Developmental phenotypic plasticity can allow plants to buffer the effects of abiotic and biotic environmental stressors. Both water availability, through increased drought stress, and light availability, through changes in competitive environments, may be altered by climate change. Therefore, it is vital to improve our understanding of how phenotypic plasticity in ecological functional traits is coordinated with variation in physiological performance in plants. To identify coordinated leaf responses to low-water versus low-light availability, we measured leaf mass per area (LMA), leaf anatomical characteristics, and leaf gas exchange of juvenile Populus tremuloides trees.
Spongy mesophyll tissue surface area (Asmes/A) was correlated with intrinsic water-use efficiency (WUEi: photosynthesis, Aarea: stomatal conductance, gs). Under low-water availability, these changes occurred at the cost of greater leaf tissue density and reduced expansive growth, as leaves were denser but were only 20 % the final area of control leaves, resulting in elevated LMA and elevated WUEi. Low-light resulted in reduced palisade mesophyll surface area (Apmes/A) while spongy mesophyll surface area was maintained (Asmes/A), with no changes to WUEi. These leaf morphological changes may be a plastic strategy to increase laminar light capture while maintaining WUEi. With reduced density and thickness, however, leaves were 50 % the area of control leaves, ultimately resulting in reduced LMA. Our results illustrate that P. tremuloides saplings partially maintain physiological function in response to water and light limitation by inducing developmental plasticity in LMA with underlying anatomical changes. We reveal additional implications of these results in the context of developmental plasticity, growth trade-offs, and the ecological impacts of climate change.