Grassland diversity across an aridity gradient in northern Sumatra is strongly associated with shifts in disturbance regimes

Grassy vegetation is widely spread across Southeast Asia, yet very little data exists describing the diversity in these plant communities. Groundlayer diversity data is especially rare, with previous studies focussing on woody plants. Research in grassy ecosystems worldwide suggest that there are substantial differences in grassland community composition across climate aridity gradients, which are aligned with a switch from strong disturbance by fire in wetter sites to disturbance by mammalian herbivory in drier sites. The aim of this investigation was to record the groundlayer diversity of grasslands found in northern Sumatra Island, Indonesia, and to determine whether that diversity was structured fire and mammalian herbivory. We sampled ten 50 m x 50 m vegetation plots in grassy vegetation across an aridity gradient, with soil data and grazing pressure data collected on site, and climate and fire frequency data compiled from geographic resources. We classified all species according to two morphological characteristics that related to plant evolution under fire and herbivory, namely stem form and leaf placement. We recorded 89 groundlayer species across all plots, with an average of 29 species per plot. Our analysis confirmed that the strongest axis of community differentiation was with respect to the fire-grazing trade-off axis, which selected for specific adaptive stem and leaf placement morphologies. The groundlayer diversity of grasslands in northern Sumatra is closely related to other monsoonal regions of Southeast Asia, likely reflecting grassland connectivity during the last ice age. The abundance of disturbance-adapted species suggests appropriate use of fire and herbivory will be necessary to sustain these communities.