Tropical rainforests are globally important carbon stocks; thus, significant modifications in them may have an impact in the global climate. In tropical regions, temperature is predicted to increase by 2-5 °C in the next decade, but we still lack an understanding of how plants will respond to those changes. Raising temperatures may increase the rates of biological processes, including photosynthesis, and may increase the forest sink. Alternatively, higher air temperature may also increase rates of respiration, influencing the plant’s ability to maintain a positive carbon balance. Understanding plant’s responses to temperature increments is critical. We evaluated tropical tree responses to changes in temperature in a tropical rainforest at Biosphere 2 (B2). The B2 is a mesocosm that has experience temperatures of 5°C higher than some tropical forests ~20 years, and thus provide a useful location to evaluate plant responses to increases in temperature. We collected leaves of 14 tropical tree species across a 30°C temperature gradient at B2. We collected 5 leaves per individual tree located at different heights (1-3 m, 7-11 m, 14-17 m, and top of the canopy), which also allow us to assess how tree species will respond to microclimate variation.
Results/Conclusions
We estimated leaf reflectance (R) with a spectroradiometer (400-2500 nm) and used the PROSPECT models to estimate transmittance (Tr), absorptance (A), and chlorophyll content (Chl). We measured specific leaf area (SLA) and leaf water content (LWC) to account for interspecific variation. We found that SLA and LWC correlated strongly with Chl, Tr, A and R, making the latter one’s good predictors of plant’s responses. Most species showed increases on SLA, Chl, and A across the temperature gradient at all vertical strata. We found little interspecific variation in A, but mean values of Tr differed significantly with the vertical strata, with species showing marked differences at low and medium vertical strata, but similar responses at the top of the canopy. On a leaf the photosynthetic active radiation (PAR) can be expressed in units of biomass (A), or unit chlorophyll (Chl). We found that A increased for most species across the 30°C temperature gradient, while Chl was relatively similar for all species, with some minor differences along the vertical strata. Overall, increases in temperature modified the PAR profile influencing the rates of photosynthesis of tree species. Leaf optical properties are useful to monitor plant’s responses and identify temperature thresholds where plants can stop photosynthesis.