DOI: 10.5061/dryad.kwh70rz46
title: Vascular epiphyte global distributions

description: Aim: Vascular epiphytes are ubiquitous components of wet tropical forests
 where they contribute substantially to local and regional plant diversity.
 While some basic epiphyte distribution patterns are relatively well
 studied, little effort has been made to understand the drivers responsible
 for constraining their global distribution. This study quantifies the
 substantial contribution of epiphytes to global gradients and centres of
 vascular plant diversity and explores whether epiphytes vary from
 terrestrial plants in relation to contemporary and historical
 environmental variables. Location: Global. Time period: Present. Major
 taxa studied: Tracheophyta. Methods: Using a comprehensive epiphyte
 species list (EpiList 1.0), and distribution information for 27,850
 epiphyte species derived from numerous literature sources, we describe the
 global biogeography of epiphytes. We used generalized linear mixed effects
 models to assess the relationship between epiphytic and terrestrial plant
 diversity, and contemporary and historical environmental
 predictors.  Results: We find that epiphytes substantially
 contribute to global centres of vascular plant diversity, accounting for
 up to 39% of the vascular flora in Neotropical regions. Epiphytes decrease
 in species numbers with increasing latitude at a rate three times faster
 than terrestrial plants, a trend that is driven mainly by the distribution
 of tropical forests and precipitation. Further, large regional differences
 emerge that are explained by several large endemic angiosperm families
 (e.g., Neotropical Bromeliaceae) that are absent in other tropical
 regions. Main conclusions: Our results show that epiphytes are
 disproportionately diverse in most global centres of plant diversity and
 play an important role in driving the global latitudinal diversity
 gradient for plants. The distribution of precipitation and tropical
 forests emerge as major drivers of the latitudinal diversity gradient in
 epiphyte species richness. Finally, our findings demonstrate how epiphyte
 floras in different biogeographical realms are composed of different
 families and higher taxa revealing an important signature of historical
 biogeography.
description: <strong>Epiphyte and terrestrial plant distribution
 data</strong> As a baseline list of all known
 epiphyte species, we used the EpiList 1.0 database, which contains over
 31,000 epiphyte and hemiepiphyte species names collated from 978
 literature sources (Zotz<em> et al.</em>, 2021). Distribution
 data was obtained from a variety of literature and database sources. All
 pteridophyte species distributions were obtained from the World Ferns
 database (Hassler, 2021), while seed plant distributions were mainly
 derived from the Global Inventory of Floras and Traits database (GIFT,
 Weigelt<em> et al.</em>, 2020), and the World Checklist of
 Selected Plant Families (WCSP, 2018). External literature searches were
 performed for individual species that could not be matched to any
 database, the references of which can be found in Appendix 1 of our
 corresponding manuscript GEB-2021-0338.
 <strong>Environmental predictor
 variables</strong> We related epiphyte richness to 10 contemporary and past environmental variables out of 19 variables that were initially considered. Variables included three contemporary climatic variables derived from CHELSA V1.2 (Karger<em> et al.</em>, 2017) – mean annual precipitation (mm, hereafter precipitation), precipitation seasonality (coefficient of variation in precipitation), and mean daily minimum temperature (°C, hereafter temperature), all of which have been previously hypothesised or shown at regional to global scales to be important predictors of both epiphytic and terrestrial plant species richness (Zotz, 2005; Gentry & Dodson, 1987; Kreft & Jetz, 2007; Taylor<em> et al.</em>, 2021). As a measure of habitat availability for epiphytes, we included the contemporary extent of tropical forest biomes (km<sup>2</sup>). Contemporary tropical forest biomes were extracted from a global map of terrestrial biomes (Olson<em> et al.</em>, 2001), and overlaid with our botanical country polygons in order to quantify the total area of tropical forest for each botanical country. In addition, we selected area (km<sup>2</sup>, Weigelt<em> et al.</em>, 2020) and elevational range (m, from the Global Multi-resolution Terrain Elevation data set by Danielson & Gesch, 2011 at a resolution of 30 arc-seconds), which are important geographical predictors of species diversity. We further considered three historical factors, reflecting past climate – Last Glacial Maximum (LGM) ice cover (km<sup>2</sup>, Ehlers<em> et al.</em>, 2011), and past distribution of tropical forests – LGM tropical forest area (km<sup>2</sup>, Ray & Adams, 2001) and tropical forest area during the Mid-Miocene climate optimum (km<sup>2</sup>, Henrot<em> et al.</em>, 2010). The extent of both historical tropical forest biomes was quantified in the same manner as for the distribution of contemporary tropical forested biomes. However, because biome definitions differed between datasets, we first standardised all biomes to match across datasets, delineating “tropical rainforest”, “sub-tropical forest”, and “tropical seasonal forest” to “tropical forest”. Finally, we explore continental differences in epiphyte occurrences, which allow inferences about idiosyncratic historical biogeographic patterns not captured by the environmental predictors (variable biogeographic realm). Similar to the classification of biomes, each region was assigned its respective realm following Olson<em> et al.</em>, (2001), in order to explore the richness and relative contribution of different epiphyte families among the different continents. All references and extended methods can be found in our accompanying manuscript GEB-2021-0338.