Herbivory (the process whereby an animal eats a plant or a plant-like organism) constitutes a widespread and persistent defoliation pressure on plants. Plants have evolved a range of defence traits against herbivory. Some people classify them into two broad groups that basically describe the general defence approach: architectural defences and direct leaf defences. The term “architectural defence” emphasizes that the strategy requires whole-plant morphological modification to ensure an effective defence strategy.
Together with researchers from Wageningen University, Dr. Kyle W. Tomlinson of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study to concentrate on plant defences against vertebrate mammals. They asked whether woody plant species with low nutrient contents in their leaves, a type of leaf defence, represented d distinct defence strategies from species with spinescence (thorns, spines and prickles), a type of architectural defence effective against mammals, in terms of their physical, chemical and physiological traits. They wanted to get a mechanistic explanation as to why two alternative defence strategies against mammals (architectural defence, low nutrient defence) might have evolved.
The researchers compared organ biomass partitioning and leaf physiology of 20 week-old juvenile trees of 28 tree species (all Fabaceae), 16 non-spinescent and 12 spinescent, which are abundant in the semi-arid and humid savannas on southern continents, Africa, Australia and South America. They first predicted that spinescent species had small leaves and leaf mass fraction relative to non-spinescent species, and non-spinescent species have low leaf nitrogen per unit mass. They then predicted that spinescent and non-spinescent species are found at opposite ends of a trade-off between leaf mass fraction and leaf nitrogen per unit mass, enforced by vertebrate herbivory.
The study showed that saplings of savanna trees with and without spinescence have distinct plant trait suites that may indicate contrasting defence strategies against vertebrate mammalian herbivores, namely architectural defence versus low nutrient defence. Architectural defence against vertebrates (which includes spinescence) limits berbivore access to plant leaf materials, and partly depends on leaf-size reduction, thereby compromising leaf mass fraction. Low nutrient defence requires that leaf material is of insufficient nutrient value to support vertebrate metabolic requirements, which depends on low leaf nitrogen per unit mass. Thus there is an enforced trade-off between leaf mass fraction and leaf nitrogen per unit mass, leading to distinct trait suites for each defence strategy.
The study entitled “Defence against vertebrate herbivores trades off into architectural and low nutrient strategies amongst savanna Fabaceae species” has been published online in Oikos.
