Diversity in habit expands the environmental niche of Ziziphus (Rhamnaceae)

Abstract

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. 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, permutational analysis of variance was 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. Liana species are restricted to closed forests 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. 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.