DOI: 10.5061/dryad.pnvx0k6rb
title: Data from: Evidence for divergence in phenology over morphology in response to limiting similarity in montane communities of Rhododendron

description: 1. The coexistence of closely related species is key to understanding the
 nature of biodiversity hotspots where in situ diversification has yielded
 rich communities of close relatives. Limiting similarity predicts that
 co-occurring species are differentiated in their niches; identifying the
 axes of differentiation in sympatric close relatives can thus help reveal
 the eco-evolutionary dynamics of community assembly. For flowering plants,
 these axes may be temporal (related to reproductive phenology) or
 morphological (related to functional traits). 2. We collected fine-scale
 data on abundance, morphology, and phenology over a flowering season for
 34 species of Rhododendron (Ericaceae) spanning a 2700 m elevation
 gradient in a nature reserve in the eastern Hengduan Mountains, China. We
 used null models to test for patterns of clustering versus overdispersion
 in species’ abundances, phylogenetic relatedness, and functional traits
 across sites, and applied joint distribution models to examine the
 correlates of pairwise associations. 3. We found that species tended to be
 spatially aggregated, indicating that communities are not strongly
 structured by competitive exclusion. At higher elevation sites, species
 tended to be vegetatively more similar (clustered) and closely related.
 Environmental variables, including climate and topography, were strong
 predictors of species' ranges. No evidence of niche differentiation
 was detected along spatial or morphological (functional) axes, but along
 the temporal axis, the phenology of co-occurring species showed
 significant divergence, and was less phylogenetically conserved compared
 to morphological traits. 4. Synthesis. Local communities of
 Rhododendron in its center of diversity are structured by environmental
 filtering and the effects of limiting similarity. Evidence for the latter
 is apparent in the pervasive phenological divergence of co-occurring
 species, likely driven by reproductive interference from shared
 pollinators. The evolutionary lability of flowering time appears to render
 it the quickest path to coexistence for recently diverged species that
 experience secondary contact in this biodiversity hotspot.
description: Between April 25 and June 28, 2019, we censused Rhododendron
 species in the Gongga Mountain National Nature Reserve (29°10’–30°10’N,
 101°10’–102°10’E, an area ca. 1×10<sup>4</sup>
 km<sup>2</sup>) in western Sichuan province, China. In several
 major valleys surrounding Mt. Gongga, we surveyed 128 sites spaced at
 intervals of approximately 100 m change in elevation, or distances
 &gt; 1 km in the absence of elevation change. The sites spanned
 elevations from 1790--4580 m, with habitats ranging from deciduous and
 mixed forests to alpine shrublands and meadows. We conducted four
 censuses, each involving a circuit of visits to all 128 sites, at
 intervals of 10--17 days over the fieldwork period. We estimated the
 population densities of Rhododendron species using variable area transects
 (VAT), an efficient and robust method suitable for dense populations used
 in previous studies of Rhododendron. For each species at a site, we
 measured population density once by setting up two transects from
 different random starting points within the boundaries of their
 occurrence, each with a fixed width (ω = 1.5 m). The transect length (x)
 was recorded as the distance to the fourth individual encountered (r = 4).
 Density was estimated using the formula (nr - 1)/(ω ∑xi). We translated
 densities to abundances by multiplying by a constant factor of 100
 m<sup>2</sup>. We then constructed a species abundance-by-site
 matrix and a corresponding binary presence-absence matrix. At each visit,
 we recorded the phenological phase of each species by visual estimation of
 the proportion of flowering inflorescences out of the total inflorescences
 across 10--20 individuals. These values were later normalized by the peak
 value observed for the species over the duration of the census. To assess
 functional variation related to growth, stress tolerance, and pollination,
 we measured three vegetative traits and six floral traits. Plant height
 and leaf area are indicators of light acquisition ability and correlate to
 overall plant biomass; specific leaf area (SLA) is a measure of
 photosynthetic capacity and leaf longevity, commonly used to quantify
 species response to environmental resources. We collected mature leaves
 from three individuals per species per site in sealed plastic bags to
 maintain freshness, and later the same day scanned them on a flatbed
 scanner (Epson Perfection V19) and dried them in silica gel. We measured
 leaf area from the scans using ImageJ, and measured leaf mass by an
 electronic analytical scale after oven-drying at 70°C for 24 hours. Flower
 size and shape are are pollination-related traits that influence the
 potential for reproductive interference via heterospecific pollen
 transfer. We measured corolla tube length and width, corolla lobe length
 and width, stamen length, and style length on fully-opened flowers of
 three individuals per species chosen randomly from representative sites
 (86 in total, ranging 1--10 sites per species, median = 4).
description: All data are in comma-separated values (csv) files, and can be
 easily read and processed in R.