Ecological Archives A025-124-A1
Colin M. Beier, Jesse Caputo, and Peter M. Groffman. 2015. Measuring ecosystem capacity to provide regulating services: forest removal and recovery at Hubbard Brook (USA). Ecological Applications 25:2011–2021. http://dx.doi.org/10.1890/14-1376.1
Appendix A. Additional information regarding methods used to calculate measures of water flow regulation and tree species composition.
1. Identification of thresholds for water flow regulation
The high flow threshold was estimated based on the structural capacity of the Franklin Falls Dam, which controls the outflow of the Pemigawasset River watershed into which Hubbard Brook ultimately drains. Based on technical documentation, we determined the maximum daily discharge of the dam and translated this volume of water to a height over the entire area of the Pemigawasset River watershed. This value represents the average maximum daily area-weighted discharge that catchments within the larger watershed can attain without the dam exceeding its maximum discharge. The low flow threshold was estimated based on historical water use data provided by the U.S. Geological Survey (USGS). In this case, the annual volume of surface water extracted from Grafton County, NH, was translated to a daily height over the entire area of the county. This value constitutes the average minimum area-weighted streamflow that catchments must maintain before daily surface water use exceeds supply. USGS water-use records are reported on a 5-year time step starting in 1985. We assumed water use in a given year was equivalent to water use for the most recent record prior to that year. For all dates prior to 1990, we used the 1985 value.
2. Tree species composition index
To provide a measure of the forest’s potential to yield valuable wood products, we calculated a tree species composition index by multiplying the relative density of each tree species by its relative stumpage value and summing these individual values. Species value was estimated based on recent regional stumpage values reported by the states of New York, New Hampshire, and Maine, USA. Values for each species were scaled by the value of the most valuable tree in the data set, in this case, black walnut (Juglans nigra L.). A value of 1.0, therefore, would indicate that all stems in a given watershed were J. nigra stems. We note that this choice of species composition metric is relevant only to measuring the capacity for provision of future sawtimber and related forest products. It is likely not the case that a monoculture of black walnut, or other high-value species, would yield optimal conditions for providing wildlife habitat, carbon sequestration, recreational opportunities, and other benefits. In fact, it is likely that a more diverse species mix would result in a greater value of most of these services than would a monoculture of any single species.