Hydraulic dysfunction and carbon depletion are two of the determining factors in tree mortality. In temperate forests, deciduous trees are either diffuse or ring porous and have diverse hydraulic characteristics and carbohydrate requirements. The physiological mechanisms involved in the carbon-water trade-off differ between these wood types, but previous studies have found conflicting results regarding their responses; we seek to resolve this conflict by measuring sap flux in mature canopy species of similar ages and sizes in comparable sites. Specifically, we are interested in how the physiological response to a drying atmosphere differs between these two wood types.
We determined the hydraulic response by quantifying how the sap flux of ring- and diffuse-porous canopy species vary in response to an increasing vapor pressure deficit. We predicted that ring-porous trees would maintain higher sap flux despite increasing atmospheric dryness because of their higher carbohydrate requirements. We measured sap flux with custom-built thermal dissipation probes using established protocols and calculated vapor pressure deficit from recorded temperature and humidity values. Diffuse-porous species included Fagus grandifolia and Acer saccharum, while ring-porous species included Quercus rubra and Quercus alba.
Results/Conclusions
We found that sap flux significantly varied with vapor pressure deficit (p < 0.001), and that as expected, sap flux decreased as vapor pressure deficit increased. We also found that sap flux significantly differed between wood types (p < 0.01). Furthermore, and most interestingly, we found that wood type significantly affected the relationship between sap flux and vapor pressure deficit (p < 0.05). Although both wood types showed an overall decrease in sap flux rates at higher vapor pressure deficits, the diffuse-porous species maintained a higher rate of sap flux than the ring-porous species. These results support our hypothesis that ring- and diffuse-porous species respond differently as the atmosphere becomes drier. However, contrary to our predictions, the diffuse-porous species maintained a higher sap flux rate than the ring-porous species as atmospheric water availability decreased. This finding may be due to the substantial late season precipitation but may also indicate that hydraulic response changes seasonally within a wood type. Though preliminary, these results suggest that trees with contrasting hydraulic structures differ in their physiological response to short-term drying, but in a pattern opposite of what was previously predicted.