Effects of Anthropogenic CO₂ Emissions on the Growth and Water Use Efficiency of Western U.S. Ponderosa Pine Trees

Authors Soulé and Knapp (2011), as they describe it, "examined changes in and relationships between radial growth and intrinsic water-use efficiency (iWUE) of ponderosa pine (Pinus ponderosa) trees, climate, and atmospheric CO₂ in the western United States since the mid-nineteenth century," developing "tree-ring chronologies for eight sites in three climate regions and using carbon isotope data to calculate pentadal values of iWUE," after which they "examined relationships among radial growth, climate, iWUE, and CO₂ via correlation and regression analyses." So what did they learn?

Soulé and Knapp say their results demonstrate that "trends toward higher rates of iWUE for ponderosa pine are panregional, occurring at eight sites within three distinct climatic regimes and for two subspecies," which results, in their words, "are similar to those reported by Feng (1999) for several coniferous tree species found throughout western North America." They also note that "increasing iWUE has been reported for conifers at other northern hemisphere locations (e.g., Bert et al., 1997; Saurer et al., 2004)," concluding that "future increases in iWUE are likely for ponderosa pine within our study regions as CO₂ levels increase." In addition, they state that they found "significant improvements in radial growth rates during drought years after 1950."

The two researchers say their findings suggest that "increased iWUE associated with rising CO₂ can positively impact tree growth rates in the western United States and are thus an evolving component of forest ecosystem processes." And these CO₂-induced iWUE increases are a mighty positive component, it should be noted.

Soulé and Knapp also state that "if potential climate changes lead to increasing aridity in the western United States, additional increases in iWUE associated with future increases in CO₂ might ameliorate growth declines associated with drought conditions." Yet in spite of all this good news, the U.S. Environmental Protection Agency and the Intergovernmental Panel on Climate Change strive to do all within their power to reduce anthropogenic CO₂ emissions to essentially nothing.

Additional References
Bert, D., Leavitt, S. and Dupouey, J.-L. 1997. Variations of wood δ¹³C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1596.

Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Saurer, M., Siegwolf, R. and Schweingruber, F. 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years. Global Change Biology 10: 2109-2120.