In order to survive a changing climate, trees will need to acclimate over the span of very long lives. Because woody material is made to last, it is not particularly responsive to rapid environmental change. Leaves, on the other hand, may be a tree’s best hope to acclimate. They can be produced rapidly and continuously within the span of a season, and their complex biochemical and anatomical traits allow for plasticity along numerous gradients. Since tree leaves also need to be able to acclimate to extreme within-canopy changes in microclimate, we can look to the plasticity in leaf traits across vertical canopy gradients to gain insight about different species’ potential ability to acclimate to a climate changing across time. We measured numerous leaf functional traits across vertical canopy gradients in both tropical and temperate broadleaved forests to assess within-canopy plasticity in leaf form and function. We present these data and propose a framework to use these vertical trait patterns to indicate a species’ resilience or resistance to climate change.
Results/Conclusions
Within a Puerto Rican tropical wet forest, we found both stomatal density and size to increase with height for two species. The upper canopy was abruptly hotter and brighter than the lower canopy, and we hypothesize that this vertical acclimation allows for greater evaporative cooling at the canopy top. On the other hand, in a temperate hardwood forest, a tropical moist forest, and a tropical wet forest, we found leaf temperature maxima to exceed temperatures optimum for photosynthesis only in the upper canopy. These results suggest a reduced ability to uptake carbon in the canopy top for all three forests. Similarly, we found both quantum yield and respiratory thermal acclimation to decrease with canopy height in a leaf warming experiment in a wet tropical forest canopy. These trends indicate higher stress in the upper canopy with potential implications for reduced canopy carbon uptake, and/or greater respiratory efflux of CO2 with warming. Since upper canopy leaves are responsible for a disproportionate amount of overall forest photosynthesis, our data indicate that because these vulnerable upper canopies are not able to fully acclimate to extreme microclimates with height, these species may also be less able to acclimate to a warming climate.