Fire, grazing and the evolution of New Zealand grasses

Less than 4% of the non-bamboo grasses worldwide abscise old leaves, whereas some 18% of New Zealand native grasses do so. Retention of dead or senescing leaves within grass canopies reduces biomass production and encourages fire but also protects against mammalian herbivory. Recently it has been argued that elevated rates of leaf abscission in New Zealand’s native grasses are an evolutionary response to the absence of indigenous herbivorous mammals. That is, grass lineages migrating to New Zealand may have increased biomass production through leaf-shedding without suffering the penalty of increased herbivory. We show here for the Danthonioideae grasses, to which the majority (c. 74%) of New Zealand leaf-abscising species belong, that leaf abscission outside of New Zealand is almost exclusively a feature of taxa of montane and alpine environments. We suggest that the reduced frequency of fire in wet, upland areas is the key factor as montane/alpine regions also experience heavy mammalian grazing. Without frequent fire to remove dead leaves, detritus and woody competitors, evolution of leaf abscission is favoured, especially in large, long-lived tussocks with sclerophyllous leaves. As fire frequency was low in New Zealand before the arrival of humans, grass leaf-abscission was an evolutionary advantage whenever large tussock species shared habitat with tall woody plants.