Carbon isotope values for grasses in Madagascar's Central Highlands establish baselines for historical and paleoecological research

Societal Impact StatementGrasses are significant drivers of fires and are the primary food source for cattle in Madagascar's Central Highlands. However, their extent and importance to animals and people in the past remain poorly understood. Clarifying the history of Malagasy grasslands is necessary for building climate resilient food systems and supporting carbon stores that also conserve biodiversity. We generated chemical data for grasses that grow in open habitats in central Madagascar, which will help improve our understanding of the ecological and economic importance of modern grassy ecosystems, reconstruct the regional history of grasses, and anticipate how vegetation may respond to changing climate and rising atmospheric carbon dioxide levels. Stable carbon isotope (delta C-13) data for Malagasy grasses are needed to establish expected values for C-3 and C-4 grasses from particular regions in Madagascar, and possible differences among different grass lineages, or species with different distributions or adaptations. These data, in turn, may help inform how widespread grasses were in the past, and the importance of grasses to endemic and domesticated animals as well as people over time.We analysed both delta C-13 and weight %C:N from 63 Poaceae species that grow in open grassy biomes in Madagascar's Central Highlands and explored how these values relate to multiple variables, including encounter frequency, distribution, lineage, adaptations to grazing and fire and the typical floral assemblage in which each species occurs.Of the species sampled, 56 are C-4 and seven are C-3. There are no differences in delta C-13 or weight %C:N among either C-3 or C-4 species with different distributions or adaptations, from different assemblages, or that are frequently or infrequently encountered. However, there are differences in both delta C-13 and weight %C:N among C-4 lineages, and the single C-3 arundinoid (Styppeiochloa hitchcockii) has larger weight %C:N than C-3 Paniceae.Our results provide a foundation for evaluating reliance on C-4 resources by people, as well as domesticated and endemic animals both today and in the past. We encourage gathering additional comparative data for co-occurring individual plants from the same open grassy biome localities, as well as other species, habitats and regions in Madagascar.