Biogeography constrains acclimation to warming in two Australian Eucalypts: A climate shift experiment

Background/Question/Methods

The ability of forest trees to cope with rapid and widespread climate warming is critically dependent on their ability to physiologically acclimate to increased temperature. Previous studies suggest that taxa originating from different regions (e.g., populations, provenances) exhibit predictable intra-specific variation in physiological traits, suggesting that the ability to acclimate to climate warming may be constrained by biogeographical patterns. Here, we predict that warm-origin provenances have a constrained ability to acclimate to warming relative to cool-origin provenances of the same species, as the warm-origin provenances are operating near their thermal limit. We tested this prediction with a “climate shift” glasshouse experiment using many provenances of two widely distributed Australian Eucalypts (12 provenances of Eucalyptus tereticornis and 8 of E. grandis) originating from a latitudinal gradient in eastern Australia (from -15.5 to -38.0^o latitude). To simulate a realistic climate-shift scenario, we grew each provenance in conditions approximating the temperature at seed origin and a warmed temperature (+3.5^oC) using a series of climate-controlled glasshouse bays. We measured a suite of physiological traits (e.g., leaf-level photosynthesis, respiration, and biomass production) across all provenances and growth conditions to test the hypothesis that the degree of acclimation to warming is constrained in warm-origin taxa.

Results/Conclusions

We observed a strong biogeographic pattern regarding acclimation to warming that was consistent with our hypothesis. In response to warming, cold-origin taxa increased biomass production by ~60%, while warm-origin taxa showed reduced biomass production by ~20%. Furthermore, the degree to which warming stimulated biomass production was negatively correlated with the latitude of origin (p < 0.01, r² = 0.6). Measurements of light-saturated leaf photosynthesis at ambient CO₂ (A_sat) and saturating CO₂ (A_max) were consistent with the biomass measurements. Warming increased A_sat and A_max by ~60% in the cold-origin provenances but reduced A_sat and A_max by 20-40% in the warm-origin provenances of both species. These responses were strongly correlated with the latitude of origin in the same manner as biomass production (p < 0.01, r² values of 0.4 to 0.5). In contrast, rates of leaf dark respiration were remarkably homeostatic in response to warming, indicating nearly complete acclimation of respiration. These results indicate that there is substantial intra-specific variation in plant responses to climate warming that follows predictable biogeographic patterns. Thus, the effects of climate warming may vary substantially across the range of a tree species, with growth declines in warm-origin trees and increased growth of cool-origin trees.