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Amazon Forest Dynamics
Reference
Laurance, S.G.W., Laurance, W.F., Nascimento, H.E.M., Andrade, A., Fearnside, P.M., Rebello, E.R.G. and Condit, R. 2009. Long-term variation in Amazon forest dynamics. Journal of Vegetation Science 20: 323-333.

Background
The authors write that "old-growth tropical forests were once widely thought to be in long-term equilibrium, but some now believe that they are being altered by global-scale drivers," noting that "Amazon forest productivity and biomass have apparently increased at many sites (Baker et al., 2004)." More specifically, they state that "rising CO2 levels might lead to higher primary productivity and plant growth, and also to increasing tree mortality and turnover as a consequence of elevated competition among individuals," due to the CO2-induced increase in growth. And they say that the "higher productivity, in turn, could potentially increase forest carbon storage (Phillips et al., 1998)."

What was done
Working within 20 one-hectare plots scattered over approximately 300 km2 of intact Amazon rainforests, Laurance et al. further explored these ideas by evaluating forest dynamics over the period 1981-2003, based on data obtained from 21,667 individual trees.

What was learned
The seven scientists say their "large-scale, long-term study appears to illustrate two contrasting patterns: (1) long-term trends in which tree mortality, recruitment, turnover, and basal area are progressively increasing over time in most (80-100%) of our study plots; and (2) shorter-term fluctuations in which strong pulses of tree mortality and poor growth have more transitory impacts on forest dynamics."

What it means
With respect to the first of these findings, Laurance et al. conclude that "the increasing forest dynamics, growth and basal area observed are broadly consistent with the CO2 fertilization hypothesis," while with respect to the second finding they say that "tree mortality peaked, and tree recruitment and growth declined during atypically wet [our italics] periods," and that "tree growth was fastest during dry periods, when reduced cloudiness might have increased available solar radiation." Consequently, it would appear that the historical increase in the atmosphere's CO2 concentration, driven by the burning of fossil fuels, has been good for the Amazon's trees, and (very likely) for the rest of the region's plants and animals, even in the face of the local warming of 0.26°C per decade that they report for the region since the mid-1970s.

References
Baker, T.R., Phillips, O.L., Malhi, Y., Almeida, S., Arroyo, L., Di Fiore, A., Erwin, T., Higuchi, N., Killeen, T.J., Laurance, S.G., Laurance, W.F., Lewis, S.L., Monteagudo, A., Neill, D.A., Núñez Vargas, P., Pitman, N.C.A., Silva, J.N.M. and Vásquez Martínez, R. 2004. Increasing biomass in Amazonian forest plots. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 353-365.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical forests: Evidence from long-term plots. Science 282: 439-442.

Reviewed 22 July 2009