Leaf Temperatures in an Indian Tropical Forest Exceed Physiological Limits but Durations of Exposures Are Currently Not Sufficient to Cause Lasting Damage

Abstract

Increasing temperatures in the tropics will reduce performance of trees and agroforestry species and may lead to lasting damage and leaf death. One criterion to determine future forest resilience is to evaluate damage caused by temperature on Photosystem-II (PSII), a particularly sensitive component of photosynthesis. The temperature at which 50% of PSII function is lost (T₅₀) is a widely used measure of irreversible damage to leaves. To assess vulnerability to high temperatures, studies have measured T₅₀ or leaf temperatures, but rarely both. Further, because extant leaf temperature records are short, duration of exposure above thresholds like T₅₀ has not been considered. Finally, these studies do not directly assess the effect of threshold exceedance on leaves. To understand how often, and how long, leaf temperatures exceed critical thresholds, we measured leaf temperatures of forest and agroforestry species in a tropical forest in the Western Ghats of India where air temperatures are high. We quantified species-specific physiological thresholds and assessed leaf damage after high-temperature exposure. We found that leaf temperatures already exceed T₅₀. However, continuous exposure durations above critical thresholds are very skewed with most events lasting for much less than 30 min. As T₅₀ was measured after a 30-min exposure, our results suggest that threshold exceedances and exposure durations for lasting damage are currently not reached and will rarely be reached if maximum air temperatures increase by 4°C. Consistent with this, we found only minor indications of heat damage in the forest species. However, there were indications of heat-induced reduction in PSII function and damage in the agroforestry leaves which have lower T₅₀. Our findings suggest that, for forest species, while high-temperature thresholds may be surpassed, durations of exposure above thresholds remain short, and therefore, are unlikely to lead to irreversible damage and leaf death, even under 4°C warming.

Keywords: agroforestry; climate change; leaf temperature; photosynthesis; thermotolerance; tropical forests.

FIGURE 1

Mean leaf temperature records across leaves of the four forest species—Memecylon, Psydrax, Olea, and Terminalia—examined in the study for a representative 4‐day period (left column). Gray shade shows the variance across the three to four leaves of each species at a given time. Right column shows the corresponding leaf‐to‐air temperature differences across the same period for one of the leaves of each species.

FIGURE 2

Daily maximum leaf temperatures of forest species measured continuously during the dry season (typically several months) using thermistors attached to leaves (Table 1 and Figure S2) and species specific T ₅₀ , T ₅ , T _max and T _opt .

FIGURE 3

Leaf temperatures measured daily around mid‐day using a handheld thermal camera (FLIR C2) during a two‐week period in May 2023 in relation to species specific threshold measures T ₅ (purple) and T ₅₀ (red). Measurements were made on 13 agroforestry species and the four forest species investigated in this study (Table S1). Per species at least six sun exposed and shaded leaves from three to five individuals were probed.

FIGURE 4

Frequency distribution of duration of uninterrupted leaf exposure above threshold temperature for two threshold measures (T ₅ , T ₅₀ ) during the measurement period in 2023 and for two future scenarios of 2°C and 4°C air temperature increases relative to 2023.

FIGURE 5

Dark‐adapted quantum efficiency F _v /F _m measured on shade (gray) and sun exposed (white) leaves at end of dry season of four forest species and seven agroforestry species. Red asterisks denote species with significantly different sun and shade F _v /F _m from a nested ANOVA across species and leaf types (shade vs. sun exposed).