A. Weather and soil water data
The following mean hourly environmental variables were monitored: mean overstory and understory PAR, air temperature and relative humidity, soil temperature, rainfall, and soil water content. Continuous measurements of forest litter water content based on the electrical resistance characteristics of wet vs. dry litter were also made for estimating forest floor surface evaporation. Towers at the WB-oak, GSMNP, and MMSF sites were available for measurements of the above-canopy environment. At FEF, WB-pine, BSFNRA similar instrumentation was located in a nearby clearing to simulate above canopy climatic data. Cellular phones or direct phone service were installed at all sites (except FEF) for direct access to the data loggers for remote automated downloading of data and periodic checks on the instrumentation.
B. Measurement of forest transpiration
Measurements of sap flow velocities were made using thermal dissipation probes (Dynamax, Inc., Houston, TX) consisting of two probes of 1.1 mm diameter, each of which were inserted 3 cm into the sapwood of the tree. Individual sensors were installed on 8 or 9 dominant canopy trees in each plot . Total sap flow (m3 s-1) for each tree was calculated as the product of measured sap velocity and the cross-sectional area of sapwood at the heating probe (m2).
C. Unique Features of the Sapflow Based Approach
The sapflow approach for estimating stand transpiration provides spatially explicit and species-specific information (see figure) on transpiration within a forest stand. It allows for continuous and automated direct measure of plant response to atmosphere and soil environments. The approach can be applied to forest sites that would otherwise be inappropriate for eddy correlation measurements (e.g., steep slopes, small plots with inadequate fetch). When combined with observations of soil surface evaporation and changing soil water content, sapflow based approaches provide an effective means of characterizing forest hydrologic budgets with high temporal resolution (see figure).
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