Faculty of 1000

Post-publication peer review

Branching out

Posted by Callum Anderson on 22 February, 2010

We all know that given the right conditions, forests are prone to grow, but until now, it has been very difficult to define what can be considered normal re-growth (i.e. as a forest ages), and what might be considered exceptional growth.

In a paper appearing in PNAS, a team led by Sean McMahon and Geoffrey Parker, both of the Smithsonian Institute have been able to discover evidence for a recent increase in forest growth. Sean and his team were able to access a dataset of biomass from 55 temperate forest plots collected over 22 years.

In putting together weather and CO2 data collected over 100 years and recorded biomass levels over the last 22 years, Sean and his team were able to conclude that 1987-2009 represents a period of exceptional growth in the forests sampled. The interesting thing about this study is that is was perhaps the first to use large datasets in comparison with predicted growth, calculated by using the Monod function.

Once they realised that the biomass was growing much more quickly than expected, McMahon, Parker and the rest of the team attempted to hypothesize why this might be the case.

According to the team, the most likely factors affecting biomass are

1. A small increase in mean temperature over the last 100 years
2. Longer growing seasons over the same period
3. Increased atmospheric CO2 from 1970-2009

Yet again we see evidence suggesting that climate change (note the small ‘c’s) can have a wide effect on the ecosystem. Although this study is only limited to the forests around Maryland, McMahon and Parker believe that the phenomenon is representative of the Eastern deciduous forest biome as a whole.

Faculty of 1000 member Richard Houghton has also noted in his evaluation of the article that the findings here are contrary to results of previous studies. He says

The results are in sharp contrast to the study by Caspersen et al. {1} that found no evidence for increased rates of growth from forest inventory data across five eastern states. On the other hand, Caspersen et al. analyzed growth over the decade ending in the early to mid 1990s, while McMahon et al. analyzed data obtained between 1987 and 2009. Perhaps the different findings help to define ‘recent’ as post-1990.

So it looks like as well as growing at a higher rate than expected, the forests in question also confined growth almost entirely to the period between the mid 1990s and 2009. Should we therefore be looking at the 1990s as an ecological turning point in terms of biomass growth?


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