SCALES OF ABOVEGROUND AND BELOW-GROUND COMPETITION IN A SEMIARID WOODLAND DETECTED FROM SPATIAL PATTERN

Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy p atches. We hypothesized that both aboveground competition (within cano py patches), and below-ground competition (between canopy patches), wo uld be important structuring processes in these communities. We invest igated the spatial pattern of trees in a Pinus edulis-Juniperus monosp erma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of bot h species. Crown centers of both species were always less aggregated t han stem centers at scales less than canopy patch size, indicating mor phological plasticity of competing crowns. In the smallest size classe s of both species, aggregation was most intense, and occurred over a l arger range of scales; aggregation decreased with increasing size as i s consistent with density-dependent mortality from intraspecific compe tition. Within canopy patches, younger trees were associated with olde r trees of the other species. At scales larger than canopy patches, yo unger trees showed repulsion from older conspecifics, indicating below -ground competition. Hence, intraspecific competition was stronger tha n interspecific competition, probably because the species differ in ro oting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- an d below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest c ontinuum (a gradient of increasing woody canopy cover) and hence we ex pected, and were able to detect, the effects of both above-and below-g round competition.