Variation in plant responses to neighbors at local and regional scales

Differences in the responses of plant species to neighbors may determine th eir distribution among contrasting environments, but no studies have compar ed variation in competitive or facilitative abilities both within and among environments. We determined whether the responses of plants to interspecif ic interactions varied at large scales (between environments) and small sca les (among sites within an environment) across a tree line. We separated th e effects of above- and belowground interactions on seedlings of grasses an d trees grown in prairie or forest using vegetation removals at several sit es. Species interactions generally had no significant effect on transplant survival. Competition reduced seedling growth by about 33%-89% in both prai rie and forest environments. Despite the strong suppression of growth by ne ighbors, environment and species effects contributed more to variation in t ransplant performance than did neighbor removals. Responses to neighbors va ried among transplant species but generally did not vary significantly betw een environments or among sites. With vegetation removed, grasses grew sign ificantly faster in prairie and trees grew faster in forest. Thus, in the a bsence of neighbors, species showed distinct preferences for the environmen t in which they are most abundant. In summary, the responses of grasses and woody species to neighbors did not vary significantly at either large (bet ween environments) or small (among sites) scales. These results suggest tha t species responses to interspecific interactions do not vary strongly with environment or smaller-scale site effects.