The Hope of Trees Part II: Growth Boom

This tulip poplar, Liriodendron tulipifera, growing in central New Jersey, is big for its age. According to a guide for estimating ages of trees based on diameter, it is about 40 years old. But it’s only 23. I know because I planted it as a one-year-old seedling in 1988. Other more extensive evidence indicates that trees are growing faster than they used to. A recent study (McMahon, et al., 2010, Evidence for a recent increase in forest growth, PNAS, http://www.pnas.org/content/early/2010/02/02/0912376107.abstract) measured growth of trees in temperate forest plots in the eastern U.S. and found that the trees are adding biomass at a rate much higher than formerly. The article references a number of other studies with similar findings. A higher growth rate for trees is consistent with data, pictured in the chart below from James Hansen’s recent book, Storms of My Grandchildren (p. 119), showing that, despite large increases in anthropogenic carbon emissions since 1950, the fraction of emitted carbon that ends up in the atmosphere has remained at about 50%. The rest of the emitted carbon, now totaling about five gigatons each year, is absorbed in approximately equal portions by oceans and terrestrial plants (Watson, et al., 2009, Tracking the variable North Atlantic sink for atmospheric CO2, Science, 326, 1391-1391).

This means that despite widespread deforestation, the world’s forests and other plants are taking up more than twice as much carbon today than they did in 1950. How is this happening? McMahon et al. suggest that the higher growth rate is driven by fertilization from increased CO2 in the atmosphere, warmer temperatures, a longer growing season, or a combination of some or all of these. More research is needed, but it seems likely much of the increase is due to CO2. It is food for plants. For millions of years, plants have been accustomed to levels of CO2 in the range of 225 to 280 ppm. Its atmospheric concentration today is nearly 400 ppm. As long as their growth is not limited by something else, plants could be expected to grow much faster in today’s CO2-rich world.

Among the concerns about climate change, the booming growth of trees is a ray of hope. It's becoming clear that lowering emissions and halting the growth of CO2 in the atmosphere won't be enough to avert dangerous climate change; the CO2 concentration must be reduced, probably to the vicinity of 350 ppm. Trees and other plants can help do this by eating up atmospheric CO2, and their capacity to do this seems to be accelerating.