The macroecology of spines on woody plants

Spines are a major ecological innovation supporting plant defence and diversification. Spine anatomy is diverse, having arisen in multiple plant lineages from many different plant organs and parts, which may differ in relative advantages across environmental gradients. Systematic analyses of the correlates of spiny plant diversity are limited, but climate and soil properties may be important. We analysed spatial patterns of the proportional richness of spiny woody plant species (fraction of total woody species richness) and the proportional richness of species with particular spine types (fraction of richness of spiny plants) across three regions with high plant geolocational data density spanning three continents, China (Asia), South Africa (Africa), and Australia. Spiny plants accounted for 12% of woody species, but there are strong phylogenetic biases in the evolution of spiny lineages and lineages bearing different spine types. The proportion of spiny plants increased towards drier environments and higher soil clay contents, and decreased towards soils with greater total N. Species bearing different spine types appear to be distributed differently across climate and soil gradients, suggesting trade-offs across productivity gradients, specialization for climate space, and constraints on environmental adaptability. The spatial richness of spiny plants was positively correlated with estimated historical richness of large herbivorous mammals (body mass >20 kg, diet >90% plant material), and species bearing different spine types also mostly show positive relationships with mammal richness. Plants with spines appear to be advantaged over non-spiny species when exposed to high mammal browsing pressure in arid environments or over certain soil conditions, and species bearing different spine types are differentially advantaged across climate and soil gradients.