Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis)

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37 °C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30 ± 0.5 °C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4 ± 0.38 °C for HEAT and 48.2 ± 0.24 °C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30 ± 0.5 °C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.