Evolutionary and ecological controls over leaf-level traits in New Zealand’s tussock grasses

Removal of a strong selective pressure allows for modification of traits previously requisite for survival, which may enable species to take advantage of new ecological opportunities. Globally many traits shared by grasses are a byproduct of strong selection due to mammalian herbivores and fire, including non-dehiscence of senesced leaves. We used two genera of grasses (Danthoniiae), Chionochloa and Rytidosperma, that have radiated in New Zealand in the absence of mammalian herbivory and low fire frequency and have unusually high proportions (>50%) of species with dehiscent leaves to examine the ecological consequences of novel growth strategies. We integrate phylogenetic, niche modelling, and common garden studies to understand the evolutionary, biogeographic, and physiological consequences of leaf dehiscence. The Chionochloa and Rytidosperma phylogenies show a complex pattern of the development of dehiscence, with multiple lineages developing leaf dehiscence and several species or subspecies showing reversion to non-dehiscent states. Ecologically, the non-dehiscent species have broader physiological and geographical niches, likely representing continued adaptations to ancestral New Zealand habitats, while dehiscent species have narrower and distinctive niches in colder and wetter habitats. Both modelling and laboratory studies show that dehiscent species are more low temperature adapted than non-dehiscent species. Additionally, dehiscent species have lower quality leaves for herbivores, based on stoichiometry and silica content, suggesting that invertebrate and avian herbivory may have also been an important evolutionary factor in leaf-level traits in these genera. Both dehiscent and non-dehiscent species have similar sensitivity to shading. Our results indicate that dehiscence evolved as species changed their defence strategies and allowed species to respond to the new ecological opportunity presented with the appearance of cold, wet alpine environments in the Pliocene-Pleistocene.