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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.

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 leaves close to a temperature threshold?

Are tropical forest canopies close to a temperature threshold?

How hot do tropical forest leaves get?

Would a decrease in clouds increase or decrease tropical forest NEE?

Introduction

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 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 5 – Averages for all temperature curves taken on 3 species at the top of the canopy over a 3 month period.

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 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 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 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 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.

Are tropical forests sensitive to temperature?

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 NASA’s LBA (Large–Scale 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?

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.

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.

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 (light>1000 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.

Half hourly averaged tower flux data show a similar pattern to leaf level data with decreasing CO 2 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 CO 2 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 CO 2 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.

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.

Chris Doughty (cdoughty@uci.edu), Mike Goulden, Scott Miller, University of California at IrvineHumberto da Rocha, Augusto Maia, University of Sao Paulo, Brazil