Searching for a Relationship Between Forest Water Use and Increasing Atmospheric CO2 Concentration with Long-Term Hydrologic Data from the Hubbard Brook Experimental Forest

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Abstract
Increases in atmospheric C02 concentration from mid-1956 through mid-1997 were compared with hydrologic records from five forested, gaged watersheds in the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, U.S.A. The purpose of the comparison was to assess whether a relationship between increasing atmospheric CO2 concentration and whole-ecosystem evapotranspiration (ET) could be determined. The HBEF is particularly well suited to this type of study because of the length of the hydrologic record and the physical properties of the watersheds. This analysis is based on HBEF water years (which begin 1 June and end the following 31 May) rather than calendar years. Hydrologic records from individual watersheds used in this analysis ranged from 28 to 41 water years. During the full 41-water-year period, it is estimated that water-year mean atmospheric CO2 concentration increased more than 15% (from about 314 to 363 ppm). In one south-facing watershed (i.e., HBEF watershed 3), there was a statistically significant negative relationship between atmospheric C02 concentration and ET. This translated into a nearly 77 rnndyear reduction in ET as a result of a 50 ppm increase in atmospheric C02 concentration, a result of practical significance. Evapotranspiration from the other watersheds was also negatively related to atmosphericmore » CO2 concentration, but with smaller (and statistically insignificant) magnitudes. Evapotranspiration from the three south-facing (but not the two north-facing) watersheds included in the analysis was "abnormally" low during the most recent 2 years (i.e., water years beginning in 1995 and 1996), and this affected the trends in ET. This recent and abrupt, reduction in ET deserves further analysis, most importantly by an extension of the hydrologic record through continued long-term monitoring in the HBEF (which is ongoing). If ET remains relatively low during the coming years in south-facing watersheds, studies of the physical and/or biological cause(s) of that reduction will be in order. On the whole, this analysis indicates only modest effects, or none at all, of recent increases in atmospheric C02 concentration on water use by temperate forests. The results are broadly supported by available (albeit limited) experimental results for "large" trees. Nonetheless, and in spite of the length of the HBEF hydrologic record, firm conclusions cannot be drawn about effects of past increases in atmospheric C02 concentration on HBEF ET, in part because of inter-watershed differences in temporal trends in ET. Moreover, because atmospheric C02 concentration increased monotonically during the study period, any other environmental or biological/ecological factor also changing monotonically might be confused with atmospheric C02 concentration in this type of analysis. To my knowledge, this is the first reported analysis of long-term (i.e., up to 41 years) forest ET trends in relation to increasing atmospheric CO2 concentration using complete and precise whole-ecosystem water budgets. This analysis should be extended as future hydrologic data become available for the HBEF. In addition, similar analyses should be conducted for other terrestrial ecosystems with complete water budgets to expand empirical understanding of hydrologic responses of whole ecosystems to increasing atmospheric CO2 concentration and other ongoing enviromnental changes.« less

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