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Real-World Growth Response of Ponderosa Pines to Post-1950 Atmospheric CO2 Enrichment
Volume 9, Number 22: 31 May 2006

In the opening sentence of the scientific paper describing their latest research findings, Soule and Knapp (2006) note that "two major environmental issues have arisen regarding the increasingly CO2-rich world of the late 20th and early 21st centuries: climatic change, and plant responses to the environment," and in their second sentence they write that "while the implications of atmospheric CO2 for potential climatic change have received the majority of attention, the potential role of atmospheric CO2 fertilization in plant growth and subsequent ecosystem dynamics may be equally important." Hence, they focused their attention on this latter topic in a study of ponderosa pine trees growing at eight different sties within the Pacific Northwest of the United States, in order to see how they may have responded to the increase in the atmosphere's CO2 concentration that occurred after 1950.

The two geographers say they chose study sites that "fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance." In addition, they selected locations with "a variety of climatic and topoedaphic conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years." Also, they say that all sites were located in areas "where ozone concentrations and nitrogen deposition are typically low."

At each of the eight sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included "the potentially oldest trees on each site," so that their results would indicate, as they put it, "the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations." Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine (Pinus ponderosa Laws. var. ponderosa) radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers report finding a post-1950 radial growth enhancement that was "more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site." As for the magnitude of the response, they determined that "the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%."

With respect to the significance of their observations, Soule and Knapp say that their results, "showing that radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged, suggest that nonclimatic driving forces are operative." In addition, they say that "these radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005)." Hence, they say "these findings suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions."

Summarizing their findings, Soule and Knapp recount how they had "hypothesized that ponderosa pine...would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites," and in the following sentence they say that their results "support these hypotheses." Hence, they conclude their paper by stating it is likely that "an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the [USA's] interior Pacific Northwest," providing yet another important, specific and real-world example of the ongoing CO2-induced "greening of the earth."

Sherwood, Keith and Craig Idso

Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in naturally occurring ponderosa pine trees: a late-20th century CO2 fertilization effect? New Phytologist doi: 10.1111/j.1469-8137.2006.01746.x.