Volume 9, Number 43: 25 October 2006
In an important new study, Soule and Knapp (2006) sought "to determine if gradually increasing levels of atmospheric CO2, as opposed to 'step' increases commonly employed in controlled studies, have a positive impact on radial growth of ponderosa pine (Pinus ponderosa) in natural environments." Working in the interior U.S. Pacific Northwest in pursuit of this goal, they "developed a series of tree-ring chronologies from minimally disturbed sites across a spectrum of environmental conditions," after which "a series of difference of means tests were used to compare radial growth post-1950, when the impacts of rising atmospheric CO2 are best expressed, with that pre-1950." Breaking the 20th century into these two equal parts thus allowed them to see if the 3.5 times greater increase in the air's CO2 concentration over its last half (~58.5 ppm) compared to its first half (~16.7 ppm) led to greater rates of radial tree growth post-1950 compared to pre-1950.
This work revealed, in the words of the authors, that (1) "significant increases in radial growth rates occurred post-1950, especially during drought years, with the greatest increases generally found at the most water-limited sites," and that (2) "site harshness [was] positively related to enhanced radial growth rates." More specifically, they found that when comparing the growth responses of trees during matched dry and wet years pre- and post-1950, the relative change in growth was upward at seven of their eight sites, "ranging from 11 to 133%, with responses during matched wet years less pronounced." In addition, they "found similar results when analyzing the data by Palmer Drought Severity Index category, with the greatest absolute and relative increases in radial growth post-1950 occurring during the years when soil moisture was most limiting."
The two researchers say that their results, "showing that radial growth has increased in the post-1950s period at all sites, and significantly at 50% of the sites, while climatic conditions have generally been unchanged [our italics], suggest that nonclimatic [our italics] driving forces are operative," and that "these findings suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine [our italics]."
Much the same thing had earlier been demonstrated by the two North Carolina scientists for another tree species. As they describe it, "our earlier work in the interior Pacific Northwest examining possible atmospheric CO2 fertilization (Knapp and Soule, 1996, 1998; Knapp et al., 2001a,b; Soule et al., 2003, 2004) suggests that the effect is operative for the tree species western juniper (Juniperus occidentalis var. occidentalis)." More specifically, it suggested that "the 25% increase in atmospheric CO2 during the past century, most of which occurred since 1950, contributed to both ecotonal expansion into drier areas and enhanced radial growth of western juniper."
In evaluating the careful and persistent work of Knapp and Soule, it is clear that it has provided some of the best real-world evidence to date that the historical increase in the atmosphere's CO2 concentration has enhanced the growth rates of two species of long-lived woody plants and, by implication, the growth rates of other such plants as well. This finding has two important ramifications. First, it strengthens the CO2-induced Greening of the Earth hypothesis put forth by Idso (1986). Second, it weakens the "hockeystick" representation of earth's temperature history over the past millennium that was put forth by Mann et al. (1998, 1999), as it indicates that the large 20th-century warming they deduced from tree-ring studies likely contains a component that is CO2- (and not temperature-) driven. Consequently, the work of Soule and Knapp goes a long way to helping us better understand the true state of earth's environment, as it suggests that (1) the degree of warming experienced over the 20th century may be somewhat over-estimated, and that (2) the aerial fertilization effect of gradual real-world atmospheric CO2 enrichment may be somewhat under-estimated.
Sherwood, Keith and Craig Idso
References
Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22.
Knapp, P.A. and Soule, P.T. 1996. Vegetation change and the role of atmospheric CO2 enrichment on a relict site in central Oregon: 1960-1994. Annals of the Association of American Geographers 86: 387-411.
Knapp, P.A. and Soule, P.T. 1998. Recent Juniperus occidentalis (Western Juniper) expansion on a protected site in central Oregon. Global Change Biology 4: 347-357.
Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001a. Detecting the potential regional effects of increased atmospheric CO2 on growth rates of western juniper. Global Change Biology 7: 903-917.
Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001b. Post-drought growth responses of western juniper (Juniperus occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.
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 171: 379-390.
Soule, P.T., Knapp, P.A. and Grissino-Mayer, H.D. 2003. Human agency, environmental drivers, and western juniper establishment during the late Holocene. Ecological Applications 14: 96-112.
Soule, P.T., Knapp, P.A. and Grissino-Mayer, H.D. 2004. Comparative rates of western juniper afforestation in south-central Oregon and the role of anthropogenic disturbance. Professional Geographer 55: 43-55.