Differences in plant cover and species composition of semiarid grassland communities of central Mexico and its effects on net ecosystem exchange

Changes in land use across the semiarid grass- lands of northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of dif- ferent plant communities have resulted from these changes. Their implications, however, on the carbon (C) cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO2 exchange (NEE) and their biotic and abiotic controls. NEE was measured in five representative plant community types within a semiarid grassland by temporarily enclosing the entire aboveground ecosystem using a chamber method (i.e., geodesic dome). Sites included an oat crop (crop), a moderately grazed grass- land (moderate grazing), a 28 yr-old grazing exclosure (ex- closure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub en- croachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were simi- lar (P > 0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly to- tal nighttime NEE (carbon loss) was more than double (P < 0.05) for sites with high plant cover compared to sites with low cover, resulting to slight C sinks for the low plant cover sites, and neutral or sources for the high plant cover sites as accounted by daytime and nighttime NEE annual bal- ance. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, day- time NEE in low plant cover sites reached light compensation points at lower photosynthetic photon flux density than those from high plant cover sites. Differences in species composi- tion did not influence NEE rates even though there were tran- sient or permanent changes in C3 vs. C4 functional groups. Our results allowed the detection of the large variability and contribution of different plant communities to regional C bal- ance in patchy landscapes. Identification of the role of land- scape patches in the regional C balance as either sinks or sources may provide tools allowing land use management strategies that could favor C uptake in patchy landscapes.