The rapid conversion of grasslands into shrublands has been frequently observed worldwide, and such an ecosystem change is particularly well documented in the desert grasslands of the southwestern United States. However, the exact trajectory of vegetation change in these ecotones is rarely studied, preventing the accurate estimations of the historical carbon pool dynamics and soil nutrients loss/gain. Here we investigated the vegetation change at a typical grass-shrub transition zone in the Northern Chihuahuan Desert, NM, where extensive woody shrub encroachment has been reported during the past 150 years. Soil samples were collected at various depths from three 60-cm profiles, and their corresponding ages were estimated using radiocarbon (δ14C) dating. Since the C4 grasses and C3 shrubs in the study area utilize different photosynthetic pathways, SOC (Soil Organic Carbon) and δ13C of soil and plant samples were used to calculate the relative contributions of grasses and shrubs. Soil charcoal was collected and counted in each sample to indicate the fire frequency. The dynamics of soil carbon and nitrogen cycling were also depicted with soil TC (Total Carbon), SOC (Soil Organic Carbon), soil TN (Total Nitrogen), and soil δ15N results.
Results/Conclusions
The results revealed sudden increases in shrub proliferation rate in approximately 1400 years ago and 400 years ago. The first remarkable vegetation cover transition in ~1400 years ago might be related to human setting fires to modify the landscape when native American tribes flourished in New Mexico. From ~400 years ago, fire frequency gradually reduced, which is consistent with the time when European settlers came, who conducted fire suppression and grazing activities, stimulating the rapid increase of shrub biomass in the study area.