University of Nebraska-Lincoln Lincoln, Nebraska, United States
Abstract: Background/Methods/Question: Examination of plant functional traits along environmental gradients often reveals a shift in trait averages, implying that ecological strategies are changing based on the environmental context. For instance, specific leaf area (SLA) for most species often declines with increasing elevation, a pattern attributed to abiotic stressors becoming of greater importance relative to biotic pressures. Here, we address the question, “Is it possible to maintain a consistent ecological strategy, expressed via functional or reproductive traits, along an elevation gradient?” We used the genus Rhododendron (Ericaceae) as the model taxonomic group, focusing on species from Malesia. Tropical Rhododendron species occur from Borneo to Papua New Guinea, and across different habitat types spanning from sea-level to mountain summits. We examined leaf functional traits of 156 Rhododendron species of known provenance grown in botanical garden greenhouses. Reproductive characteristics, and information on elevational distributions, habitat characteristics, height, and growth forms, were obtained from botanical monographs, for a total of 22 traits. We compared these traits among Rhododendron species to other plants on the leaf economic spectrum (LES), and to literature. Then, we evaluated how Rhododendron traits vary with species’ elevational distributions using linear regression and phylogenetic independent contrasts.
Results/
Conclusion: The combination of Rhododendron functional trait values, and trade-offs, provides evidence of a stress-tolerant ecological strategy, especially relative to other woody species. We propose that low leaf trait variation in part enables Rhododendron species to occupy a broad array of resource-limited environments, including ultramafic (serpentine) soil, subalpine zones, and areas of recent disturbance. Tropical Rhododendron species have especially low SLA values and tougher, thicker leaves relative to other tropical species or taxonomic groups. Despite the wide range of elevational provenances of these Rhododendron species, their leaf functional traits were remarkably consistent, which is in stark contrast to most plant communities. However, reproductive traits were more variable across species and with respect to elevation, with changes in seed-tail length and flower characteristics corresponding with elevation, which most likely reflects differences in pollinator assemblages and dispersal modes with topography. Maintaining consistent functional traits, as an indicator of ecological strategy, requires avoidance of competition exclusion, with expansion to lower elevational areas most likely facilitated by transitioning from terrestrial to epiphytic growth forms, or by utilizing novel habitat types.
