Fig 1

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Are tropical forests sensitive to temperature?
Chris Doughty (cdoughty_at_uci.edu), Mike Goulden,
Scott Miller, University of California at
Irvine Humberto da Rocha, Augusto Maia,
University of Sao Paulo, Brazil
Introduction
How hot do tropical forest leaves get?
Much work has been done on leaf
temperatures and leaf energy budgets (Berry and
Bjorkman 1980). Rising temperatures due to
global warming has given this area a renewed
importance as leaf temperature controls a range
of physiologically important states such as
photosynthesis, respiration, photorespiration and
isoprene emissions. Leaf temperatures are
especially important in the tropics due to their
important role in the global carbon cycle.
We used a combination of leaf level gas
exchange measurements and eddy covariance data to
look at the effect of high canopy level leaf
temperatures and a variable light environment on
CO2 exchange and stomatal conductance. We used 3
years of eddy covariance data from the Tapajos
national forest in the Brazilian Amazon as part
of NASAs LBA (LargeScale Biosphere-atmosphere)
project and compared these to micrometeorological
and photosynthetic leaf level data taken at the
same site in the years of 2003-2004. We
want to answer the following questions at both
the leaf and canopy level How warm do canopy
level leaves get? How does the length of the
sunny period affect CO2 uptake and stomatal
conductance? Is the pattern the same at both the
leaf and canopy level?
Light regimes in tropical forests are
organized into bimodal patterns of light
distribution. As conditions move from cloudy to
sunny conditions, radiation energy almost
triples, so it is natural that leaf temperatures
should likewise increase substantially and have
bimodal patterns similar to light levels (figure
1). A time series for a single gap leaf records
the environmental conditions present during an
hour long period (figure 2). Changing light
conditions account for most of the variation
between leaf temperature and air temperature but
wind speed and air temperature also play a role.
Air temperature in this gap ranged by about a
degree between cloudy and sunny conditions and
would heat up more slowly than leaves. It is not
just the amount, but the length of the sunny
periods which will influence air and leaf
temperature. We binned all daytime
sunny leaf temperature data for three canopy
level species for one dry season (figure 3).
There is wide variation in the temperatures, but
by removing temperatures that are less than 1
above air temperature which are likely partially
shaded, we find on average sunlit leaves are 4
( 2) above air temperature.
Fig 3 Leaf minus air temperature for 12 canopy
level from 3 species leaves during sunny periods
over a 3 month period in the dry season of 2004.
Fig 1 The top figure shows canopy PAR during the
dry season of 2001 between 11-12 AM. The bottom
two figures show leaf level light and temperature
in a gap for a one hour period.
Are tropical leaves close to a temperature
threshold?
Light conditions in the tropics are
extremely variable. Using a tower top PAR
sensor, we calculated average length of sunny
periods (figure 4). Light conditions were highly
variable with 60 of sunny conditions (lightgt1000
micromoles/m2s) occurring in intervals of less
than 10 minutes in the dry season. During the
three month period when we recorded leaf
temperatures for the canopy level species, we
took regular temperature photosynthesis curves
with the LiCor 6400. Photosynthesis showed a
gradual decline with temperature until between
35-37 where there was a sharp decline (figure
5). Stomatal conductance was flat until 33
followed by a slight decline until 35-37 where
there was a sharp decline which probably caused
the decline in photosynthesis. In
response to a simultaneous increase in light and
temperature, g (stomatal conductance), A
(photosynthesis), and E (transpiration) remained
flat and then after 1-2 minutes begin to decline
(figure 6). For the first minute the LiCor was
not in equilibrium and therefore the data was not
included. There were substantially lower levels
of A, g, and E at 42 versus 37. The
stomata typically require a few minutes to
respond to temperature induced VPD changes and
longer periods to fully respond. However, since
light is so variable in the tropics, it is only
during longer sunny periods that stomata have
time to completely respond to sunny hot
conditions.
Methods
We made leaf level measurements on
two platform scaffold towers near the km 67 and
83 eddy flux towers. Leaf temperature
thermocouples were initially placed on 50 leaves
of three different canopy level species during
the dry season of 2004. We made photosynthesis
measurements using the Li-Cor 6400 portable
photosynthesis machine (Li-Cor Nebraska).
The tower measurements are described in
detail by Miller et al. 2004. We wanted to find
periods of tower data where intervals of cool
shady conditions preceded hot sunny conditions.
Using an algorithm, we screened the tower top PAR
sensor data to look for intervals of 10 minute
cloudy conditions followed by 20 sunny
conditions. Typically turbulent fluxes are
calculated using a period length of 30 minutes
however, this study uses a shorter 5 minute flux
interval. Although this period length will
likely underestimate fluxes by missing the
passage of large convective cells and will not be
absolutely accurate they should be accurate for
comparing relative differences among different 5
minute flux intervals.
Fig 4 The average length of time of a sunny
interval during the dry season in a tropical
forest. More than half of all light will come in
periods of less than ten minutes.
Fig 2 A time series of a single gap leaf over
an hour. Light is the controlling factor for
leaf temperature, but wind and air temperature
play an important role.
Are tropical forest canopies close to a
temperature threshold?
Half hourly averaged tower flux data show
a similar pattern to leaf level data with
decreasing CO2 uptake at air temperatures above
28 (figure 7). To determine the mechanism of
this decline,100 we took 10 minute cloudy
intervals followed by 20 minute sunny intervals
and calculated 5 minute flux intervals to
determine CO2 exchange, latent and sensible heat
exchange, and canopy conductance. Latent and
sensible heat increase with increasing light but
there is a slight drop in both towards the end of
the light interval (figure 8). The CO2 flux and
canopy conductance both initially increase and
then decline as the sunny interval continues
(figure 9). A possible cause of the decreasing
CO2 exchange and canopy conductance is that
canopy leaf temperature passes a temperature
threshold. During the first five minute
interval, the pyrgeometer data (measures outgoing
long wave radiation) shows the canopy temperature
rising quickly by about a degree which is due to
increasing leaf temperature (figure 10). Then,
for the following ten minutes, it heats up with
air temperature which increases steadily by
almost a degree over the entire 20 minute period.
However, in the final 5 minute period, as
turbulent motions increase (u), canopy
temperature no longer heats up with air
temperature.
Fig 6 Canopy leaves of three species over a
three month period placed in a warm (37 or 42
degree) bright (1000 micromole/m2s) chamber. A
(photosynthesis), g (stomatal conductance), and E
(evaporation) all declined with time.
Fig 5 Averages for all temperature curves taken
on 3 species at the top of the canopy over a 3
month period.
Would a decrease in clouds increase or decrease
tropical forest NEE?
Light limitation has been theorized to be
the major limitation on NPP in tropical forests
(Nemani et al. 2003). However, as light
intervals become longer, the canopy has more time
to heat up and the stomata have more time to
react to high temperatures and close which could
potentially decrease NPP. El Niño years have
substantially fewer clouds in the tropics than
normal years (Wielicki et al. 2002). Inverse
tracer transport models show less CO2 uptake
during these same periods which have also been
correlated with higher average temperatures and
less tree growth (Clark et al. 2003). It is
possible that the long sunny periods in el Niño
years, in combination with less rainfall, cause
canopies to move past a temperature threshold.
This may cause a decline in stomatal conductance
and a subsequent decline in CO2 uptake by
tropical forests.
Fig 7 - NEE using eddy covariance data for light
intervals above 1000 micromoles/m2s where the
light curve is already saturated. NEE decreases
above 28 degrees air temperature.
Fig 9 Canopy conductance and CO2 exchange with
five minute eddy flux intervals over a 10 minute
cloudy period followed by a 20 minute sunny
period.
Fig 8 Latent heat, sensible heat, and PAR for 5
minute eddy covariance intervals over a half hour
period where a 10 minute cloudy period is
followed by a 20 minute sunny period.
Fig 10 During a 20 minute sunny interval, u
(top figure) increases with time as does air and
canopy temperature (middle figure). Aerodynamic
canopy temperature (bottom) initially increases
but then decreases as u increase.