 percent o f the Kopta
Slough study site, but contained 11 percent o f A. negundo seedlings and 3 percent o f J.
californica seedlings (Table 7).
Juglans californica seedling density was significantly higher under canopy than in
canopy gaps (Mann-Whitney U test; exact P = .0058) (Table 8). However, in contrast to
the chi-square goodness-of-fit test results, A. negundo seedling density was not
significantly different between gaps and canopy (Mann-Whitney U test: exact P = .0873)
(Table 8). Seedling densities for P. frem ontii and all seedlings combined were also not
8— Seedling densities per 0.1 ha with respect to canopy gaps at the Kopta
Slough study site. Data are means ± 1 S O . Different superscript letters within a row indicate
significantly different values (P < .05, Mann-Whitney Ü test), n = number of plots occupied by
T a b le
Cover Type
Species
all species combined
Juglans californica
Populus fremontii
Acer negundo
canopy gap
n = 10
17.6 ±31.6"
4.5 ±11.6"
7.4 ±23.3"
5.7 ± 12,7"
under canopy
M= 33
20,2 ±22.7"
8.4 ± 11.0”
0.8 ±3.0"
7.6 ± 13.8"
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
18
significantly different between gaps and canopy (Mann-Whitney U tests: all species
combined, exact P = .0630; P. frem ontii, exact P = .9662) (Table 8).
DISCUSSION
Seedling- Groundcover Associations
The results o f this study support the hypothesis that plant-plant interactions
influence woody plant regeneration in portions o f the floodplain with low flood
frequencies. Seedling distributions among groundcover classes were significantly
nonrandom: seedlings o f all species examined were negatively spatially associated with
both forb and liana cover and were positively spatially associated with litter-covered
substrate. These results imply that at the Kopta Slough and Pine Creek study sites,
certain vegetative cover types might limit successful seedling establishment and survival.
Negative spatial associations between seedlings and both forb and liana cover
could reflect inhibitory plant-plant interactions. These patterns could be caused by
below-ground resource competition, light competition, or allelopathy (Maguire and
Forman 1983, Ponder 1987, DeSteven 1991, Clinton et al. 1994, Berkowitz et al. 1995).
Liana cover at the two study sites was usually extremely dense. Lianas can inhibit
sapling growth in tropical forests (Putz 1984), and liana removal has been linked to
increased survivorship o f tropical trees (Brokaw 1985). Strahan (1984) hypothesized that
in Sacramento River riparian systems, within-floodplain regeneration was primarily
limited by dense liana cover. Forb cover varied in density but was dominated by tall
forbs such
Artemisia douglasiana and Solanum americanum, both o f which often
exceeded 1 m in height. Artemisia douglasiana was the most abundant forb species
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
sampled at both locations, and other congeneric species have been found to
allelopathically inhibit herbaceous plant growth (Schlatterer and Tisdale 1969, Halligan
1976).
Given the strong negative association between seedlings and liana cover at both
study sites, it is worth noting that a substantial proportion o f liana cover was comprised
o f exotic species. Twenty-one percent o f liana cover at the Pine Creek study site was
occupied by exotic species, predominately Ruhus discolor. At the Kopta Slough site, 60
percent o f liana cover was occupied by two exotic species, K discolor and Ficus carica.
Both R. discolor and F. carica are invasive, weedy species, and the proportion of
groundcover occupied these species at both study sites may increase over time.
Correlations between seedlings and vegetative cover were not uniformly negative.
Seedlings o f most species were positively spatially associated with graminoid cover, and
for some species, seedling density was significantly higher in graminoid cover than in
other vegetative cover types. These findings are in contrast to other studies, which have
found that competition for soil moisture between grasses and woody seedlings can
significantly decrease seedling growth and survival (Gordon and Rice 1993, Kolb and
Robberecht 1996). Furthermore, Goldberg and Fleetwood (1987) and Gordon et al.
(1989) found that the competitive effects o f grasses on woody seedlings were more
intense than the competitive effects o f forbs. It is unlikely that the positive spatial
associations between graminoid cover and seedlings reflect facilitative interactions. In
contrast to the density o f forb and liana cover, which was often high, the density o f
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
graminoid cover was usually very low. It is possible that the density o f graminoid cover
was too low for competitive interactions between seedlings and graminoids to be strong.
It is also possible that seedling distributions among vegetative cover types are
independent o f the cover type itself and instead reflect variation in the physical
environment. For example, forbs and lianas might grow preferentially in microsites not
amenable to woody seedlings, whereas graminoids might grow preferentially in
microsites amenable to woody seedlings.
Positive spatial associations between seedlings and litter-covered substrate could
reflect decreased biotic interference and/or increased resource availability in this cover
type. If species’ regeneration niches are limited by inhibitory interactions with certain
vegetative cover types, higher seedling density in litter could reflect the absence o f these
interactions. In old-field communities, Facelli (1994) found that litter indirectly
facilitated woody seedling growth by reducing the density and biomass, and thus the
competitive effect, o f herbaceous groundcover. The physical environment in litter cover
might also be more amenable to seedling establishment and survival because of higher
below-ground resource levels or higher light regimes. Similar mechanisms could also
explain positive associations between seedlings and woody debris.
Overall seedling distribution patterns at both study sites imply that seedling
distributions may be influenced by plant-plant interactions. Overall patterns (when
seedlings were combined across species) were similar between the Kopta Slough and
Pine Creek study sites. However, seedling distribution patterns were not always similar
between study sites for particular species. Juglam californica seedling densities were
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
21
significantly different among groundcover classes at the Kopta Slough study site but not
at Pine Creek. This difference is probably due to the low number o f J. californica
seedlings (n = 28 seedlings) found at the Pine Creek site.
Seedling-Overstory Associations
Juglans californica, P. frem ontii, and A. negundo seedlings were nonrandomly
distributed with respect to overstory composition. Seedling distributions were strongly
species specific and could reflect different phenologies or dispersal mechanisms.
Juglans californica seedlings showed a strong affinity for conspecific overstory. Other
studies have found similar patterns for heavy-seeded species (Andersson 1991). The
positive spatial association between J. californica seedlings and conspecific overstory
could reflect gravity-mediated seed dispersal. Though the allelopathic potential o f J.
californica has not been studied, many studies suggest that the congeneric Juglans nigra
is allelopathic to many plant species (Rice 1984, Talley et al. 1996). However, this study
found no evidence that adult J. californica interferes with the establishment or survival
o f J. californica seedlings. This study encompassed only one field season, and I have no
data on proportionate seedling mortality rates among overstory classes. However, if
seedling distributions observed in this study reflect long-term seedling survivorship
patterns, then it appears likely that J. californica recruitment occurs primarily under
established conspecific overstory, Juglans californica stands, therefore, may be self­
replacing.
Populus frem ontii seedlings were strongly associated with canopy openings.
Ribiero (1991) found that low light levels did not affect P. frem ontii seed germination,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
22
but that light was a limiting factor for P, frem ontii seedling growth. He hypothesized that
because o f insufficient light regimes, P. frem ontii could not regenerate under the canopy
o f mature riparian forest. This study supports that hypothesis. Though P. frem ontii was
a substantial component o f the overstory at Kopta Slough and was the dominant
overstory species at Pine Creek, only 46 seedlings were recorded at both locations, and
32 were recorded at one plot. In contrast to their rarity under established riparian forest, I
observed that P. frem ontii seedlings were abundant on early successional landforms
adjacent to both study sites. Populus frem ontii seedlings were strongly negatively
associated with conspecific overstory. However, this could be an artifact of the small
num ber o f P. frem ontii seedlings found and their localized distribution.
The most striking pattern for A. negundo was the pronounced negative spatial
association between seedlings and conspecific overstory. Negative associations between
seedlings and adult conspecifics have been observed for other members of this genus,
including Acer saccharum in northern temperate forest (Woods 1984) and Acer rubra in
southeastern floodplain forest (Streng et al. 1989, Jones et al. 1994). High densitydependent seedling mortality, either through herbivory, pathogenic fungi, or intraspecific
competition, has been suggested as a possible explanation for similar patterns in tropical
tree species (Augsperger 1984, Clark and Clark 1985). However, Streng et al. (1989)
found no support for any o f these mechanisms as causative agents, and it remains unclear
what causes this pattern. Beatty (1984) and Crozier and Boemer (1984) both found that
some overstory trees affected the distribution and composition of herbaceous cover by
altering soil cation exchange capacity and organic matter and nutrient content. It is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
possible that adult A. negundo trees alter microsites under their canopy, creating
conditions unfavorable for either seed germination or seedling survival.
Interpreting associations between A. negundo seedlings and all other overstory
species is limited by my sampling method. I only recorded the tallest overstory trees
along line-intercepts; however, A. negundo was often present in the subcanopy. Given
the pronounced negative spatial association between seedlings and conspecific adults,
patterns between A. negundo seedlings and non-conspecific overstory might be more a
reflection o f the relative presence o f adult A. negundo in the midstory.
SeedUng-Canopy Gap Associations
Canopy gaps have been found to be important to woody seedling growth and
survival in many forest systems (Augsperger 1984, Canham 1988, Holmes 1995).
However, at the Kopta Slough study site, seedling distributions with respect to canopy
gaps varied by species. Acer negundo seedlings were positively spatially associated with
canopy gaps while J. californica seedlings were negatively spatially associated with
canopy gaps and also occurred at lower densities in gaps . It is unclear what mechanisms
cause these species-specific responses.
It is possible that soil moisture is limiting at the Kopta Slough study site for some
woody riparian species. Ribiero (1991) found that soil moisture accounted for more
variability in germination o f A. negundo and J. californica seeds than light in controlled
experiments. The Sacramento Valley experiences regular summer drought: virtually no
precipitation occurs between May and October, and summer temperatures often exceed
35®C. Drought effects in riparian communities may be ameliorated by proximity to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
24
groundwater; however, due to limited root development, seedlings o f some species may
be unable to access this resource, and soil moisture may be a critical factor for these
species. It is possible that J. californica seedling survivorship at the Kopta Slough study
site is soil moisture limited, and that for this species, the beneficial effects o f higher light
regimes in canopy gaps might be outweighed by decreases in soil moisture in these
microsites (Holmgren et al. in press).
If soil moisture is limiting in canopy gaps, the positive correlation between A.
negundo seedlings and these microsites could reflect more efficient water relations for
this species. It is possible that for A negundo seedlings, decreases in soil moisture do not
outweigh the beneficial effects o f higher light regimes in canopy gaps. Though soil
moisture was more important, higher light did increase seed germination ofW. negundo
seeds (Ribiero 1991). Other mechanisms, such as greater seed dispersal, could also
explain A. negundo distributions with regard to canopy gaps observed in this study.
Conclusion
This study supports the hypothesis that biotic interactions are important factors
for within-floodplain regeneration. Seedlings were nonrandomly distributed among both
groundcover and overstory classes, implying that regeneration for woody riparian species
in floodplain landforms might be influenced by overstory and groundcover composition.
Seedling distributions were strongly correlated with cover type. Whether mediated
directly by the composition o f groundcover or overstory or by variation in physical
environment, microsite variability appears to delimit, at least in part, woody species’
regeneration niche at the two study sites. If long-term seedling survivorship is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
25
proportionate to distribution patterns observed in this study, microsites amenable to
seedling establishment and survival may be critical to successful regeneration by seed.
However, given the dominance o f invasive exotic species such as Rubus discolor
and Ficus carica in portions o f the floodplain, the proportion o f groundcover classes may
change over time. Cepello (1991) found F. carica was strongly correlated with terrace
landforms (flood recurrence interval > 1 0 yr). The development o f water resources,
especially the construction o f Shasta Dam upstream of the study sites, has dampened the
intensity and frequency o f flood events along the mainstem o f the Sacramento River
(Kahrl 1979). This may create conditions more favorable to F. carica expansion. If
liana cover limits regeneration opportunities for native woody species, an increase in the
proportion o f this cover type may alter long-term species composition in remnant
floodplain forest on the Sacramento River.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
26
REFERENCES
Andersson, C. 1991. Distribution o f seedlings and saplings o f Quercus robur in a grazed
deciduous forest. J. Veg. Sci. 2: 279-282.
Augsperger, O.K. 1984. Seedling survival o f tropical tree species: interactions o f
dispersal distance, light gaps, and pathogens. Ecology 65: 1705-1712.
Beatty, S.W. 1984. Influence o f microtopography and canopy species on spatial patterns
o f forest understory plants. Ecology 65: 1406-1420.
Berkowitz, A.R., C D. Canham and V.R. Kelley. 1995. Competition vs. facilitation o f
tree seedling growth and survival in early successional communities. Ecology 76:
1156-1168.
Brokaw, N.V.L. 1985. Treefalls, regrowth, and community structure in tropical forests.
Pp. 53-69 in S.T.A. Pickett and P.S. White (eds.). The ecology o f natural disturbance
and patch dynamics. Academic Press, Inc., London.
Brower, J.E., J.H. Zar and C.N. von Ende. 1990. Field and laboratory methods for
general ecology. 3rd ed. Wm.C. Brown Publishers, Dubuque.
California Department o f Fish and Game. 1988. Sacramento River riparian atlas.
W ildlife Conservation Board, Nongame Heritage Program, Sacramento.
California Resources Agency. 1989. Upper Sacramento River fisheries and riparian
habitat management plan. Sacramento.
Callaway, R.M. 1992. Effect o f shrubs on recruitment of Quercus douglasii and
Quercus lobata in California. Ecology 73: 2118-2128.
Canham, C D 1988. Growth and architecture o f shade tolerant trees: response to canopy
gaps. Ecology 69: 786-795.
Cepello, S. A. 1991. Riparian vegetation distribution along the middle Sacramento River
in relation to flood frequency. MS Thesis, California State University, Chico.
Clark, D.B. and D A. Clark. 1985. Seedling dynamics o f a tropical tree: impacts o f
herbivory and meristem damage. Ecology 66: 1884-1892.
Clinton, B.D., L.R. Boring and W.T. Swank. 1994. Regeneration patterns in canopy
gaps o f mixed-oak forests o f the southern Appalachians: influence o f topographic
position and evergreen understory. Am. M id i Nat. 132: 308-319.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
Conard, S.G., R.L. MacDonald and R.F. Holland. 1977. Riparian vegetation and flora o f
the Sacramento Valley. Pp. 47-55 in A. Sands (ed.). Riparian forests in California.
Institute of Ecology Publication No. 15, University o f California, Davis.
Crozier, C.R. and R.E.J. Boemer. 1984. Correlations o f understory herb distribution
patterns with microhabitats under different tree species in a mixed mesophytic forest.
Oecologia 62: 337-343.
Darley-Hill, S. and W.C. Johnson. 1981. Acorn dispersal by the blue jay {Cyanocitta
cristata). Oecologia 50: 231-232.
Debussche, M. and P. Isenmann. 1994. Bird-dispersed seed rain and seedling
establishment in patchy Mediterranean vegetation. Oikos 69; 414-426.
DeSteven, D 1991. Experiments on mechanisms o f tree establishment in old-field
succession: seedling survival and growth. Ecology 72; 1076-1088.
Facelli, J.M. 1994. Multiple indirect effects o f plant litter affect the establishment o f
woody seedlings in old-flelds. Ecology 75: 1727-1735.
Gibbons, J.D. 1976. Nonparametric methods for quantitative analysis. Holt, Reinhart &
Winston, New York.
Goldberg, D.E. and L. Fleetwood. 1987. Competitive effect and response in four annual
plants. J. Ecol. 75; 1131-1143.
Gordon, D R. and K.J. Rice. 1993. Competitive effects o f grassland annuals on soil
water and blue oak {Quercus douglasii) seedlings. Ecology 74: 68-82.
Gordon, D R., J.M. Welker, J.W. Menke and K.J. Rice. 1989. Competition for soil
water between annual plants and blue oak {Quercus douglasii) seedlings. Oecologia
79: 533-541.
Grubb, P.J. 1977, The maintenance o f species-richness in plant communities: the
importance o f the regeneration niche. Bio. Rev. 52: 107-145.
Halligan, J.P. 1976. Toxicity o î Artemisia californica to four associated herb species.
Am. Midi. Nat. 95; 406-421.
Harris, R.R. 1987. Occurrence o f vegetation on geomorphic surfaces in the active
floodplain of a California alluvial stream. Am. Midi. Nat. 118: 393-405.
Harrison, J.S and P.A. Werner. 1984. Colonization by oak seedlings into a
heterogeneous successional habitat. Can. J. Bot. 62: 559-563.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28
Hickman, J.C. (éd.). 1993. The Jepson manual: higher plants o f California. University
o f California Press, Berkeley.
Holmes, T.H. 1995. Woodland canopy structure and the light response of juvenile
Quercus lobata (Fagaceae). Am. J. Bot. 82; 1432-1442.
Holmgren, M., M. Scheffer and M.A. Huston. In press. The interplay o f competition and
facilitation in plant communities. Ecology.
Holstein, G. 1984. California riparian forests: deciduous islands in an evergreen sea.
Pp. 2-22 in R.E. W arner and K.M. Hendrix (eds.). California riparian systems.
University o f California Press, Berkeley.
Hupp, C.R. and W.R. Osterkamp. 1985. Bottomland vegetation distribution along
Passage Creek, Virginia, in relation to fluvial landforms. Ecology 66: 670-681.
Johnson, W.C., R.L. Burgess and W.R. Keammerer. 1976. Forest overstory and
environment on the Missouri River floodplain in North Dakota. Ecol. Monogr. 46:
59-84.
Jones, R.H., R.R. Sharitz, P.M. Dixon, D.S. Segal and R.L. Schneider. 1994. Woody
plant regeneration in four floodplain forests. Ecol. Monogr. 64: 345-367.
Kahrl, W.L. (ed.). 1979. The California water atlas. California Govemor’s Office of
Planning and Research and California Department of Water Resources, Sacramento.
Kolb, P.F. and R. Robberecht. 1996. Pinus ponderosa seedling establishment and the
influence o f competition with the bunchgrass Agropyron spicatum. Int. J. Plant Sci.
157: 509-515.
Maguire, D.A. and R.T.T. Forman. 1983. Herb cover effects on tree seedling patterns in
a mature hemlock-hardwood forest. Ecology 64: 1367-1380.
McAuliffe, J R. 1988. Markovian dynamics o f simple and complex desert plant
communities. Am. Nat. 131: 459-490.
McBride, J R. and J. Strahan. 1984. Establishment and survival o f woody riparian
species on gravel bars o f an intermittent stream. Am. Midi. Nat. 112: 235-245.
McGill, R.R. 1987. Land use changes in the Sacramento river riparian zone. Redding to
Colusa, third update, 1982 to 1987. California Department of W ater Resources,
Sacramento.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
Nanson, G.C. and H.F. Beach. 1977. Forest succession and sedimentation on a
meandering-river floodplain, northeast British Columbia, Canada. J. Biogeog. 4;
229-251.
Ponder, Jr., F. 1987. Effect o f three weeds on the growth and mycorrhizal infection o f
black walnut {Juglans nigra) seedlings. Can. J. Bot. 64: 1888-1892.
Putz, F.E. 1984. The natural history o f lianas on Barro Colorado Island, Panama.
Ecology 65: 1713-1724.
Ribiero, J.F. 1991. Environmental heterogeneity in space and time and plant life history
traits on zonation o f five riparian woody species o f the California Central Valley.
Ph.D. Dissertation, University o f California, Davis.
Rice, E.L. 1984. Allelopathy. 2nd ed. Academic Press, New York.
Robertson, P.A., G.T. Weaver and J.A. Cavanaugh. 1978. Vegetation and tree species
patterns near the northern terminus o f the southern floodplain forest. Ecol. Monogr.
48: 249-267.
Sacchi, C.F. and P.W. Price. 1992. The relative roles o f abiotic and biotic factors in
seedling demography o f arroyo willow {Salix lasiolepis: Salicaceae). Am. J. Bot. 79:
395-405.
Schlatterer, E.F. and E.W. Tisdale. 1969. Effects o f litter of Artemisia, Chrysothamnus,
and Tortula on germination and growth o f three perennial grasses. Ecology 50: 869873.
Sokal, R.R. and F.J. Rohlf. 1995. Biometry; the principles and practice o f statistics in
biological research. 3rd ed. W.H. Freeman and Co., New York.
Strahan, J. 1984. Regeneration o f riparian forests o f the Central Valley. Pp. 58-67 /n
R.E. Warner and K.M. Hendrix (eds.). California riparian systems. University of
California Press, Berkeley.
Streng, D R., J.S. Glitzenstein and P. A. Harcombe. 1989. Woody seedling dynamics in
an east Texas floodplain forest. Ecol. Monogr. 59: 177-204.
Talley, S.M., R.O. Lawton and W.N. Setzer. 1996. Host preference o f Rhus radicans
(Anacardiaceae) in a southern deciduous hardwood forest. Ecology 77; 1271-1276.
USAGE [U.S. Army Corps o f Engineers], 1986. Sacramento River Flood Control
System: Sacramento River and tributaries, Collinsville to Red Bluff. 1" = 400' aerial
photographs. File No. 50-18-5805. Sacramento District, Sacramento.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
Wissmar, R.C. and F,J. Swanson. 1990. Landscape disturbances and lotie ecotones. Pp.
65-89 in R.J. Naiman and H. Décamps (eds.). The ecology and management o f
aquatic-terrestrial ecotones. UNESCO, Paris.
Woods, K.D. 1984. Patterns o f tree replacement: canopy effects on understory pattern in
hemlock-northern hardwood forests. Vegetatio 56: 87-107.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
APPENDIX A
Table A-1— Proportion of groundcover at Kopta Slough (derived from proportionate
cover of 50-m line-intercepts, n =
________________________________________
Proportion
Groundcover
litter
0.259
(duff, leaves)
graminoid
(grasses and Carex spp.)
0.142
Rubus discolor
(Himalayan blackberry)
0.129
Vitis californica
(California wild grape)
0.096
Ficus carica
0.096
(edible fig)
0.071
(<
5
cm
diameter)
small woody debris
0.053
(> 5 cm diameter)
large woody debris
0.047
Artemisia douglasiana
(mugwort)
0.043
Rubus ursinus
(California blackberry)
0.019
(American black nightshade)
Solanum americanum
0.012
(mineral soil)
bare
0.007
Conyza spp.
(horseweed)
0.006
Phytolacca americana
(American pokeweed)
0.004
Urtica dioica ssp. holosericea
(hoary creek nettle)
0.004
Bidens frondosa
(sticktight)
0.003
Juglam californica var. hindsii
(northern California black walnut)
0.002
(species
unknown)
forb
0.002
Xanthium strumarium
(cocklebur)
0.001
Aristolochia californica
(California pipevine)
0.001
Clematis ligusticifolia
(virgin’s-bower)
(box-elder)
0.001
Acer negundo var. californicum
0.001
Sambucus mexicana
(blue elderberry)
(Oregon
ash)
trace
Fraxinus latifolia
trace
Heliotropium curassavicum
(wild heliotrope)
Polygonum lapathifolium
(wiUow-weed)
trace
Toxicodendron diversilobum
(poison oak)
trace
Datura stramonium
trace
(jimsonweed)
T a b le A-2— Proportion of overstory at Kopta Slough (derived from proportionate
cover of 50-m line-intercepts, n = 34)
Overstory
Proportion
Juglam californica var. hindsii
(northern California black walnut)
0.490
Populus fremontii ssp. fremontii
(Fremont cottonwood)
0.272
Acer negundo var. californicum
(box-elder)
0.087
open
(canopy gap)
0.072
0.027
Platanus racemosa
(western sycamore)
0.019
Ficus carica
(edible fig)
0.017
Juglans regia
(English walnut)
0.011
Salix gooddingii
(Goodding’s black willow)
0,003
Ptelea crenulata
(hoptree)
Sambucus mexicana
(blue elderberry)
0.002
Salix exigua
0.001
(sandbar willow)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
32
APPENDIX B
T a b le B-1— Proportion of groundcover at Pine Creek (derived from proportionate
cover of 50-m line-intercepts, n = 14)
Groundcover
Proportion
litter
(duff, leaves)
0.232
Artemisia douglasiana
(mugwort)
0.141
small woody debris
(< 5 cm diameter)
0.115
graminoid
(grasses and Carex spp.)
0.109
(> 5 cm diameter)
0.068
large woody debris
Solanum americanum
(American black nightshade)
0.063
Vitis californica
(California wild grape)
0.033
Rubus ursinus
(California blackberry)
0.032
forb
(species imknown)
0.033
Conyza spp.
0.029
(horseweed)
0.027
(soil texture dominated by sand or silt)
deposition
0.026
Equisetum spp.
(horsetail)
Rubus discolor
(Himalayan blackberry)
0.023
Clematis ligusticifolia
(virgin’s-bower)
0.015
Arundo donax
(giant-reed)
0.015
(box-elder)
0.007
Acer negundo var. californicum
Vinca major
(periwinkle)
0.005
(cocklebur)
Xanthium strumarium
0.005
Bidens frondosa
0.004
(sticktight)
Chenopodium spp.
(goosefoot)
0.004
(mineral soil)
0.003
bare
Datura wrightii
(thorn-apple)
0.003
Juglans californica var. hindsii
(northern California black walnut)
0.002
Fraxinus latifolia
(Oregon ash)
0.002
(arroyo willow)
Salix lasiolepis
0.001
Salix exigua
(sandbar willow)
trace
Nicotiana glauca
(tree tobacco)
trace
Polygonum lapathifolium
(willow-weed)
trace
Nicotiana quadrivalvis
(Indian tobacco)
trace
T a b le B-2— Percent cover of overstory at Pine Creek (derived from proportionate
cover of 50-m line-intercepts, n = 14)
Overstoiy
Proportion
Populus fremontii ssp. fremontii
(Fremont cottonwood)
0.764
0.100
Acer negundo var. californicum
(box-elder)
0,046
Salix gooddingii
(Goodding’s black willow)
0.031
Platanus racemosa
(western sycamore)
0.028
open
(canopy gap)
0.025
Juglans californica var. hindsii
(northern California black walnut)
0.005
Arundo donax
(giant-reed)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.