A long-term perspective on woody plant encroachment in the desert southwest, New Mexico, USA

Objective To assess potential causes of woody plant encroachment into desert grassland systems using a 5500-yr-old sediment record from a cienega in New Mexico. Location Cloverdale Cienega, southwestern New Mexico. Methods Utilizing fossil pollen and charcoal preserved in wetland sediments, a long-term record of variations in vegetation composition and fire activity was generated. The record was compared to published data on drought, the El Nino/Southern Oscillation (ENSO), carbon dioxide concentrations and known dates of grazing impacts to evaluate various hypotheses regarding the timing and causes of woody plant encroachment in desert grasslands. Results The woody plant encroachment observed in the past 200yrs is unprecedented in the context of the previous 5500yrs of vegetation history. The observed increase in woody plant abundance was not related to droughts or changes in ENSO event frequency, and was contemporaneous with the rise in atmospheric CO2 and known grazing impacts. Charcoal influx increased in conjunction with historic woody plant pollen abundance and therefore the encroachment was not related to fire exclusion. Conclusions The long-term sediment record from Cloverdale Cienega provides the context for evaluation of the various hypotheses for woody plant encroachment. At Cloverdale Cienega we demonstrate that woody plant encroachment of the last 200yrs is unprecedented and outside of the historic natural range of variability of the previous five millennia. We also demonstrate that woody plant encroachment is not related to climatological changes such as drought or increases in ENSO frequency. Increases in woody plant pollen are associated with increases in carbon dioxide, and high-intensity grazing during the late 19th and early 20th centuries.