Background/Question/Methods Carbon-mediated hydraulic failure is the current leading hypothesis for tree mortality. However, the physiological mechanisms of this process are complex and vary among species and environment. The way in which a tree responds to drought is defined as its hydraulic behavior, which is described using the isohydric/anisohydric continuum. Theoretically, diffuse-porous and ring-porous trees should fall at opposite ends of this continuum due to their contrasting xylem anatomy and associated carbon requirements. While previous studies have documented this trend, the relationship between wood type and hydraulic behavior is still unresolved, particularly in temperate forests. One reason for the conflicting results in previous work may be that some studies examined mature trees while others measured saplings. The overall goal of my research was to describe hydraulic behavior in ring- and diffuse-porous species in saplings in a temperate forest and compare the patterns in saplings to those we previously observed in mature trees in the same forest. The diffuse-porous species included Acer saccharum, Fagus grandifolia and Platanus occidentalis, while the ring-porous species included Quercus rubra, Quercus alba and Carya ovata. We measured gas exchange and leaf water potential throughout the season, along with soil moisture and vapor pressure deficit. Results/Conclusions Our results demonstrate that hydraulic behavior differs between wood types but is more of a continuum than a dichotomy. Diffuse-porous A. saccharum exhibited distinct isohydry with conservative changes in stomatal conductance and leaf water potential throughout the season. Opposingly, ring-porous C. ovata exhibited distinct anisohydry with variable patterns in diurnal and seasonal stomatal conductance, despite decreasing water availability. Data from the other four species demonstrate a spectrum of hydraulic behaviors between these two ‘extremes.’ Most importantly, we found that the patterns in hydraulic behaviors among ring- and diffuse-porous saplings paralleled hydraulic behaviors we observed in mature trees in the same forest. Our research suggests that age class does not influence hydraulic behavior, and that specific environment is more important in determining whether a species responds isohydrically or anisohydrically. Therefore, we conclude that comparing studies that measured saplings to those that measured mature trees is valid, so long as the studies were conducted in similar environments with similar climate conditions. Overall, this work provides new insights into the physiological mechanisms responsible for carbon-water trade-offs in ring- and diffuse-porous trees in temperate forests, and resolves previous discrepancies involving the possible influence of age class on a tree’s hydraulic behavior.
