Tropical forest temperature thresholds for gross primary productivity

Abstract

Tropical forests are hyper‐diverse and perform critical functions that regulate global climate, yet they are also threatened by rising temperatures. Canopy temperatures depart considerably from air temperatures, sometimes by as much as air temperatures are projected to increase by the end of this century; however, canopy temperatures are rarely measured or considered in climate change analyses. Our results from near‐continuous thermal imaging of a well‐studied tropical forest show that canopy temperatures reached a maximum of ~34°C, and exceeded maximum air temperatures by as much as 7°C. Comparing different canopy surfaces reveals that bark was the warmest, followed by a deciduous canopy, flowers, and coolest was an evergreen canopy. Differences among canopy surfaces were largest during afternoon hours, when the evergreen canopy cooled more rapidly than other canopy surfaces, presumably due to transpiration. Gross primary productivity (GPP), estimated from eddy covariance measurements, was more strongly associated with canopy temperatures than air temperatures or vapor pressure deficit. The rate of GPP increase with canopy temperatures slowed above ~28–29°C, but GPP continued to increase until ~31–32°C. Although future warming is projected to be greater in high‐latitude regions, we show that tropical forest productivity is highly sensitive to small changes in temperature. Important biophysical and physiological characteristics captured by canopy temperatures allow more accurate predictions of GPP compared to commonly used air temperatures. Results suggest that as air temperatures continue to warm with climate change, canopy temperatures will increase at a ~40% higher rate, with uncertain but potentially large impacts on tropical forest productivity.

Thermal imaging in plant and ecosystem ecology: applications and challenges

“…3). Pau et al (2018) also showed, using thermal imaging, that GPP was more strongly associated with canopy temperature than to T air or vapor pressure deficit. This response suggests that leaf photosynthesis is maintained at fairly cold leaf temperatures, even while respiration is suppressed.…”

“…For example, Kim et al (2016) showed that leaf canopy temperature was a better predictor of variations in net ecosystem exchange. Pau et al (2018) also showed, using thermal imaging, that GPP was more strongly associated with canopy temperature than to T air or vapor pressure deficit. This study also used canopy thermal imaging to identify temperature-GPP dynamics, such as the temperature of peak GPP and the high-temperature threshold, beyond which GPP declined sharply.…”

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

“…Thermography has been used to estimate Esoil (Haghighi and Or, 2015;Nachshon et al, 2011;Shahraeeni and Or, 2010) and T from plant canopies (Jones, 1999;Jones et al, 2002), often in agricultural settings (Ishimwe et al, 2014;Vadivambal and Jayas, 2010). Researchers are increasingly using tower and UAV-mounted thermal cameras to measure the temperatures of different ecosystem 10 components at high temporal and spatial resolution (Hoffmann et al, 2016;Pau et al, 2018), which could revolutionize the measurement of T from plant canopies (Aubrecht et al, 2016) or even individual leaves in a field setting (Page et al, 2018).…”

Spatiotemporal Patterns and Phenology of Tropical Vegetation Solar-Induced Chlorophyll Fluorescence across Brazilian Biomes Using Satellite Observations

“…Environmental conditions, including water availability and VPD, have also been examined in relation to satellite-based SIF in the Amazon and correlations were found between water stress conditions and increased variability in SIF measurements, yet specific relationships among VPD, T, and SIF remain uncertain in Amazon tropical forests and unexamined for other vegetation and biomes [22,35,83,84]. Measurements of canopy temperature (T can ) have been shown to differ from air temperature (T air ) in a tropical forest canopy by as much as 7 degrees Celsius and display better agreement with GPP [85].…”