Tropical forests are approaching critical temperature thresholds

The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages ~46.7°C (Tcrit) 1.  However, it remains unclear whether leaf temperatures experienced by tropical vegetation approach this threshold or soon will under climate change. We found that pantropical canopy temperatures independently triangulated from individual leaf thermocouples, pyrgeometers, and remote sensing (ECOSTRESS) have midday-peak temperatures of ~34°C during dry periods, with a long high-temperature tail that can exceed 40°C.  Leaf thermocouple data from multiple sites across the tropics suggest that even within pixels of moderate temperatures, upper-canopy leaves exceed Tcrit 0.01% of the time. Further, upper-canopy leaf warming experiments (+2, 3, and 4°C in Brazil, Puerto Rico, and Australia) increased leaf temperatures non-linearly with peak leaf temperatures exceeding Tcrit 1.3% of the time (11% >43.5°C, 0.3% >49.9°C).  Using an empirical model incorporating these d..., Field Data - We estimate canopy temperature at the km 83 eddy covariance tower in the Tapajos region of Brazil 1–3 using a pyrgeometer (Kipp and Zonen, Delft, Netherlands) mounted at 64 m to measure upwelling longwave radiation (L↑ in W m-2) with an estimated radiative-flux footprint of 8,000 m2 4.  Data were collected every 2 seconds and averaged over 30-minute intervals between August 2001 and March 2004. We estimated canopy temperature with the following equation:
Eq 1 – Canopy temperature (°C) = (L↑/(E*5.67e-8))0.25-273.15
We chose an emissivity value (E) of 0.98 for the tower data, as this was the most common value used in the ECOSTRESS data (SDS_Emis1-5 (ECO2LSTE.001) and the broader literature for tropical forests 5. We compared canopy temperature derived from the pyrgeometer to eddy covariance derived latent heat fluxes (flux footprint ~1 km2), air temperature at 40 m, which is the approximate canopy height (model 076B, Met One, Oregon, USA; and model 107, Campbell Scientific, Lo...,