Drought special part 6. Tree time

There has been a change in perspective over the past 10 years, from expectations of enhanced forest growth under enriched atmospheric CO2 to the more sobering prospect of damage or decimation of standing forest caused by an increase in the drying rates of leaves and soil in a hotter climate.

This from Timothy Brodribb et al. — article six of the Science special issue on drought. It’s about the effects of droughts on forests generally, but the most intriguing section describes current knowledge on the likely effects of increased CO2 concentrations on forest growth. It has long been my assumption that trees and plants would benefit from increased CO2, because CO2 is used in photosyntheses (organic matter is of course made from carbon that is fixed from CO2 in the air). This is known as CO2 fertilization. It’s estimated that up to half of Earth’s vegetated land has become more green in the last 35 years, due to rising levels of atmospheric carbon dioxide. (Source: NASA).

The interesting take-away from the Brodribb et al. article is that things are a little more complicated. It has been theorized that the effects of warming and possible drought frequency and intensity (bad news for leaf stomata) will eventually dominate and outweigh the effects of CO2 fertilization, leading to an eventual reversal in global greening. This is still total speculation, but it’s a reminder of how little we really know about even the basic drivers of plant growth.