Woody Plants Invading Grasslands

Introducing their study, authors Cable et al. (2009) write that "woody plant expansion into native grasslands is a near-global phenomenon" that affects "the decomposability, turnover, and amount of soil carbon." Against this backdrop Cable et al. endeavored to understand how encroachment by mesquite (Prosopis velutina, a nitrogen-fixing shrub) "may affect soil carbon processes in semi-arid ecosystems," focusing on "a medium-dense mesquite shrubland (e.g., intermediate between a grassland and closed-canopy woodland in terms of shrub density) located along the San Pedro River, southeast of Tucson, Arizona [USA]." As such, they conducted an experiment in which "soil was collected from several depths beneath large-sized shrubs, medium-sized shrubs, grass, and bare ground -- the four primary microsite-types found in this ecosystem," after which they measured respiration rates from substrate-induced incubations, which were accompanied by measurements of soil microbial biomass, soil carbon, and soil nitrogen, in the process of which work they developed "a hierarchial Bayesian model for analyzing soil incubation data and complementary field data to gain a more mechanistic understanding of soil carbon turnover." So what did they learn?

The five researchers discovered that as the invading shrubs grow larger, "the soil beneath their canopies changes whereby all of the following tend to increase: soil carbon stocks, microbial biomass, substrate-use efficiency, and soil respiration." More specifically they report that "total organic carbon under mesquite (big and medium shrubs) was 1.8 and 1.5 times greater than total carbon under bare and grass microsites, respectively," that "microbial biomass was 1.9 and 2.3 times greater under mesquite (big and medium shrubs) compared to grass and bare microsites, respectively," and that "microbes associated with big mesquite had approximately threefold greater carbon substrate-use efficiency than the other three microsites."

Given such findings, it can likely be concluded that as the air's CO₂ content continues to rise, promoting the spread of woody plants into semi-arid grasslands around the globe, ever more carbon will likely be captured and stored, both above- and below-ground, which phenomenon tends to slow the rate-of-rise of the atmosphere's CO₂ concentration and reduce the ultimate degree of warmth that might otherwise be attained by the planet.