PLANT SPATIAL PATTERN AND NUTRIENT DISTRIBUTION IN PINYON-JUNIPER WOODLANDS ALONG AN ELEVATIONAL GRADIENT IN NORTHERN NEW-MEXICO

Plant and soil resource spatial patterns were measured in pinyon-juniper communities in northern New Mexico over an elevational gradient that also served as a water-availability gradient to examine the role of resource competition and resource availability in determining plant spatial patterns. Total canopy coverage increased with increasing elevation. Percent coverage of juniper declined with elevation and, with the exception of one site, that of pinyon increased. Water appeared to be a strong factor in the maintenance of stand structure and plant distribution in this pinyon-juniper ecosystem. Plant water stress was greater during the dry season at the low-elevation, low-density sites than at the upper-elevation, higher-density sites. At the upper-elevation sites, plant distributions were significantly more clumped than at the drier sites; only at these low-elevation sites was there a significantly negative relationship between plant size and distance to nearest neighbor and a trend toward regular spacing. Juveniles were significantly clumped with respect to adults at all sites, with this trend most significant in seedlings, but seedlings did not show an affinity for the canopy of conspecific adults. Nitrogen availability was greater in canopy soils than in intercanopy soils, although a significant water-availability gradient was not seen across a canopy-intercanopy transect. Thus seedling establishment patterns could result from nutrient-availability differences, shade protection, or seed-dispersal and germination patterns, but probably not from microsite water differences.