Leaf heat tolerance acclimation to warming in tropical tree species is not affected by vapor pressure deficit

Mitigating the negative impact of climate change on tropical forests requires a comprehensive understanding of the direct and indirect effects of climate variables such as temperature and vapor pressure deficit (VPD). We investigated the effects of warming at two levels of VPD on the leaf heat tolerance of nine species of tropical forest trees varying in successional status in Panama. Seedlings were grown either under ambient conditions (control), at elevated temperatures (5°C daytime warming and increased relative humidity), and at elevated temperature and elevated VPD (5°C warming and decreased relative humidity). Plants were maintained under the treatment conditions for 32-103 days, allowing full development of new leaves prior to measurements. We quantified chlorophyll fluorescence-based leaf heat tolerance with two parameters derived from Fv/Fm decline curves: Tcrit and T50. Treatments significantly (P < 0.001) affected Tcrit and T50 across species. The average Tcrit value across species increased from 47.5°C (ambient) to 48.6°C in each of the two high temperature treatments, while T50 increased from 50.1 to 51.1°C. The rate of increase was 0.20°C-1Tgrowth for both Tcrit and T50. However, VPD did not significantly alter the leaf heat tolerance thresholds (P > 0.05). On average, late successional species had lower heat tolerance than early successional species, but moderately higher acclimation rate, suggesting a tradeoff between constitutive heat tolerance and upregulation of heat tolerance. The study shows that tropical species, irrespective of successional status, can partially acclimate to warming and that VPD does not play a significant independent role.