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A Recent Analysis of Ecosystem Water Use Efficiency Trends in an Old-growth Forest

Paper Reviewed
Jiang, Y., Still, C.J., Rastogi, B., Page, G.F.M., Wharton, S., Meinzer, F.C., Voelker, S. and Kim, J.B. 2019. Trends and controls on water-use efficiency of an old-growth coniferous forest in the Pacific Northwest. Environmental Research Letters 14: 074029.

Writing as background for their study, Jiang et al. (2019) say that water use efficiency (WUE) is "an important aspect of carbon and water cycling and has been used to assess forest ecosystem responses to climate change and rising atmospheric CO2 concentrations." Indeed, and over the years we have documented numerous studies highlighting the positive response of increasing atmospheric CO2 on the WUE of trees and forests on our website here.

As their contribution to this topic, the team of eight researchers set out to assess changes in WUE in an old-growth evergreen coniferous forest in the Pacific Northwest of the United States, as well as the factors driving that change, over the short-term period 1998-2015.

The study site was located in a forest stand at the Wind River Experimental Forest of the T.T. Munger Research Natural Area in the southern Cascade Range of Washington State, USA. The forest, according to the authors, is approximately 500 years old and is dominated by Douglas fir and western hemlock. Inside the forest is an 87 m tower, where micrometeorological and eddy covariance instrumentation has been gathering data since 1998. Using such data, Jiang et al. calculated three different metrics of WUE: (1) photosynthetic WUE, defined as "the ratio of gross primary production to evapotranspiration," (2) inherent WUE, defined as "the ratio of the product of gross primary production and vapor pressure deficit to evapotranspiration," and (3) ecosystem WUE, represented by "the ratio of net ecosystem exchange to evapotranspiration." The key results are presented in Figure 1 below.

Each measure of ecosystem-scale WUE derived from the eddy covariance flux estimates revealed an approximate 26% increase in water use efficiency over the 18 year period, though the increases were not statistically significant at the p < 0.05 level. Additional analyses on the data series demonstrated in the words of the authors that "the strength of associations among individual meteorological variables and WUE varied according to the scale of temporal aggregation." And in this regard, they report that vapor pressure deficit and air temperature exhibited the greatest control on WUE at half-hourly and daily timescales, whereas atmospheric CO2 concentration was "identified as the most important indicator of monthly WUE."

In light of the above findings, it is expected that rising atmospheric CO2 concentrations in the years and decades ahead will continue to exert a positive influence on the ecosystem WUE of this old-growth forest. And this increasing influence of atmospheric CO2 may well increasingly offset and overpower those influences that act to reduce WUE, such as drought or low soil moisture.

Figure 1. Photosynthetic WUE (left), inherent WUE (center) and ecosystem WUE (right) during the growing-season (March-September) from 1998 to 2015. The solid red lines represent linear regressions of the full data sets. Adapted from Jiang et al. (2019).

Posted 3 February 2020