Mesquite and grass relationships at two spatial resolutions

Relationships between woody plants and grass are typically described at either the stand (≥100 m²) or plant (≤10 m²) spatial resolutions. Descriptions of the Prosopis velutina (mesquite)-grass relationships in the North American Desert Grassland differ between spatial scales: P. velutina often inhibits grass at the stand scale compared to facilitation or neutral relations at the plant scale. We use simultaneous measures of grass density at both spatial resolutions (made at 3 y intervals from 1997 to 2003) to evaluate the influence of the spatial resolution on detecting P. velutina-grass relationships. We confine our observations to the parsimonious conditions of (1) two distinct P. velutina age classes that are exclusive occupants of study sites and (2) total cover of P. velutina was not different between study sites in 2000 and 2003. Thus, total P. velutina cover is constant in those 2 years but P. velutina ages contributing to that cover differed between the study sites. We confirmed the importance of spatial scale because no grass species was related to P. velutina age at the stand scale, but 2 of 5 species were significantly related at the plant scale. Aristida spp. was greater under young than old P. velutina, and Muhlenbergia porteri was greater under old P. velutina. These results suggest that the influence of old P. velutina on grass abundance is restricted to the area under the canopy and does not extend into inter-canopy areas, and therefore the observations at the stand scale may not detect the P. velutina-grass relationships at the plant scale.