Extreme drought induces rapid declines in co-occurring native Bouteloua eriopoda and invasive Eragrostis lehmanniana

Abstract

Global climate change is expected to result in increased temperatures and variation in precipitation. In the Desert Southwest region of the United States, climate change may result in drier conditions. Eragrostis lehmanniana (Lehmann lovegrass) is an invasive species that has extensively colonized the southwestern United States, including Bouteloua eriopoda (black grama) grasslands. While both E. lehmanniana and B. eriopoda evolved in arid conditions, drier conditions may influence this invader–native system in unforeseen ways. Using a combination of precipitation pivot point (PPP) and drought sensitivity (DS) analysis, we compared responses of B. eriopoda and E. lehmanniana to altered precipitation regimes including long-term drought treatments in the northern Chihuahuan Desert, New Mexico. We defined PPP as the critical point in precipitation amount below which plant percent cover decreased and above which it increased. We used DS, defined as the square millimeters of cover lost when precipitation decreased by 1 mm, to elucidate the direct response of biomass to drought on either side of the PPP. We estimated mean monsoon PPPs of 62.5 and 63.7 mm for B. eriopoda and E. lehmanniana, respectively. We estimated mean annual (1 October–30 September) PPPs of 108.6 and 108.5 mm for B. eriopoda and E. lehmanniana, respectively. Bouteloua eriopoda mean DS was 4.71 mm2/mm. Eragrostis lehmanniana mean DS was 1.56 mm2/mm. Results suggest that it is unlikely that extended drought will benefit E. lehmanniana over B. eriopoda and both species may decline under extreme drought.