News / Science & Technology
Forest Growth Response to CO2
By examining various properties of tree rings, researchers can deduce how historical increases in the air's CO2 concentration have already affected tree productivity and water use efficiency. Duquesnay et al. (1998), for example, analyzed the relative amounts of 12C and 13C present in yearly growth rings of beech trees raised in silviculture regimes in northeastern France, which enabled them to discover that their intrinsic water use efficiencies rose by approximately 33% during the prior century, as the atmosphere's CO2 concentration rose from approximately 280 to 360 ppm. In another case, Rathgeber et al. (2000)1 used tree-ring density data to create an historical productivity baseline for forest stands of Pinus halepensis in southeastern France, from which they determined that the net productivity of such forests would likely increase by 8 to 55% with a doubling of the air's CO2 content.
One year earlier, when running a forest growth model, but one that was based on empirical observations reported in the literature, Lloyd (1999)2 determined that the rise in the atmospheric CO2 concentration since the onset of the Industrial Revolution had likely increased the net primary productivity of mature temperate deciduous forests by about 7%. In addition, he determined that a proportional increase in anthropogenic nitrogen deposition likely increased forest net primary productivity by 25%. And when he combined the two effects, the net primary productivity stimulation rose to 40%, which due to synergetic interactions was actually more than the sum of the growth enhancements resulting from the individual increases in CO2 and nitrogen acting by themselves.
Contemporaneously, Medlyn et al. (1999)3 had conducted a meta-analysis of data obtained from 15 atmospheric CO2 enrichment studies of European forest species growing in field environments, in order to determine their overall photosynthetic response to elevated (approximately doubled) atmospheric CO2 concentrations. And the resulting meta-analysis of the 21 researchers indicated that the twice-ambient CO2 concentrations utilized in the studies were found to have stimulated the trees' net photosynthetic rates by an average of 51%.
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