What was done
The authors examined patterns of production, standing biomass, carbon (C) and nitrogen (N) storage, community composition, and soil moisture along a 25-year chronosequence of sites within an annual, exotic-dominated grassland at Stanford University's Jasper Ridge Biological Preserve in the interior foothills of the central coast range south of San Francisco, California (USA), various parts of which had been invaded at a number of different times over the past quarter-century by Baccharis pilularis shrubs.

What was learned
Zavaleta and Kettley report that increasing above- and below-ground biomass along the chronosequence "drove increases in ecosystem N sequestration of ~700% and in C storage of over 125%," including a 32% increase in total soil C over the 25-year period. What is more, they found that "increases in carbon storage also did not appear to be saturating at 25 years after shrub establishment in any pool, suggesting the potential for additional carbon gains beyond 25 years." In addition, they found that Baccharis shrubs began to decline in prominence after about 20 years, as native oaks "with life spans of centuries" and the potential to drive even larger ecosystem changes began to grow in the shrub-dominated areas.

Of further interest, the two researchers say they "initially hypothesized that Baccharis-invaded sites would experience increasing N limitation as N was immobilized in biomass and litter." However, as they continue, they found that "total soil N increased rapidly with shrub age" and that "the magnitude of increase in total soil nitrogen was much larger than the increase in nitrogen immobilization in biomass and litter over time."

What it means
Zavaleta and Kettley conclude by saying that their findings "illustrate the potential for important vegetation-mediated ecosystem responses and feedbacks to atmospheric CO2 and climate change," and indeed they do. To us, in particular, they highlight the great potential for CO2-induced range expansions of trees to totally transform the face of the planet as the greening of the earth continues. In addition, they pretty much lay to rest the claim of Hungate et al. (2003) that the availability of nitrogen, in forms usable by plants, will probably be too low for large future increases in carbon storage driven by CO2-induced increases in plant growth and development. Clearly, the biosphere has ways of transforming itself in response to the ongoing rise in the air's CO2 content that are not yet fully appreciated by any of us.

Reference
Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.