Scales of aboveground and below‐ground competition in a semi‐arid woodland detected from spatial pattern

Abstract

Semi‐arid woodlands are two‐phase mosaics of canopy and inter‐canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below‐ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in aPinus edulis‐Juniperus monospermawoodland in New Mexico using Ripley'sK‐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 both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density‐dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below‐ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above‐ and below‐ground competition, and t…