Global ChangeIntersection of Nature and Culture

Global change is causing forests to grow more

Wednesday, February 3rd, 2010

Decades of research have shown that increasing carbon dioxide in the atmosphere can cause trees to grow better.  However, what we don’t know as well is how much rising temperatures and CO₂ impact forest growth over longer time scales, such as the entire 20th century.

This is a harder question to answer for one big reason:  When you look back that long, you have to rely on things like tree rings to measure growth rate.  This also means you have to contend with natural regeneration cycle of individual trees.  Young trees often grow fast, and growth slows as the trees get older.  If you cut down a tree and look at a cross section of tree rings, you can often see wide rings fading to narrow rings over the lifespan of an individual.

In a forthcoming article¹ in the Proceedings of the National Academy of Sciences (open access), Sean McMahon and colleagues investigated the question of long-term forest response to global change in Maryland forests.

Using statistical techniques, they were able to factor out the messiness of these aging trends to look for effects caused mainly by a changing physical environment.

They found that 80% of the trees grew more than you would expect by stand-level growth dynamics alone.  However, they found it difficult to pin this trend on any single environmental factor, concluding that temperature, increased lengths of growing seasons, and increased CO₂ were likely synergistic drivers.

This is an interesting result because it contrasts with the results of elevated CO2 experiments, which show that forest growth typically slows a few years after trees are subjected to experimentally raised CO₂.  What those studies are finding is that nitrogen in soils could become limiting and essentially shut off extra growth caused by CO₂ fertilization.

The implications are fairly significant:  Either the Maryland site is unusually nutrient rich, and we have to discount the ability to generalize from that one study, or the elevated CO₂ experiments may not fully capture the dynamics of how forests responding to climate change.   This should spur an interesting debate.

¹McMahon, S.M. (in press) Evidence for a recent increase in forest growth.  Proceedings of the National Academy of Sciences

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