January 4, 2007

Global Forests Love Global Warming

Filed under: Adaptation, Climate History, Plants

Over the past 20 years, approximately 5,000 articles have been published in major scientific journals showing how plants benefit from higher levels of atmospheric carbon dioxide (CO2) with or without elevated temperatures. As CO2 concentration increases, plants substantially increase the rate of photosynthesis, rate of growth above and below ground, the water use efficiency, the production of fruit and seeds, and resistance to a variety of stresses. Critics of this positive response to elevated CO2 claim that many of these experiments are conducted in highly controlled laboratory conditions that may have little in common with what is happening in the real world. Outdoor experiments are also conducted, but in most cases, in something far less than “real-world” conditions. In the special case of forests, researchers must create clever experiments to overcome the obvious problems of waiting around a few decades or centuries to see the outcome of an experiment.

We keep our eyes open for reports in the scientific literature on what is happening to the great forests of our planet. An article in a recent issue of the South African Journal of Science brings us wonderful news from the famous and very popular Knysna forest of southern Africa. The Knysna forest comprises 80,000 hectares (200,000 acres) and is the largest indigenous forest in South Africa. The fauna and flora are abundant and include many exotic species, including yellow-wood, stinkwood, ironwood, blackwood, Cape chestnut, and white alders; some of the trees have been dated to be 400 to 800 years old. The forest is dense in places and almost impossible to penetrate. The forest houses the Knysna Elephant and is a favorite tourist attraction in southern Africa.

A pair of botanists from the University of Cape Town and South African National Parks began their article on the Knysna forest noting “The present and predicted future impacts of global environmental change on intact forests are both alarming and contentious” and that “some local models have predicted the demise of South Africa’s only significant extent of indigenous forest, the Knysna forest, by 2050.” Midgley and Seydack then state “There is thus a need for a local perspective on this debate, which we aim to provide here by an analysis of a decade of growth of the Knysna forest.”

Well, everything seems just fine in the forest. They found that basal area and above ground biomass had increased by 2% over their 10-year study. They also found that rainfall was 5% below average during the study period which led them to conclude “Changes in these rates may have been the effect of the increase in global atmospheric carbon dioxide, rather than to enhanced local precipitation because precipitation was average.” Midgley and Seydack end by stating “At this stage, therefore, there appears to be no sign of the effect of environmental change on the above-ground biomass of the Knysna forest.”

Critics may say that in the vast literature on forest demise, we found the one article from the one forest with a positive response to environmental change. OK then, how about this one—a recent issue of Global Change Biology contains an article on natural forests all over the world. In the abstract, the pair from the University of Montana writes “globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting. Of the 49 papers reporting forest production levels we reviewed, 37 showed a positive growth trend, five a negative trend, three reported both a positive and a negative trend for different time periods, one reported a positive and no trend for different geographic areas, and two reported no trend.” Boisvenue and Running have to score right from the professional scientific literature: 37 positive, 5 negative. However, based on popular presentations of global warming, you would think positive articles on forest response are rare, when in reality, they dominate the literature on the subject.

Regarding global trends in photosynthetic rates based on the normalized difference vegetation index (NDVI), they conclude “an increasing trend in photosynthetic activity during 1982–1999 (from 0.0015 to 0.0045 NDVI units per year), with trends generally higher in the 1990s than in the 1980s at global latitude bands from 35° to 75° north.” Can the news on forests get any better? Growth of forests is increasing, and the rate of increase is increasing!

Regarding forest net primary production (NPP) in the USA, they state “Regional studies in North America and in the USA reported increases in NPP of 2–8% between 1982 and 1998.” Finally they conclude “Data support forest productivity increases across temperate North America, Northern Europe, most of Central Europe, some parts of Southern Europe, and Japan.”

We can see the forest and we can see the trees, and we can see from the science literature that the forest and the trees are doing better than ever before thanks directly and indirectly to the buildup of atmospheric CO2 concentrations.

References:

Boisvenue, C. and S.W. Running. 2006. Impacts of climate change on natural forest productivity – evidence since the middle of the 20th century, Global Change Biology, 12, 862–882,

Midgleya, J.J. and A. Seydackb. 2006. No adverse signs of the effect of environmental change on tree biomass in the Knysna forest during the 1990s, South African Journal of Science, 102, 96-97.




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