Previous research indicates that ring- and diffuse-porous trees differ in their hydraulic response to a drying environment, but the seasonal pattern of this response is still unresolved. Furthermore, some studies examined saplings while others measured mature trees, making comparisons across studies difficult. Our objectives were: 1) to understand how seasonal patterns in hydraulic responses differ between ring- and diffuse-porous trees coexisting in a forest and 2) to understand how age class may influence hydraulic patterns within these wood types.
We measured environmental variables and hydraulic parameters on six tree species across the 2019 growing season; we included both saplings and mature trees of the same species. The ring-porous species included shagbark hickory (Carya ovata), white oak (Quercus alba) and red oak (Quercus rubra), while the diffuse-porous species included sugar maple (Acer saccharum), American beech (Fagus grandifolia) and American sycamore (Platanus occidentalis). We installed custom-built sap flux sensors in the mature trees, conducted gas exchange measurements on the saplings and measured leaf water potential for both age classes. We also collected soil moisture measurements throughout the season and calculated vapor pressure deficit from temperature and humidity values recorded by a data logger installed in the canopy of our study site.
Results/Conclusions
Overall, our results suggest that the seasonal pattern of a tree’s hydraulic response to drying does significantly differ between wood types (p < 0.01). However, species-level differences indicate that these patterns in hydraulic response really fall across a continuum rather than into a dichotomy. Many scientists have proposed that ring- and diffuse-porous trees may exhibit opposing hydraulic strategies, with one wood type behaving isohydrically, while the other responds more anisohydrically. However, empirical evidence is conflicting. Our study implies that while there is a difference between wood types overall, the species fall across a spectrum with sugar maple representing the most isohydric response and hickory representing the most anisohydric response. Sycamore, beech, red oak and white oak spread across the middle of this spectrum, respectively.
Furthermore, our data indicate that age influences the hydraulic response of a tree but may be more influential in certain species. Understanding the patterns in hydraulic responses between species and wood types informs fields across multiple levels of ecology. For example, studies such as ours will allow us to better predict forest-level responses to changes in moisture availability, while also informing studies that focus on the physiological mechanisms involved in regulating stomatal behavior.