Altitudinal variation in size effects on plant reproductive effort and somatic costs of reproduction

Abstract

Effects of plant size and altitude on reproductive effort (RE) and somatic costs of reproduction were examined for Ranunculus acris L. and Trollius europaeus L. (Ranunculaceae) in subarctic Swedish Lapland. We estimated investment and cost during one reproductive season in terms of biomass (indexed B), nitrogen (N), and phosphorus (P). Above the tree line, plants of both species had smaller size, and thus, smaller resource pools than at lower altitudes. For Ranunculus, reproductive effort did not depend on plant size. In terms of biomass, RE increased, while REn decreased, with altitude. For Trollius at lower altitudes, REb and REp varied inversely with size and altitude. However, above the tree line, REb and REp showed no relationship with size. In terms of nitrogen, RE decreased with size for all populations. Somatic costs of reproduction generally did not vary with plant size and/or altitude in a similar manner as RE. Only in terms of P for Ranunculus, somatic costs varied proportionally with reproductive effort. For plants of all sizes, somatic costs were often lower as compared to RE. Different effects of size and altitude on RE may result from varied environmental and morphological constraints along the altitudinal gradient. However, patterns of variation in RE were not reflected in the variation in somatic costs. We found an overall lack of conformity between reproductive effort and somatic costs of reproduction.