Tropical forests are hyperdiverse and perform critical functions that regulate global climate, yet they are also threatened by climate change. Tropical forests may be especially sensitive to rising temperatures because these species have evolved within a narrow temperature range, and furthermore exist closer to their upper temperature maxima. Additionally, tropical regions may experience air temperatures well outside of their historical range much sooner than other regions. Canopy temperatures should depart considerably from air temperatures, however canopy temperatures are rarely measured or considered in climate change analyses and their relationship with ecosystem functions such as gross primary productivity (GPP) are unknown.
Results/Conclusions
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. GPP estimated from eddy covariance 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 and began to decline above ~31 °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 temperature 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.