Ecological studies at the Shale Hills Observatory include patterns of tree water use across the water shed, and how water availability may influence trees at the physiological, community and evolutionary time scales. Measurements include:

• Mapping of tree species distribution, sapwood area and leaf area index
• Transpiration by sap flux
• Tree hydraulic parameters for conducting water and tolerating water stress
• Sources of water uptake using natural abundance of ¹⁸O and ²H

From this work we hope to improve hydraulic models and our understanding of how water availability shapes the ecology of temperate forest trees.

We characterized the woody vegetation in the watershed by surveying all canopy trees greater than 8” diameter at breast height (DBH). From this data we gained information about the presence, distribution, and dominance of species within the watershed. These data are being used to better understand how water availability may influence the ecology of tree species as well as permit scaling of individual estimates of tree transpiration (Figure 1).

Sap flow sensors (heat dissipation probes) have been installed at multiple locations in the watershed in multiple tree species. Data are being continuously logged and transmitted to the web. Trees of different vascular anatomy (ring-porous vessels, diffuse porous vessels, trachieds only - coniferous) seem to have very different responses to wetting and drying events (Figure 2).

Small branch samples have been collected to measure specific conductivity, capacitance, and xylem vulnerability. These parameters make it possible to characterize the hydraulic architecture of the tree and to determine which species make more or less conservative use of water. Six species within the three dominant genera of the watershed were sampled at four different sites. By sampling all species at both wet and dry sites, environmental variation is reduced and we are able to begin to differentiate between genetic adaptation by a species and an individual’s plasticity.

When trees take up water from the soil, the water maintains the same concentration of ¹⁸O and ²H isotopes while it is transported through the tree. By extracting the xylem water from small branches in the canopy, it is possible to determine the isotopic signature of the water taken from the soil. This signature is then compared with the signatures of surface-shallow soil water and deeper ground water. Because each source has a unique signature, we can determine which water source the tree is relying on by how similar the tree’s signature is to a source’s signature.

Contact: David Eissenstat (PI)