Tropical Rhododendron diversity and leaf traits along elevation gradients
One of the most well-known ecological patterns is the dramatic increase in species diversity with decreasing latitude. Species diversity can vary across other gradients, such as elevation, although these patterns are not as well described. The concept of 'diversity' can also encompass functional traits, which are morphological and physiological characteristics associated with ecological niches or life history trade-offs. One knowledge gap is whether species and functional diversity are parallel along these gradients. If so, this would indicate a comparable response to biotic or abiotic selective pressures. We quantified taxonomic and functional trait diversity of vascular plants along a regional elevation gradient, focusing on the genus Rhododendron (Ericaceae) compared to other highly diverse genera. We used >1.5 million herbarium records archived in GBIF from the Malay Archipelago to calculate the elevation range for 26,540 tropical plant species (including 285 Rhododendron species). In addition, we quantified functional leaf traits, specific leaf area (SLA) and lamina area, for 71 Rhododendron species. We then determined if trait values, averaged across species occurring per 100m interval, were correlated with elevation, and whether functional and species diversity varied concordantly.
Species diversity of selected tree or woody genera was highest at low elevations (0 - 500m), diversity of orchid and fern genera peaked at low to mid elevations (1000 - 1500m), Ericaceae diversity peaked at mid to high elevations (2000 - 2500m), and grass diversity was highest above the tree-line (3500 - 4000m). After accounting for land area, we found that tree diversity peaked at mid elevations (2000m), whereas Rhododendron diversity increased with elevation and peaked at 3500m. Rhododendron species had a wide range of growth forms, and occurred across a variety of habitats (low elevation wet forests, montane cloud forests, and alpine meadows). Rhododendron lamina area declined with increasing elevation, whereas SLA was not correlated with elevation. Functional diversity also differed between leaf traits. Our analyses suggest that whether or not taxonomic and functional diversity of Rhododendrons display concordance along elevation gradients depends on the functional trait examined. For lamina area they are concordant, but for SLA they are decoupled. The peak of Rhododendron species diversity also supports greater lamina area diversity. Therefore, this elevation range may represent a possible ecotone between lower elevation communities structured by biotic interactions and higher elevation communities structured by abiotic factors.