Global patterns in plant defence: Are tropical plants nastier?

Background/Question/Methods

Previous information on herbivory and plant defence suggests that tropical species might be more heavily defended than temperate species. However, this idea has not been comprehensively tested at a global scale. We will present a global quantification of latitudinal gradients in plant chemical and physical defence. Our dataset includes 258 species, from 86 families. These species were sampled from 75 different ecosystems from all around the world, including deserts in Australia, Arizona and Israel; rainforests in Peru, Panama and the Congo; savannas in Zambia, South Africa and Australia, and tundra in Greenland and Alaska. We quantified a wide range of physical and chemical traits, including specific leaf area, presence of spines and hairs, delayed greening, force to shear the leaf, leaf water content, cyanogenic capacity, nitrogen and carbon concentration, PEG binding capacity (a functional measure of tannins), dry matter digestibility, lipid concentration and ash concentration. 

Results/Conclusions Many traits were correlated with latitude. For example, carbon concentration, dry matter digestibility, lipid concentration and PEG binding capacity are higher in species from higher latitudes, while nitrogen concentration, % of species with delayed greening and % of species with the ability to fix nitrogen are all higher towards the equator. However, many defensive traits (including presence of spines, hairs on leaves and cyanogenic capacity) were not related to latitude, and even the significant relationships were generally weak (most had R²s less than 0.1). Overall, it seems that latitudinal gradients in traits associated with plant defence are weaker than are latitudinal gradients in traits associated with leaf economics.