So what did the Wisconsin scientists find? As they describe it, and contrary to their initial hypothesis, "liana abundance and diameter did not [our italics] increase in the interiors of Wisconsin (USA) forests over the last 45 years." In fact, they report that Toxicodendron radicans - a liana popularly known as poison ivy that they say "grew markedly better under experimentally elevated CO2 conditions than did competing trees (Mohan et al., 2006)" - actually decreased in abundance over this period, and did so significantly.

How did it happen that what seemed to be so logical turned out to be so wrong? Londre and Schnitzer say that "the lack of change in overall liana abundance and diameter distribution in [their] study suggests that lianas are limited in the interiors of deciduous forests of Wisconsin by factors other than increased levels of CO2." Paradoxically, it is likely that the interior-forest lianas were limited by the historical increase in atmospheric CO2 via the enhanced tree growth provided by the CO2 increase, which likely resulted in the trees becoming more competitive with the vines because of CO2-induced increases in tree leaf numbers, area and thickness, all of which factors would have led to less light being transmitted to the lianas growing beneath the forest canopy, which phenomenon likely negated the enhanced propensity for growth that likely was provided the vines by the historical increase in the atmosphere's CO2 concentration.

Support for this net-zero competing effects hypothesis is provided by Londre and Schnitzer's finding that "compared to the forest interior, lianas were >4 times more abundant within 15 m of the forest edge and >6 times more abundant within 5 m of the forest edge," which "strong gradient in liana abundance from forest edge to interior," in the words of the two researchers, "was probably due to light availability." In addition, they say their results "are similar to findings in tropical forests, where liana abundance is significantly higher along fragmented forest edges and within tree fall gaps," and, we might add, where the interior tropical trees have also not suffered what some have claimed would be the negative consequences of CO2-induced increases in liana growth, as we describe in our review of the study of Phillips et al. (2002).

In commenting on the significance of their findings, Londre and Schnitzer write that because "forest fragmentation (and thus edge creation) has increased significantly over the last half-century, particularly in the northeastern and midwestern United States (e.g., Ritters and Wickham, 2003; Radeloff et al., 2005), liana abundance has likely increased in temperate forests due to forest fragmentation." Consequently, they say that "as forest fragmentation continues, liana abundance will also likely continue to increase, and the effects of lianas on temperate forests, such as intense competition with trees (Schnitzer et al., 2005), reduced tree growth rates and biomass sequestration (Laurance et al., 2001), and the incidence of arrested gap-phase regeneration (Schnitzer et al., 2000) may become even more pronounced."

In light of these latter observations, it is clear that it is not rising CO2 concentrations that are to be feared in this regard, it is the encroachment of man upon the world of nature (Waggoner, 1995; Tilman et al., 2001, 2002; Raven, 2002); for it is this phenomenon that is destined to desecrate the globe's forests and drive innumerable species of both plants and animals to extinction, unless we can dramatically increase the water use efficiency of our crop plants, so we are not forced to encroach further upon the forests of the world to obtain the additional land and water resources (Wallace, 2000) we will otherwise need to grow the greater quantities of food that will be required to sustain our larger projected population at the midpoint of the current century. Clearly, the most effective means of ensuring that the needed increase in plant water use efficiency actually comes to pass (in contrast to the grandiose schemes of men that promise much but produce little, especially where it is really needed) is to allow the atmosphere's CO2 concentration to continue its natural upward course, which will truly give crops throughout the world the productivity boost they will need to supply us with the food we will require but a few short decades from now without usurping further land and water resources from "wild nature," enabling us to thereby preserve for future generations - and for their own sakes - what yet remains of the world's forests and the great profusion of lifeforms they contain (Idso and Idso, 2000).

Sherwood, Keith and Craig Idso

References
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