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

[摘要] Changes in land use across the semiarid grasslands of northern Mexico havedriven a decline of plant cover and alteration of plant species composition.A number of different plant communities have resulted from these changes.Their implications, however, on the carbon (C) cycle and regional carbonbalance are still poorly understood. Here, we examined the effects of plantcover loss and changes in species composition on net ecosystem CO₂exchange (NEE) and their biotic and abiotic controls. NEE was measured in fiverepresentative plant community types within a semiarid grassland bytemporarily enclosing the entire aboveground ecosystem using a chambermethod (i.e., geodesic dome). Sites included an oat crop (crop), a moderatelygrazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed sitewith low perennial grass cover (overgrazed), and an overgrazed site presenting shrubencroachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE forsites with a high plant cover (exclosure and moderate grazing) were similar(P > 0.05) ascompared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly totalnighttime NEE (carbon loss) was more than double (P < 0.05) for siteswith high plant cover compared to sites with low cover, resulting to slightC sinks for the low plant cover sites, and neutral or sources for the highplant cover sites as accounted by daytime and nighttime NEE annual balance.Differences in plant cover and its associated biomass defined thesensitivity to environmental controls. Thus, daytime NEE in low plant coversites reached light compensation points at lower photosynthetic photon fluxdensity than those from high plant cover sites. Differences in speciescomposition did not influence NEE rates even though there were transient orpermanent changes in C3 vs. C4 functional groups. Our results allowed thedetection of the large variability and contribution of different plantcommunities to regional C balance in patchy landscapes. Identification of therole of landscape patches in the regional C balance as either sinks or sourcesmay provide tools allowing land use management strategies that could favor Cuptake in patchy landscapes.

[学科分类] 地球化学与岩石