Tropical tree seedling responses and acclimation potential to elevated temperatures

Background/Question/Methods

Current climate projections suggest that tropical lowland forests will soon see the emergence of unprecedented thermal regimes. Although these conditions maybe beyond the realized thermal-niche of many species that dominate today’s forests, our ability to predict how the structure and function of forests may change is constrained by the paucity of data on tropical plant responses and their potential to acclimate to these future conditions. We used growth chamber studies (EGC-15, Environmental Growth Chambers Chagrin Falls, OH) to investigate how both day and night time thermal regimes impact tropical tree seedling performance. In addition to net growth we monitored changes in biomass allocation, morphological characteristics and acclimation of both dark respiration and photosynthesis.

Results/Conclusions

In the 10 tropical tree species tested there were substantial differences in their fundamental thermal niche, with pioneer species (e.g. Ficus insipida and Ochroma pyramidale) exhibiting optimal growth rates at day-time air temperatures of 36 °C − markedly above the long-term average temperatures seen in lowland Panama. In contrast a range of late successional and montane species showed only limited carbon gain or were killed when exposed high growth temperatures. We demonstrate that transpirational cooling may play a fundemental role in allowing pinoeer species to thrive under high air temperatures. As well as highlighting the interplay between vapour pressure deficit, air temperatures and energy balance at the leaf level. In addition to differences in integrated biomass accumulation species tested showed a range of morphological and physiological responses to increased temperature, including altered biomass allocation (i.e. root-shoot ratio), stem elongation, changes in leaf morphological traits, and photosynthetic capacity. Plasticity in physiological and morphological traits resulted in altered thermal response of leaf level gas fluxes, with significant shifts in respiration and photosynthetic thermal response curves. We conclude that while tropical tree species may have a greater thermal niche than previously thought, and that species may show substantial acclimation potential, such capacity varies significantly between species and plant functional types.