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Demise of Earth's Tropical Forest Carbon Sink Greatly Exaggerated
Climbing woody plants or vines - the lianas of tropical forests - are known to experience large growth enhancements when exposed to elevated concentrations of atmospheric CO2 (Condon et al., 1992; Granados and Korner, 2002).  Hence, since the air's CO2 content has risen substantially over the past several decades, one might expect to see evidence of this phenomenon in the world of nature; and in a recent study dedicated to this objective, Phillips et al. (2002) report apparent success.

Based on data from several unique, long-term, multi-regional studies of liana and tree populations, Phillips et al. tested the specific prediction that lianas experienced enhanced growth over the last two decades of the 20th century.  Analyzing reams of observations acquired from 47 interior-forest sites in four Amazonian regions (North Peru, South Peru, Bolivia and Ecuador), they found that non-fragmented Amazon forests have indeed seen significant increases in the density, basal area and mean size of climbing woody plants, and that "over the last two decades of the twentieth century the dominance of large lianas relative to trees has increased by 1.7-4.6% a year."

What is driving this change?  In asking themselves this question, Phillips et al. describe "the degree of internal consistency within and between data sets across differing sample unit sizes, target variables, minimum plant sizes, climatic regimes, edaphic conditions, regional locations and spatial scales," noting that (1) these ubiquitous observations suggest a widespread phenomenon is at work and that (2) the historical rise in the air's CO2 content is thus a logical candidate for the ultimate cause of the phenomenon.  We agree; but with respect to what they believe this phenomenon portends for the future, we disagree.

Noting that the presence of lianas can substantially suppress tree growth, as indicated by Laurance et al. (2001) and Schnitzer and Bongers (2002), Phillips et al. say the rapid and possibly-CO2-induced increase in liana growth "implies that the tropical terrestrial carbon sink may shut down sooner than current models suggest," as CO2-enhanced liana growth begins to negatively impact contemporary tree growth.  Other observations, however - many of them from Phillips himself - suggest that tropical trees may well be rising to the liana challenge and will not succumb to the parasitic vine's increasing size and presence.

For starters, Phillips et al. note that "ecological orthodoxy suggests that old-growth forests should be close to dynamic equilibrium;" and for many years, such appeared to be the case, which would suggest that any significant threat to their well-being would manifest itself in reduced growth rates.  Just the opposite trend, however, has recently been observed in nature.

In one of the first studies to illuminate this new reality, Phillips and Gentry (1994) analyzed the turnover rates - which are close correlates of net productivity (Weaver and Murphy, 1990) - of forty tropical forests from around the world.  They found that the growth rates of these already highly productive forests have been rising even higher since at least 1960, with an apparent pantropical acceleration since 1980, the period of time over which Phillips et al. say liana growth has also accelerated, which suggests that the latter phenomenon has not been an impediment to the former.

Phillips et al. additionally note that several subsequent studies have verified that neotropical forests are indeed accumulating both carbon (Grace et al., 1995; Malhi et al., 1998) and biomass (Phillips et al., 1998, 2002), "possibly in response to the increasing atmospheric concentrations of carbon dioxide (Prentice et al., 2001; Malhi and Grace, 2000)."  Consequently, it would appear that tropical trees and their parasitic lianas are both being benefited by the ongoing rise in the air's CO2 content, with the ultimate consequence that rather than seeing the tropical terrestrial carbon sink "shut down" in the near future, we can expect it to continue to gradually increase in magnitude.

Dr. Sherwood B. Idso Dr. Keith E. Idso

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