Aim: Ziziphus (Rhamnaceae) is a widely distributed genus across the Australasian and African tropics with unusual diversity in habit, and many species of significance to people. Here, we quantify the environmental limits of Ziziphus species and examine inter-specific relationships among functional traits, environment, biome and range size.

Methods: We developed a curated geolocation database for Ziziphus and used it to examine the environmental limits of the genus relative to temperature, rainfall and seasonality. To assess the relationship between biome and habit, Permanova were used, while hierarchical clustering was used to determine whether habit, leaves and fruit traits were related to biome. For 40 species with adequate geolocation data, range size was calculated to assess its relationship with habit, biomes, and cultivation. Finally, niche identity tests were used to determine niche equivalency among cultivated and non-cultivated species.

Results: Liana species are restricted to closed forest and the geoxylic habit is found only in open grasslands. Further, habit is significantly associated with range size, with trees having on average larger range sizes than shrubs, lianas and geoxyles, but biome was not correlated with range size. Cultivated species have ranges ~10 times that of non-cultivated tree species and with significantly different and broader environmental niches.

Conclusion:  The unusually wide distribution of Ziziphus can be explained by its diversity of habits associated with different biomes spanning continents. This, along with the usage of many Ziziphus species by people for their fruits, expands the range and environmental occupation of the genus.

We curated species occurrence records that resulted in a taxonomically verified database of 1847 unique records for 50 species of Ziziphus. We primarily used the BIEN package v1.2.4 (downloaded 9 December 2020) which integrates global botanical observation data from sources such as plot data, herbaria, and specimen records and where data have undergone additional taxonomic and spatial verification to that supplied by GBIF (The Global Biodiversity Information Facility 2021) (Maitner et al 2017). Following Meyer, Diniz-Filho and Lohmann (2017), where species had <10 geolocations, records were added from a combination of georeferenced herbarium specimens and from additional GBIF data (Table S1). For three species (Z. apetala, Z. hoaensis, and Z. linnaei) no geographic information was available, possibly due to a combination of factors; the tropics are undercollected (Prance et al 2000), the availability of digitized herbarium specimens varies greatly, and digitized resources often suffer from a lack of maintenance and updating (Lughadha et al 2018). Records without latitude and longitude were excluded as were specimens for the same occurrences, and the remaining spatially georeferenced records were cleaned by removing occurrences outside of the known distribution of the genus. Taxonomy was checked using the Taxonomic Name Resolution Service, Plants of the World Online, GBIF, and floras.

Species of Ziziphus were assigned to biomes based on data from floras, peer-reviewed articles, grey literature such as doctoral dissertations, United Nation Development Programme (UNDP) reports, herbarium labels, and expert knowledge from Dr. Gopal Rawat (personal correspondence) (Table S4). Biomes were categorised as ‘open’, ‘closed’, and ‘desertic’. Following the typology of Earth’s ecosystems by Keith et al (2020) that groups ecosystems into biomes for the purpose of enabling comparative work, in the analyses here, closed biomes are analogous to T1 ‘tropical and subtropical forests’ that include lowland rainforests, tropical montane forests, tropical dry forests, and tropical heath forests. Desertic biomes are analogous to T5 ‘deserts and semi-deserts’, that includes semi-desert steppes, thorny deserts and semi-deserts, sclerophyll hot deserts and semi-deserts, cool deserts and semi-deserts, and hyper-arid deserts. Open biomes are analogous to T3 ‘shrublands and shrubby woodlands’ and T4 ‘savannas and grasslands’ following Bond (2019) as both ecosystems are mediated by similar processes (Keith et al 2020). These include seasonally dry tropical shrublands, seasonally dry temperate heaths and shrublands, cool temperate heathlands, rocky pavements, screes and lava flows, trophic savannas, pyric tussock savannas, hummock savannas, temperate woodlands, and temperate subhumid grasslands.

Cultivated species and primary human uses were identified from peer-reviewed articles, floras, and grey literature such as a United States Department of Agriculture Handbook (Table S4). Following Flower et al (2021), we distinguished between species with no use or unknown uses, species which are widely used but not cultivated (harvested in situ), and species which are deliberately cultivated. Uses for non-cultivated species included spiritual, medicinal, edible, and wood for building or forage. Where multiple uses were listed for a species, the most common usage was selected.