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Twentieth Century Changes in California's Sierra Nevada Forests

Paper Reviewed
Dolanc, C.R., Safford, H.D., Dobrowski, S.Z. and Thorne, J.H. 2014. Twentieth century shifts in abundance and composition of vegetation types of the Sierra Nevada, CA, US. Applied Vegetation Science 17: 442-455.

With all of the negative effects predicted to occur in response to the ongoing rise in the air's CO2 concentration, it is only natural to want to determine what may have happened to Earth's many ecosystems over the past century or so during which time climate alarmists claim Earth's temperatures and CO2 concentration rose to levels unprecedented in modern times. A new study by Dolanc et al. (2014) does just that for the Sierra Nevada forests of California, USA.

So what exactly did they do and what did they find?

First off, the team of four researchers analyzed two forestry data sets, one comprising historic measurements made between 1929 and 1936 and the other consisting of modern data obtained during the period 2001-2010. From these two data sets Dolanc et al. "classified 4,321 historical plots and 1,000 modern plots into nine broad groups of vegetation types that are widely used by land managers and researchers in the region," after which they compared tree density and composition between the historical and modern plots both across and within these nine types of forest. The results are presented in the figure below.

Figure 1. Percent change in tree density by forest type in the Sierra Nevada Range, USA, as determined from historic (1929-1936) and modern (2001-2010) measurements. Source: Dolanc et al. (2014).

As illustrated in Figure 1, Dolanc et al. report that "in the modern data set, tree density was significantly higher in eight of nine vegetation types." In addition, "total density was significantly higher in modern plots for all west slope types, especially for montane hardwood, where modern forests are 128% denser, and mixed conifer forests, which are 69% denser." They also say that density was "significantly higher in high-elevation types such as sub-alpine forest (+20%)."

With respect to why there was a significant increase in tree density over the past several decades, Dolanc et al. offer that the changes in the density and composition of lower-elevation forests are consistent with fire suppression; but they note that the density increases in high-elevation vegetation types are "more likely to be caused by changing climate." Given this judgment, it can logically be concluded that if the CO2 emitted to the atmosphere by the burning of fossil fuels over the past century or so has been having an effect on the forests of California's Sierra Nevada, that effect has been not only positive, but hugely positive. At a minimum, the observations of this study do not support climate alarmist fears of widespread forest decline in the face of rising temperatures and atmospheric CO2.

Posted 18 October 2014