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Summary Distinct survival strategies can result from trade‐offs in plant function under contrasting environments. Investment in drought resistance mechanisms can enhance survivorship but result in conservative growth. We tested the hypothesis that the widespread oaks (Quercus spp.) of the Americas exhibit an interspecific trade‐off between drought resistance and growth capacity. Using experimental water treatments, we isolated adaptive trait associations among species in relation to their broad climates of origin and tested for correlated evolution between plant functional responses to water availability and habitat. Across all lineages, oaks displayed plastic drought responses – typically acclimating through osmolyte accumulation in leaves and/or employing conservative growth. Oaks from xeric climates had higher osmolytes and reduced stomatal pore area index, which allows for moderated gas exchange and limits tissue loss. Patterns suggest drought resistance strategies are convergent and under strong adaptive pressure. Leaf habit, however, mediates the growth and drought resistance strategies of oaks. Deciduous species, and evergreen species from xeric climates, have increased drought tolerance through osmoregulation, which allows for continuous, conservative growth. Evergreen mesic species show limited drought resistance but could enhance growth under well‐watered conditions. Consequently, evergreen species from mesic environments are especially vulnerable to chronic drought and climate change.

Determining tree response to climate stress is critical for predicting changes in forest carbon dynamics as well as tree mortality. In temperate deciduous forests, describing this response is complicated by the complex diversity of leaf and wood characteristics among co-existing species. Furthermore, because of the inherent logistical limitations of measuring mature forest trees, many carbon models and stress-response studies are informed by physiological data collected from juvenile trees (seedlings or saplings). However, the extent to which juvenile and mature trees differ in their physiological responses to water stress is not well documented under natural conditions. The majority of carbon sequestered in a forest is in mature trees; therefore, direct canopy measurements comparing responses to climate in juvenile and mature trees would allow us to more accurately predict changes in ecosystem carbon uptake. Here, we present data describing the physiological responses to summer water stress in juvenile trees of six temperate deciduous species. Our results indicate that species exhibited variation and plasticity in stress hydraulic parameters yet maintained similar rates of carbon uptake. We demonstrate how integrative photosynthetic parameters, such as photosynthetic capacity and quantum efficiency of photosystem II, are beneficial for wholistically displaying physiological responses at the leaf level. We further compared seasonal patterns of leaf water potential during decreasing soil water availability between the juvenile trees and co-existing mature trees of the same species. Our data reveal that while some species remain static in their hydraulic behavior from the juvenile to adult stage, other species are dynamic between life stages. Models, as well as experimental studies examining tree response to stressors, should plan for plasticity in physiological parameters among co-existing species, and should further allow variability between life stages for particular species. The capacity to effectively inform models from data collected in mature trees will inevitably lead to improved predictions of tree mortality and forest carbon trajectories.

Chestnut breeding programs have been using a backcross breeding technique to produce trees with a predominantly American chestnut (<i>Castanea dentata</i>) genome and chestnut blight disease resistance from Chinese chestnut (<i>Castanea mollissima</i>). The potential for other physiological changes caused by breeding has yet to be widely studied. We quantify chestnut (American, Chinese, and BC₃F₃ hybrids) responses to water-stress and measure how co-occurring drought influences disease severity. The experiment was completed using 172 bare-root seedlings organized into a completely randomized factorial design in an outdoor rain-out shelter for one growing season. BC₃F₃ hybrid gas exchange (A_sat, g_s) rates were more similar to Chinese than American chestnuts over a 20-day dry-down period, and hybrid turgor loss point showed a more intermediate (between Chinese and American) response. The relationship of stomatal conductance to mid-day leaf water potential (Ψ_md) also exhibited both American and Chinese characteristics in the hybrid trees. There was no effect of drought on the disease severity for any of the chestnut groups. We find evidence that drought physiology has been altered in some BC₃F₃ hybrids, but do not find changes in disease severity when chestnuts are under co-occurring drought.

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