The isohydric-anisohydric continuum describes the relative stringency of stomatal control of leaf water potential ( ψleaf) during drought. Hydroscape area ( HA) – the water potential landscape over which stomata regulate ψleaf – has emerged as a useful metric of the iso/anisohydric continuum because it is strongly linked to several hydraulic, photosynthetic, and structural traits. Previous research on HA focused on broad ecological patterns involving several plant clades. Here we investigate relationships of HA to climatic conditions and functional traits across ecologically diverse but closely related species while accounting for phylogeny. Across a macroclimatic moisture gradient, defined by the ratio of mean annual precipitation to mean annual pan evaporation ( P/Ep), HA decreased with P/Ep for ten Eucalyptus species. Greater anisohydry reflects lower turgor loss points and greater hydraulic safety, mirroring global patterns. More isohydric species have mesophyll photosynthetic capacity that is more sensitive to ψleaf, consistent with an earlier model for optimal stomatal behavior. Hydroscapes exhibit little plasticity in response to variation in water supply, and the extent of plasticity does not vary with P/Ep of native habitats. These findings strengthen the case that HA is a useful metric for characterizing drought tolerance and water-status regulation.

In recent decades, the rising severity of summer heatwaves has increased the co-occurrence of heat and drought stress leading to forest mortality and to reductions in crop yield. Plant responses to this combined stress can be unique from the response to either independent stress, yet few studies have investigated these responses in tree species. Our work examines adjustment of several primary metabolites and polyamine (stress indicating secondary metabolites) in paper birch and white spruce subjected to two seasons of repeated heatwaves, drought, and elevated CO2. Our objectives were to determine if the metabolic adjustments in response to heatwave+drought stress are: 1) unique or shared with either individual stress; 2) greater in birch compared to spruce; and 3) carried over into the following season. Our data show that white spruce displayed many metabolic responses that were unique to the combined stress, especially in the first year, while paper birch displayed few. Further, the unique responses in spruce seen in the first season stress exposure did not carry into the following season indicating possible stress memory. Our data highlights the importance of considering species-specific responses to multiple stressors when making predictions about forest response to future climate scenarios.