Methane and contradictions

The February 2007 edition of Scientific American contains an article on plant-methane and climate change, written by Frank Keppler and Thomas Rockmann, the researchers who discovered a year or so ago that plants actually emit large quantities of methane, a significant greenhouse gas. Keppler and Rockmann calculate that plants and trees are responsible for somewhere between 10-40% of annual global emissions of methane. They also point out that plant methane emissions explain why atmospheric methane levels vary with the same pattern as atmospheric CO2 levels. In particular, warm, inter-glacial periods coincide with high levels of atmospheric CO2 in a manner that is reasonably well understood, but an explanation was sought for why such periods also have high levels of atmospheric methane. Keppler and Rockmann state that:

Extremely high atmospheric carbon dioxide concentrations as well as rising temperatures could have resulted in a dramatic increase in vegetation biomass. Such global warming periods could have been accompanied by a massive release of methane from vegetation and by more heating. (p44-45)

Vegetation then, and forests in particular, have a positive feedback effect upon global warming. If the temperature and CO2 increases, biomass increases, methane levels increase, and temperatures increase further. Keppler and Rockmann then state, however, that:

The climatic benefits gained by establishing new forests to absorb carbon dioxide would far exceed the relatively small negative effect of adding more methane to the atmosphere...The potential for reducing global warming by planting trees is most definitely positive. (p45).

This, I don't understand. At face value, it appears to be a direct contradiction. If increasing biomass increases global warming, then because reforestation increases biomass, reforestation must increase global warming. Perhaps the difference lies in biomass gained by the growth of existing trees and plants, versus the biomass gained by the planting of new trees. Perhaps the former results in a net gain of greenhouse gas, whilst the latter results in a net decrease. New trees increase the sequestration of carbon dioxide, perhaps, whilst tree and plant growth alone do not?

The authors argue that "the large plant [positive] feedback to global climate change that most likely happened in the past, however, is probably unlikely today because so many forests have been cut down," (p45). So if the only reason why plant-methane doesn't accelerate global warming today has been deforestation, this suggests, again, that reforestation would accelerate global warming. The claim that deforestation has hidden this positive-feeback effect itself deserves close scrutiny because global forest coverage has been largely constant since 1950, according to figures from the FAO (the Food and Agriculture Organization of the UN). 20% of the world's forests have been lost, but these losses mostly occurred prior to 1950 (approximately 2% has been lost since then). Global temperatures and CO2 levels have been rising since 1975, but the biomass of the Earth has remained largely constant during that period, so surely the positive feedback mechanism should have kicked in? Apparently not, given that atmospheric methane levels have remained largely constant since the 1980s.