Journal of
Plant Ecology
VOLUME 9, NUMBER 6,
PAGES 762–772
December 2016
doi:10.1093/jpe/rtw019
Advance Access publication
12 March 2016
available online at
www.jpe.oxfordjournals.org
Leaf/shoot level ecophysiology in
two broadleaf and two needle-leaf
species under representative cloud
regimes at alpine treeline
Adriana Sanchez1,2,*, Nicole M. Hughes3 and William K. Smith2
1
Programa de Biología, Universidad del Rosario, Carrera 24 No. 63C-69, Bogotá, DC 110011, Colombia
Department of Biology, Wake Forest University, 136 Winston Hall, Winston-Salem, NC 27106, USA
3
Department of Biology, High Point University, University Station 3591, High Point, NC 27262, USA
*Corresponding address. Programa de Biología, Universidad del Rosario, Carrera 24 No. 63C-69, Bogotá, DC
111221, Colombia. Tel: +57-1-2970200 ext. 4034; E-mail: [email protected]
2
Abstract
Aims
The effects of clouds are now recognized as critically important
to the understanding of climate change impacts on ecosystems.
Regardless, few studies have focused specifically on the ecophysiological responses of plants to clouds. Most continental mountain
ranges are characterized by common convective cloud formation in
the afternoons, yet little is known regarding this influence on plant
water and carbon relations. Here we compare the ecophysiology
of two contrasting, yet ubiquitous growth forms, needle-leaf and
broadleaf, under representative cloud regimes of the Snowy Range,
Medicine Bow Mountains, southeastern Wyoming, USA.
Methods
Photosynthetic gas exchange, water use efficiency, xylem water
potentials and micrometeorological data were measured on representative clear, overcast and partly cloudy days during the summers
of 2012 and 2013 for two indigenous broadleaf (Caltha leptosepala
and Arnica parryi) and two needle-leaf species (Picea engelmannii
and Abies lasiocarpa) that co-occur contiguously.
Important Findings
Reductions in sunlight with cloud cover resulted in more dramatic declines in photosynthesis for the two broadleaf species
(ca. 50–70% reduction) versus the two conifers (no significant difference). In addition, the presence of clouds corresponded with
lower leaf conductance, transpiration and plant water status in
all species. However, the more constant photosynthesis in conifers under all cloud conditions, coupled with reduced transpiration, resulted in greater water use efficiency (ca. 25% higher)
than the broadleaf species. These differences appear to implicate
the potential importance of natural cloud patterns in the adaptive ecophysiology of these two contrasting, but common, plant
growth forms.
Keywords: gas exchange, light response curves, water use
efficiency, xylem water potential
Received: 25 August 2015, Revised: 17 February 2016, Accepted:
8 March 2016
INTRODUCTION
One of the most dramatic architectural contrasts in leaf and
branch form among vascular plants is that of species with
laminar leaves versus needle-like leaves (Reich et al. 1995).
A similarly strong contrast in whole-plant habit typically
occurs between the often abrupt transitions from subalpine
forest to the alpine plant community, across the treeline ecotone, i.e. subalpine forest trees versus the herbs, forbs, grasses
and sedges characteristic of the alpine tundra (Körner 2003).
Despite the strong contrasts in leaf/stem/crown form, and
their potentially strong effects on sunlight interception and
thermal properties, few direct comparisons of photosynthetic
gas exchange at the leaf level have been reported for broadleaf and conifer species occurring at the same location in the
field at the same time (e.g. Jordan and Smith 1993; Smith
and Carter 1988; Wyka et al. 2012). Most of the existing literature comparing needle-leaved conifers with co-occurring
broadleaf species focuses on the benefits of evergreen versus deciduous life histories as related to soil fertility, capacity
for seasonal carbon gain, or vulnerability to water loss (e.g.
Green and Kruger 2001; Wyka et al. 2012; also see Givnish
2002 for review). Here, we compare measured differences in
diurnal ecophysiology between two alpine broadleaf species,
© The Author 2016. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China.
All rights reserved. For permissions, please email: [email protected]
Sanchez et al. | Ecophysiological comparisons at alpine treeline763
the herbaceous Caltha leptosepala DC (Ranunculaceae) and
the forb/herb Arnica parryi A. Gray (Asteraceae), with two
needle-leaf, conifer tree species dominant in the contiguous
subalpine forest, Picea engelmannii Parry ex Engelm and Abies
lasiocarpa (Hook.) Nutt. (Pinaceae). These four species occur
in close proximity throughout the alpine treeline ecotone in
the Snowy Range, Medicine Bow Mountains, Wyoming, USA
(Knight et al. 2014).
Summer conditions in the central and southern Rocky
Mountains are characterized by a typical pattern of clear
mornings, followed by convective, cumulus cloud formation and sporadic thunderstorms occurring during the afternoon (Barry 2008; Sanchez et al. 2014). Although the effects
of clouds may be of primary importance in understanding
the impacts of climate change on ecosystems (IPCC 2007;
Ruddiman 2008), and are one of the greatest sources of
uncertainty in the climate change predictions (IPCC 2013),
few studies have examined the effects of cloud patterns on
plant ecophysiology. Cloudiness does appear to have both
positive and negative influences depending on cloud type,
temporal regime and sky coverage (Alton 2008; Berry and
Smith 2013; Hughes et al. 2015; Johnson and Smith 2008;
Reinhardt and Smith 2008; Sanchez et al. 2014). Furthermore,
the effects of clouds on plant ecophysiological processes may
also vary between taxa according to differences in leaf, shoot,
crown and canopy architecture (Smith et al. 2004). With the
presence of clouds, some studies have reported increases in
both photosynthetic carbon gain (due to light levels closer to
optimal) and water use efficiency (Germino and Smith 2000;
Gu et al. 2002; Johnson and Smith 2006; Monson et al. 2002;
Urban et al. 2007). However, some evidence has also shown
significant decreases in carbon gain due to suboptimal sunlight levels (Graham et al. 2003; Hughes et al. 2015; Letts and
Mulligan 2005; Sanchez et al. 2014).
The primary objective of the present study was to compare
the direct, simultaneous effects of natural regimes on the ecophysiology of selected conifer versus broadleaf species that
were considered representative of the two contiguous communities, the subalpine forest and alpine zones. In addition,
species possessing a more three-dimensional (non-laminar)
leaf, shoot and branch architectures might intercept the diffuse component of sunlight under cloud cover more than
laminar leaf forms (Brodersen and Vogelmann 2010). This
could result in less dramatic declines in photosynthesis under
cloudy skies. As opposed to the two previous studies comparing intraspecific differences in the ecophysiology within the
same two species (Hughes et al. 2015; Sanchez et al. 2014),
the present report describes the interspecific differences in
ecophysiology under typical days with natural cloud patterns.
This analysis may provide important information about future
changes in cloud regimes generated by global change impacts
(IPCC 2013). Finally, fundamental problems often neglected
in reporting values of gas exchange parameters (expressed on
a leaf area basis) for needle-leaf versus broadleaf species are
discussed.
MATERIALS AND METHODS
Study site and species
The study site was located within the Snowy Range,
Medicine Bow National Forest, Southeast Wyoming, USA (N
41°21′1.94”, W 106°16′17.34″) at approximately 3 240-m elevation within an upper treeline ecotone typical of the south
central Rocky Mountains, USA (Billings 1969; Hughes et al.
2015; Johnson et al. 2004; Peet 1988). Measurements were
made on saplings (<1.5-m height; estimated <25 years old)
of two dominant conifer tree species of the subalpine zone,
P. engelmannii Parry ex Engelm and A. lasiocarpa (Hook.) Nutt.
(Pinaceae), as well as two common perennial alpine broadleaf species, C. leptosepala DC (Ranunculaceae) and A. parryi
A. Gray (Asteraceae) that co-occur within the same ecotone
treeline location. There were substantially more P. engelmannii individuals at the study site (N = 54) compared to A. lasiocarpa (N = 5) and, although C. leptosepala and A. parryi typically
co-occur in the alpine, the former occurs most often on the
downslope of snowbanks, where both cold air and snowmelt
water accumulate (Sanchez et al. 2014). In contrast A. parryi,
as well as the two conifers, are found typically at the uphill
margins of snowbanks or located away from snowbank drainage areas in well-drained microsites. All individual plants,
shoots (conifers) and leaves (broadleaf) selected for measurement were fully exposed to the southern sky without significant shading by neighboring structures such as adjacent
canopy leaves or neighboring plants.
Microclimate
Environmental conditions at the field site were monitored
throughout the summer in an open area located near the
center of the site and no more than 15 m from measurement
plants. Total photosynthetic photon flux density (PFD, 400- to
700-nm wavelengths) was measured continuously at 1-min
intervals using a LI-190 PFD sensor (Li-Cor, Lincoln, NE)
connected to voltage amplifiers (model UTA, EME Systems,
Berkeley, CA) and HOBO data loggers (model H8 4-channel logger; Onset Computer, Pocasset, MA). Air temperature
and relative humidity (RH) were measured at 15-min intervals using a HOBO Pro Series RH/Temp data logger (Onset
Computer Corporation, Bourne, MA), shielded from the sky
using a white styrofoam radiation shield. Ambient vapor
pressure was calculated from the air temperature and relative
humidity data (List 1951) as saturated vapor pressure (SVP)
minus actual vapor pressure. Vapor pressure deficit (VPD) of
the air was computed as SVP minus vapor pressure and actual
vapor pressure was calculated as (%RH*SVP) / 100 (Murray
1967). For brevity, leaf temperatures and the leaf-to-air water
vapor difference are not presented due to a close similarity
(±2.6°C) in leaf temperatures on clear days and ±1.1°C during
cloudy day measurements. In addition to PFD measurements
taken on individual leaves during gas exchange measurements (LiCOR quantum sensor 190s), total and diffuse PFD
were also measured continuously using a BF5 sunshine
764
sensor (Dynamax Inc.) and recorded with a GP1 data logger
(Delta-T Ltd., Burwell, Cambridge, UK). Subtraction of the
diffuse PFD from the total PFD measured was used to compute direct PFD at the central location of the site. All instruments at the central location were mounted 1.5 m above the
ground, and the measurement planes of the PFD sensors were
oriented horizontally and checked several times a day for possible deviations from horizontal.
Differences in microclimate for three representative day
types based on cloud occurrence (see below; Sanchez et al.
2014) and corresponding responses in daily gas exchange
and water status for two broadleaf and two needle-leaf conifer species were monitored during the summers of 2012 and
2013. Specific measurement days were 10–18 June, 8–14 July
and 11–16 August in 2012; and 8–15 June, 9–17 July and 3–9
August in 2013. Typical afternoon-cloud cover consisted of
altostratus, cumulus and cumulonimbus clouds, often moving rapidly and occurring simultaneously.
Sunlight regimes
During June through August of 2012 and 2013, daily PFD
data were used to determine the following three categories:
clear days, overcast days and afternoon clouds only (AC).
Clear days were categorized by the absence of significant
clouds between sunrise and sunset. Overcast days were considered as days where PFD values did not equal or exceed a
maximum PFD of 1 500 µmol m−2 s−1. Days categorized as AC
were days in which no PFD values <1 000 µmol m−2 s−1 were
recorded in the morning, while afternoons were characterized
by the occurrence of only intermittent clouds. Plant measurements taken to represent cloud cover were only recorded
when cloud shade was present (PFD values <1 500 µmol m−2
s−1). Days with morning clouds only were not included in the
study because, historically, the number of days with the specific types of cloud occurrence from 2005 through 2012 were
as follows: days with afternoon clouds only (52.1%), clear
all day (24.5%), overcast all day (7.7%) and morning clouds
only (1.8%) (Sanchez et al. 2014).
Photosynthetic gas exchange
Net photosynthesis (A), leaf conductance to water vapor
(g), leaf transpiration (E) and instantaneous water use efficiency (WUE = A / E) were measured for the three day types
described above, representing typical weather conditions
for the summer growth period (June through August, 2012
and 2013). Shoot photosynthetic gas exchange for conifers
was measured at approximately 3-h intervals between 0900
and 1700 h (Solar Time, List 1987) using a Li-Cor LI-6400XT
Portable Photosynthesis System (LiCor, Lincoln, NE)
equipped with a conifer needle chamber (model 6400-05).
Approximately 5 cm of stem, containing current-year needles, was placed in the chamber. For the broadleaf species,
gas exchange was measured at 2-h intervals between 0800
and 1600 h using the LiCor LI-6400 photosynthesis system
equipped with a clear-top chamber. All measurements were
Journal of Plant Ecology
made at the same study site, and often on the same day,
using two different LI-6400 gas exchange systems. For the
two broadleaf species, measurements were taken on (i) clear
days in 2012: 18 June, 12 August; 2013: 11 July, 16 July;
(ii) overcast days in 2012: 14 July, 13 August; in 2013: 13
June, 5 August and (iii) AC days in 2012: 11 July; in 2013:
11 June, 9 July, 3 August. For the two conifers, measurements were taken on (i) clear days in 2012: 18 June, 12
August; in 2013: 9 June, (ii) overcast days in 2012: 14 July,
13 August; in 2013: 13 June and (iii) AC in 2012: 12 June,
11 July; in 2013: 3 August. All measurements were made on
south-facing branches or leaves of randomly selected plants
(N = 5–15 for each measurement period for P. engelmannii,
3–5 for A. lasiocarpa and 5 fully expanded leaves for C. leptosepala and for A. parryi). Individual plants were chosen using
a random walk technique and measured on at least 3 days
of each day type per summer. Natural leaf and shoot orientations to the sun at the time of measurement were maintained, air temperature and RH inside the chamber were
kept near (<12% deviation) ambient values and reference
CO2 were maintained at near 400 ppm. Differences in gas
exchange measurements (A, g, E and WUE) were calculated
as the percent difference between mean values for clear
and afternoon-cloud days and between overcast and afternoon-cloud days as follows: [(clear − afternoon clouds) /
afternoon clouds] * 100 and [(overcast – afternoon clouds) /
afternoon clouds] * 100.
Total needle surface area, projected leaf areas and shoot silhouette areas for sampled stem lengths were measured as described
in more detail in Hughes et al. (2015). Mean needle density
was 108 needles/5 cm (P. engelmannii) and 101 needles/5 cm
(A. lasiocarpa). Mean projected leaf area was 12.9 and 16.7 cm2
for P. engelmannii and A. lasiocarpa, respectively. Silhouette area
of the intact shoots were also determined, and mean silhouette
to total leaf area (STAR) values were 0.376 (P. engelmannii) and
0.439 (A. lasiocarpa). Impacts of these choices in the computation of gas exchange parameters are discussed below.
Light response of photosynthesis
To determine the effect of PFD on photosynthesis, light
response curves were measured for each species using a
Li-Cor LI-6400XT, equipped with either a conifer needle
chamber (for conifer measurements) or a red/blue LED light
source (6400-02B LED, LICOR Inc.) for broadleaf measurements. Variations in light intensity were provided by the LED
light source for broadleaves and using neutral density shade
screens on clear days for conifers. Measurements began at
maximum PFD and decreased stepwise (ca. 300 µmol m−2
s−1 to a PFD of ca. zero), allowing 2- to 3-min acclimation
between measurements. Quantum use (quantum yield) efficiency was calculated as the slope of the best-fit regression
line, for the linear portion between 0 and 500 µmol m−2 s−1.
Measurements were made during each month (June through
August) of 2012 and 2013 for all four species. Conifer measurements were taken on clear, cloudless mornings between
Sanchez et al. | Ecophysiological comparisons at alpine treeline765
0830 and 1330 h on the following days: 14 June, 17 June, 9
July, 11 July, 12 August for 2012, and 14 June, 10 July, 14
July, 16 July, 04 August for 2013. For the broadleaf species,
all measurements were recorded between 0800 and 1030 h
for five individuals per species taken on 20 June, 10 July, 13
July, 14 August during the summer of 2012 and 10 June, 14
July, 04 August during 2013.
used for post hoc evaluation of statistical differences between
individual means (Zar 1999). When data could not be transformed to achieve normality, means were compared using the
Kruskal–Wallis one-way ANOVA and applying Dunn’s assessment of differences between means.
Water status
Air temperature and humidity
Diurnal xylem water potential (Ψ) was measured using a
Scholander-Hammel pressure chamber (1000 model, PMS
Instruments, Corvallis, OR) in the morning (0700 and 0800 h),
midday (1200–1300 h) and late afternoon (1700–1800 h) for
all species on clear, overcast and AC days. For conifers, a
current-year shoot was excised from the plant using a razor
blade and placed immediately (within <1 min) into the chamber to estimate shoot Ψ (N = 8–15 for P. engelmannii, N = 4
for A. lasiocarpa for each measurement interval). Currentyear, south-facing shoots used for water potential measurements were chosen randomly from the field site and were not
necessarily the same individuals that had been examined for
photosynthesis measurements on the same day (except in the
case of the A. lasiocarpa, where population size was limited
to five individuals). For broadleaf species, randomly selected
healthy, fully expanded, south-facing leaves (N = 5 for each
species) were excised at the base of the petiole and placed
immediately into the pressure chamber for all Ψ measurements. Because early morning measurements often varied
significantly, probably due to precipitation on the previous
days, Ψ measurements were expressed as mean differences
from early morning values (ΔΨ). To calculate these values,
we subtracted mean early morning Ψ from mean noon Ψ and
from mean late afternoon Ψ, providing a quantitative estimate of daily changes in plant water status during a given type
of day for each species. Conifer Ψ measurements were made
on (i) clear days in 2012: 18 June, 12 August; and in 2013: 9
June, 16 July, (ii) overcast days in 2012: 14 July, 13 August;
in 2013: 13 June and (iii) AC days in 2012: 11 July; 2013: 11
June, 9 July and 3 August. Broadleaf Ψ measurements were
made on (i) clear days in 2012: 18 June, 12 August; 2013: 11
July, 16 July, (ii) overcast days in 2012: 14 July, 13 August; in
2013: 13 June, 5 August and (iii) AC days in 2012: 11 July; in
2013: 11 June, 9 July, 3 August.
Air temperature (TA) for the early mornings of clear days
(0300–0700 h) tended to be cooler than for overcast or
cloudy-afternoon days (Fig. 1a), but were generally higher
later in the day, especially between about 1400 and 1600 h.
During overcast days, the lowest daytime temperatures were
recorded between 0800 and 2000 h (Fig. 1a). Daytime VPD
values were also lowest on overcast days and highest for clear
days (Fig. 1b).
RESULTS
Photosynthetic gas exchange
Diurnal patterns of photosynthesis for the broadleaf species
C. leptosepala and A. parryi species showed high photosynthesis (A) during the morning, peak A between 1000 and 1200 h
and gradual declines during the afternoon, concomitant with
Statistics
All data sets were tested for normality using a Shapiro–Wilk
test (normality determined at P > 0.05) and transformed
when necessary (SigmaPlot v. 12.5; Systat Software Inc.). Gas
exchange measurements (i.e. A, g, E and WUE) and mean ΔΨ
were compared for each species and measurement interval
(morning, noon, late afternoon) separately, using analyses of
variance (one-way ANOVA with significance determined at
P < 0.05), or t-tests when only 2 days were compared (e.g. gas
exchange measurements for conifers at 1730 h; significance at
P < 0.05). Tukey–Kramer multiple-comparison method was
Figure 1: (a) mean air temperature (TA). (b) Mean VPD. Solid lines
represent mostly clear days (18 June 2012, 12 August 2012, 9 June
2013, 11 July 2013, 16 July 2013), dotted lines represent overcast (14
July 2012, 13 August 2012, 13 June 2013, 5 August 2013) and broken lines represent cloudy afternoons (12 June 2012, 11 July 2012,
11 June 2013, 9 July 2013, 3 August 2013).
766
declines in g and PFD at the leaf/shoot level (PFDl; Fig. 2). In
both broadleaf species, A was highest under high PFD (i.e.
clear-sky conditions). Maximum A was ca. 15 µmol m2 s−1 for
C. leptosepala and 20 µmol m2 s−1 for A. parryi. In C. leptosepala,
the presence of clouds (overcast days and cloudy afternoons)
corresponded with 30–50% reductions in A relative to clearsky measurements taken at the same time intervals (Fig. 2e).
In A. parryi, photosynthesis was similarly significantly (ca.
45–75%) reduced under overcast conditions relative to clearsky measurements made at the same time, although AC only
resulted in significant declines in A at 1600 h (Fig. 2f). When
all A values (0800–1600 h) were averaged by day type, mean
A for broadleaf species were as follows: clear days, 13.7 µmol
m2 s−1 for C. leptosepala and 16.6 µmol m2 s−1 for A. parryi;
overcast days, 7.6 µmol m2 s−1 for both broadleaf species; AC
days, 11.5 and 14.1 µmol m2 s−1 for C. leptosepala and A. parryi,
respectively.
In P. engelmannii and A. lasiocarpa, the highest A occurred
in the morning (0830 h), and steadily decreased throughout
the day, regardless of sky conditions (Fig 2g and h). In P. engelmannii, there were no significant differences in A under clear,
overcast or afternoon-cloud conditions, with the exception of
one measurement interval (A was significantly higher under
clear versus afternoon-cloud conditions at 1730 h) (Fig. 2g).
A. lasiocarpa had significantly higher A on average on clear days
relative to overcast and AC days at 0830, 1300 and 1730 h,
though no significant difference was observed at the other
Journal of Plant Ecology
two time intervals (Fig. 2h). When all A values (0900–1700 h)
were averaged by day type, mean A for conifer species were
as follows: clear days, 7.4 µmol m2 s−1 for P. engelmannii and
6.0 µmol m2 s-1 for A. lasiocarpa; overcast days, 7.2 µmol m2 s-1
and 4.6 µmol m2 s−1; AC days, 6.8 µmol m2 s−1 and 3.7 µmol
m2 s−1 for P. engelmannii and A. lasiocarpa, respectively. In general, conifer mean g did not appear to differ between totally
clear days versus days with clouds (overcast and AC) except at
midday (Fig. 2k and l), at which time g was significantly higher
under clear-sky conditions in both species. However, E tended
to decrease when clouds were present in both species (Fig. 4).
The three types of days tended to have similar WUE (A / E)
values and diurnal patterns, with WUE values near 3 µmol/
mmol in the morning followed by gradual decreases during
the day (Fig. 3). The steepest declines in WUE occurred on
AC days, where WUE dropped by 50% under afternoon-cloud
cover relative to values derived earlier in the day (Fig. 3b). On
clear and overcast days, instantaneous WUE (A / E) for conifers decreased after midday, while on AC days, WUE increased
in the late afternoon, exceeding morning values (Fig. 3). The
highest WUE observed for conifers occurred during the mornings of overcast days (mean = ca. 5), and the lowest occurred
on clear day afternoons and on the clear mornings that were
followed by afternoon clouds (mean = ca. 3 µmol/mmol for
both day types).
The two conifers had consistently lower A and g values than the broadleaf species when conifer gas exchange
Figure 2: incident sunlight (PFDl) on leaves and shoots, plus net photosynthesis (A), and leaf conductance (g) on clear, overcast and afternoon-cloud days for the broadleaf (a, e, i) C. leptosepala and (b, f, j) A. parryi, and the needle-leaf conifers (c, g, k) P. engelmannii and (d, h, l)
A. lasiocarpa. Values for time interval are means of 3–4 days ± SE with more than 30 individuals of each species recorded per measurement time.
*P < 0.05, **P ≤ 0.01. Letters indicate significant differences between times of day for each experimental day type. Solid lines represent mostly
clear days, dotted lines represent overcast and broken lines represent cloudy afternoons.
Sanchez et al. | Ecophysiological comparisons at alpine treeline767
Figure 3: WUE (A/E) comparison between broadleaf (solid circle and
line; C. leptosepala and A. parryi) and conifer (open circle and broken
line; P. engelmannii and A. lasiocarpa) species. (a) Clear days; (b) overcast days; (c) afternoon-cloud days. **P ≤ 0.01.
parameters were calculated on a projected leaf area basis
(Fig. 2, see Discussion below). Thus, instantaneous WUE (A
/ E) was significantly higher for conifers on most day types
(clear, AC or overcast) relative to broadleaf species (Fig. 3).
When mean A, g, E and WUE were calculated for each day
type and expressed as a percentage of the mean value for
the most common type of day (AC), clear days had higher
A and transpiration (E) in all four species, although it was
significantly higher only for the broadleaf species (Fig. 4;
P < 0.001). Mean g was also higher on clear days compared to
AC days in all species except A. lasiocarpa (only significantly
higher in C. leptosepala; P < 0.05). Overcast days corresponded
Figure 4: percent difference mean daily (0800–1600 h) for A, g, E
and WUE for two broadleaf (C. leptosepala and A. parryi) and two conifer (P. engelmannii and A. lasiocarpa) species. The left column indicates
the percentage calculated as the percent difference between mean
clear day values and mean values from days with afternoon clouds:
[(clear − afternoon clouds) / afternoon clouds] * 100. The right column shows the percent difference between overcast day values versus values from days with afternoon clouds: [(overcast − afternoon
clouds) / afternoon clouds] * 100. Significant differences for each species are represented by *P < 0.05, **P ≤ 0.01. Black color corresponds
to C. leptosepala, white to A. parryi, dark gray to P. engelmannii and light
gray to A. lasiocarpa.
768
with higher mean A relative to AC days in both conifer species, but lower mean A in both broadleaf species (Fig. 4). In
all four species, mean g and E were lower on overcast days
than afternoon-cloud days. Combined, this resulted in some
indication of a higher WUE in conifer species on overcast
days, compared to lower or nominal reductions in WUE in
broadleaf species (Fig 4).
Light response of photosynthesis
Photosynthetic light response curves derived for broadleaf
and conifer species are shown in Fig. 5. Maximum photosynthesis (Amax) was higher in broadleaf species (15–20 vs
7–9 µmol m2 s−1 for conifers), as well as light saturation values
(10–11 vs 4–5 µmol m2 s−1 for conifers). Conifers tended to
have lower (more negative) respiration values (−2 to −4 µmol
m2 s−1) than broadleaf species (−2 µmol m2 s−1). Quantum
use efficiency was also greater in broadleaf species (0.024–
0.027) than conifers (0.013–0.020), while conifers saturated
photosynthetically between 600 and 700 µmol m2 s−1 and no
apparent saturation of photosynthesis occurred in the two
broadleaf species. Instead, A values continued to increase
steadily beyond 500 µmol m−2 s−1 and up to 3 000 µmol
m−2 s−1. Between 750 and 3 000 µmol m−2 s−1, there was a
strong linear relationship (P < 0.001; R2 = 0.95; b = 11.14,
m = 0.002), suggesting that A was not becoming asymptotic,
i.e. approaching light saturation (Fig. 5).
Xylem water potentials (Ψ)
There was no significant difference in early morning or noon
Ψ measurements between clear, overcast and AC, for any
of the four species (P > 0.05; Table 1). In most cases, early
morning Ψ was less negative than noon measurements, with
Journal of Plant Ecology
the steepest declines in diurnal Ψ occurring between morning and noon. In all species, overcast conditions corresponded
with higher Ψ during the late afternoon (1700 h) compared
to clear days, although this difference was not significant in
A. lasiocarpa (Table 1). No apparent differences in late afternoon Ψ occurred in A. parryi and P. engelmannii between
clear and AC days, while C. leptosepala (and to some degree
A. lasiocarpa) showed recoveries to morning Ψ levels, roughly
equivalent to those under overcast conditions. Broadleaf species had consistently less negative Ψ compared to conifers
throughout the day, and on all day types (Table 1).
On clear days, mean ΔΨ between morning and noon
were the highest values recorded for all species (C. leptosepala
ΔΨ = 0.07 MPa; P. engelmannii ΔΨ = 0.25 MPa; A. lasiocarpa
ΔΨ = 0.32 MPa) except A. parryi (ΔΨ = 0.58 MPa for clear
day and ΔΨ = 0.64 MPa for afternoon-cloud days; Fig. 6).
Overcast days had significantly lower ΔΨ afternoon values
compared to other day types for A. parryi (overcast afternoon ΔΨ = −0.12 MPa) and P. engelmannii (overcast afternoon ΔΨ = −0.39 MPa) (Fig. 6; P < 0.05). During AC days,
mean ΔΨ for noon and afternoon values was relatively
stable throughout the day for conifers (P. engelmannii noon
ΔΨ = 0.08 MPa, afternoon ΔΨ = −0.002 MPa; A. lasiocarpa
noon ΔΨ = 0.05 MPa, afternoon ΔΨ = −0.09 MPa) and C. leptosepala (noon ΔΨ = 0.06 MPa, afternoon ΔΨ = −0.004 MPa),
but not for A. parryi (noon ΔΨ = 0.64 MPa, afternoon
ΔΨ = 0.43 MPa).
DISCUSSION
The general approach of the present study was to compare the
ecophysiological responses of representative alpine and subalpine species to the natural cloud regimes occurring at their
zone of overlap (the treeline ecotone). More specifically, the
current study focused on the interspecific differences between
representative pairs of species, in contrast to the intraspecific comparisons of these same species in two former papers
(Hughes et al. 2015; Sanchez et al. 2014). A better understanding of these comparisons might also serve to generate predictions about changes in the relative sizes and elevations of the
subalpine and alpine zones when more information becomes
available about changes in cloud regimes in this region (IPCC
2013).
Ecophysiology
Figure 5: light response of photosynthesis (A) for broadleaf species
C. leptosepala and A. parryi (top row) and needle-leaf species P. engelmannii and A. lasiocarpa (bottom row) during the 2012 and 2013 summer growing season (June to August). Regressions based on a single
rectangular hyperbola, y = y0 + (a × x) / (b + x). For C. leptosepala: y0 =
−2.37, a = 19.54, b = 371.51, r2 = 0.99; A. parryi: y0 = −2.59, a = 25.09,
b = 454.89, r2 = 0.99; P. engelmannii: y0 = −4.46, a = 15.32, b = 324.42,
r2 = 0.98; A. lasiocarpa: y0 = −2.55, a = 10.77, b = 337.22, r2 = 0.98.
In general, the two broadleaf species studied here had higher
computed net photosynthesis (A) and more dramatic reductions in A for overcast days compared to the two conifer species
(Fig. 2). The more dramatic declines in A under cloud cover in
broadleaf species relative to conifers is likely due to the higher
PFD required for light saturation in the broadleaf species (>3
000 µmol m−2 s−1) compared to conifers (ca. 600–700 µmol
m−2 s−1), combined with a greater quantum yield efficiency in
the broadleaf species (Fig. 5). Accordingly, a reduction in PFDl
from 1 500 µmol m−2 s−1 under clear skies to 500 µmol m−2 s−1
Sanchez et al. | Ecophysiological comparisons at alpine treeline769
Table 1: mean water potential values (Ψ, MPa) with standard error by day type and time of day
Mean Ψ (MPa)
C. leptosepala
Clear a.m.
P. engelmannii
Mean Ψ (MPa)
−0.12 (0.03)
a
Clear p.m.
−0.09 (0.03)a
−0.07 (0.03)
a
Overcast p.m.
−0.01 (0.01)b
Aft. clouds noon
−0.07 (0.04)
a
Aft. clouds p.m.
−0.01 (0.01)b
Clear noon
−0.02 (0.08)
Aft. clouds a.m.
−0.02 (0.09)
a
Clear a.m.
−0.03 (0.06)a
Clear noon
−0.84 (0.07)a
Clear p.m.
−0.81 (0.08)a
Overcast a.m.
−0.30 (0.05)a
Overcast noon
−0.80 (0.11)a
Overcast p.m.
−0.18 (0.06)b
Aft. clouds a.m.
−0.31 (0.06)a
Aft. clouds noon
−0.95 (0.10)a
Aft. clouds p.m.
−0.69 (0.09)a
−1.41 (0.10)
a
Clear p.m.
−1.30 (0.10)a
−1.39 (0.12)
a
Overcast p.m.
-0.94 (0.08)b
−1.43 (0.07)
a
Aft. clouds p.m.
−1.34 (0.09)a
−1.45 (0.11)
a
Clear p.m.
−1.38 (0.18)a
−1.28 (0.06)
a
Overcast p.m.
−1.21 (0.11)a
−1.37 (0.10)
a
Aft. clouds p.m.
−1.19 (0.09)a
Clear a.m.
Overcast a.m.
Aft. clouds a.m.
A. lasiocarpa
−0.04 (0.18)
Mean Ψ (MPa)
a
Overcast a.m.
A. parryi
a
Clear a.m.
Overcast a.m.
Aft. clouds a.m.
−1.18 (0.09)
a
−1.35 (0.08)
a
−1.35 (0.10)
a
−1.09 (0.06)
a
−1.21 (0.07)
a
−1.27 (0.08)
a
Overcast noon
Clear noon
Overcast noon
Aft. clouds noon
Clear noon
Overcast noon
Aft. clouds noon
Ψ measurements for C. leptosepala and A. parryi were made on (i) clear days in 2012: 18 June, 12 August; 2013: 11 July, 16 July, (ii) overcast
days in 2012: 14 July, 13 August; in 2013: 13 June, 5 August and (iii) days with afternoon clouds in 2012: 11 July; in 2013: 11 June, 9 July, 3
August. Ψ measurements for P. engelmannii and A. lasiocarpa were made on (i) clear days in 2012: 18 June, 12 August; and in 2013: 9 June, 16
July, (ii) overcast days in 2012: 14 July, 13 August; in 2013: 13 June and (iii) days with afternoon clouds in 2012: 11 July; 2013: 11 June, 9 July
and 3 August. Values with different letter superscripts are significantly different at P < 0.05.
Figure 6: mean change in diurnal xylem water potentials (ΔΨ) relative to morning (0700 h) values on clear, overcast and afternoon-cloud days
of 2012 and 2013. (a) C. leptosepala, (b) A. parryi, (c) P. engelmannii and (d) A. lasiocarpa. Values for time interval are means of 3–4 days ± SE with
more than 30 individuals of each species recorded per measurement time interval (0700, 1200 and 1700 h). *P < 0.05.
under overcast conditions resulted in a more dramatic decline
in A for the broadleaf species (30–50% in C. leptosepala, and
45–75% in A. parryi) compared to conifers (no significant difference at most measurement intervals). An increased capacity to utilize diffuse sunlight could also have contributed to less
dramatic reductions in A for conifer species under low PFDs
associated with overcast conditions. The cylindrical leaf shape
and bottle-brush shoot architecture of the conifer, compared
to the more laminar leaf typical of broadleaf species, could
potentially allow for more efficient light capture per unit leaf
area under enhanced diffuse irradiance (Carter and Smith
1988; Ishii et al. 2012; Smith and Brewer 1994) (Fig. 4). It has
also been reported that diffuse light may not be as effective in
driving photosynthesis as direct sunlight in broadleaf species
due to their capability for maximizing photon interception by
chloroplasts (Brodersen et al. 2008; Brodersen and Vogelmann
2010). Moreover, while Hughes et al. (2015) found no significant increase in conifer photosynthesis on partly cloudy days
(between and during cumulus cloud cover), recent data for
the same species under overcast conditions (altostratus cloud
cover) did correspond with significant increases in A relative
to clear days (N. M. Hughes, unpublished data). This could be
due to the greater diffuse light under overcast conditions relative to partly cloudy conditions. Consistent with this explanation, both broadleaf species had ca. 20% higher mean A on
clear days and 30–40% lower A on overcast days compared
with the most common, AC days (Fig. 4). In contrast, conifers
showed no significant reduction in mean A on overcast days
relative to afternoon-cloud days.
All species showed only minor fluctuations in leaf conductance to water vapor (g) under various sky conditions from
clear to cloudy (Fig. 2). However, leaf temperatures were
generally cooler on AC and overcast days, relative to clear
days (data not shown), which has effects on transpiration (E)
and instantaneous water use efficiency (WUE). The relatively
constant A in conifers under different sky conditions, coupled with reduced E on afternoon-cloud days and overcast
days compared to clear days (Fig. 4), resulted in higher WUE
770
on afternoon-cloud and overcast days relative to clear days
(Figs. 3 and 4). These results are consistent with findings of
Hughes et al. (2015), who similarly showed that a constant
A, paired with reduced E from cooler needle temperatures
on cloudy days resulted in increased WUE on AC days relative to clear-sky conditions in P. engelmannii and A. lasiocarpa.
In contrast, the greatest WUE in broadleaf species occurred
on AC days (the most common type of day). On clear days,
high transpiration reduced WUE relative to AC days, while
on overcast days, WUE was reduced due to a decrease in A
(Fig. 4).
Regardless of day type, water status indicated by water
potential measurements (Table 1) was significantly lower for
the two conifers than the two broadleaf species, despite the
greater WUE on all three day types except the mornings of
the AC days (Fig. 3). However, when comparing mean ΔΨ
for conifers and C. leptosepala, there is not much variation
between morning and afternoon measurements during clear
and AC days (maximum of 3 bar; Fig. 6). Although Ψ are
lower for conifers than broadleaf species, the variations during the day, for these two types of day, are not very high.
A. parryi, in contrast, showed more abrupt variations in mean
ΔΨ, indicating a more sensitive response to water status than
the other species (Fig. 6b). During overcast days, all species
had lower mean ΔΨ during the afternoon, highlighting water
stress relief under these conditions (Fig. 6), even if overcast
conditions translate into lower PFD and A values, especially
for the broadleaf species (Fig. 2).
The apparently lower water status during all day types may
also reflect the differences in microsites occupied by the species studied. C. leptosepala is found typically in wetter microsites
downslope of larger snowbanks, while the two conifers and
A. parryi are found in microsites away from snow deposition
areas and are not found within microtopographic depressions
where snowmelt drainage occurs. Thus, microsite selection at
the germination stage, plus the structural differences in leaf/
shoot form, may both be contributing to the ecophysiological
differences measured here.
In this study, we focused only on how certain leaf and shoot
traits are associated with differences in the ecophysiology of
two common life forms (needle-leaf and broadleaf structure)
in response to natural cloud conditions. Other life form traits,
such as whole-plant traits (e.g. crown architecture), could also
be expected to influence these responses, as well as future
responses associated with global climate change.
Leaf area comparisons
We note here that gas exchange parameters for conifers were
calculated using projected leaf area basis (individual needles
were removed from shoots and their silhouette areas measured and summated), which most likely resulted in lower
calculated A values than those which would have been
derived using the silhouette area of intact shoots (discussed
in further detail below and in Smith and Hughes, 2009). For
example, the computed values of A for the two conifers were
Journal of Plant Ecology
substantially less than the two broadleaf species. However,
these values are strongly influenced by the selection of the
leaf area term for the flux density calculations of CO2 uptake
(Smith et al. 1991). The use of total leaf area, or the projected
leaf area of detached needles, for computing A compared with
using the projected area for needles still attached to the shoot
(silhouette area), can result in a substantially larger area and,
thus, proportionally lower computed photosynthesis values
(Carter and Smith 1988; Smith et al. 1991). Thus, any comparisons of absolute values of A and E between broadleaves
and typical needle-leaf species must take these differences
into account, especially when gas exchange measurements
are made on shoots with multiple leaves. Importantly, this
omission of the leaf area choices above in the computation
of photosynthesis per unit leaf area has resulted in conclusions that rank conifers well below broadleaf species in terms
of photosynthetic rates per unit leaf area (e.g. Larcher 2003,
Lusk et al. 2003). A more recent comparison of actual net photosynthesis values of broadleaves versus needle-leaf conifer
species is found in Wyka et al. (2012). However, these authors
do not report which of the above leaf areas was used for A
calculations. Published evaluations of photosynthetic performance among plant species continue to use projected leaf area
of detached leaves for photosynthesis calculations of broadleaf
species (e.g. Long and Bernacchi 2003, Sun et al. 2014). Thus,
accurate rankings of photosynthetic performance among species, especially for needle-leaf conifers versus broadleaf species, will continue to be in question. The results reported here
show distinct differences in area-based net photosynthesis
between two broadleaf versus to two conifer, needle-leaf species occurring naturally at the same location. Expressed on
a projected leaf area basis, daily net photosynthesis and leaf
conductance was generally higher in the two broadleaf species on clear days or days with afternoon clouds. There were
smaller differences between the two pairs in response to the
increased diffuse sunlight of cloud cover, while WUE values
were even greater for the conifers compared to the broadleaves, regardless of cloud regime. Also, smaller declines in A
and g occurred during clouds compared to clear days in the
conifer species, and greater WUE during the natural cloud
patterns typical of this region was due to greater differences
in E rather than A. These differences may have important
implications to growth under changing conditions of cloud
cover and temperatures predicted for this region in the future
(Christensen et al. 2007; Scherrer and Körner 2010).
FUNDING
National Science Foundation, Physiological and Structural
Systems (1122092).
ACKNOWLEDGEMENTS
We thank Robert Musselman, John Korfmacher and John Frank
of the US Forest Service for accommodations at the Glacier Lake
Sanchez et al. | Ecophysiological comparisons at alpine treeline771
Ecosystem Experiments Site and for providing micrometeorological
data. We also thank K. L. Carpenter, D. K. Cook, T. S. Keidel, C. N.
Miller and J. L. Neal for their help with data collection.
Conflict of interest statement: None declared.
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area and photosynthetic rate of sun and shade shoots of two
Picea species in relation to the angle of incoming light. Tree Physiol
32:1227–36.
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