Rainfall pulses mediate long-term plant community compositional dynamics in a semi-arid rangeland.
Semi-arid rangelands, comprising more than 40% of the Earth's land surface, provide critical ecosystem services. Worldwide, these ecosystems are experiencing rapid degradation due to overgrazing and precipitation changes. However, how plants respond to these interacting factors remains relatively unexplored, and precisely which and how rainfall factors determine plant community dynamics in rangelands has not been well developed. We used a long-term (1953-2018) dataset from semi-arid rangeland to investigate coupled effects of grazing intensity and rainfall intensity (the total amount of precipitation) on different groups of plant cover (herbaceous, woody and cacti plants) using linear mixed-effects models, redundancy analysis and structural equation models. We examined how rainfall intensity influenced plant cover dynamics according to pulse size (intensity over time) categories, which we analysed at three scales: yearly, within the wet season only (June-September) and within the dry season only (October-May). Plant community cover showed a humpbacked trend in the last six decades, mostly through changes in woody plants. Although both grazing intensity and rainfall presented similar humpbacked trends with plant community cover, our models demonstrated that the reduction of plant cover from the 1990s has been mainly caused by a decrease of rainfall rather than grazing intensity, particularly due to profound reductions of the intensity of relatively small rainfall pulses (e.g. 5.1-15 mm/day) during the dry season. Specifically, these small rainfall pulses can increase plant cover of all subgroups of woody and herbaceous species, thereby increasing plant community cover. Moreover, rainfall pulses during the wet season had negative effects on herbaceous species and positive effects on woody plants. These results suggest a phenological niche partitioning between woody plants and herbaceous in subtropical rangelands. Synthesis and applications. Our results show how critical seasonal rainfall pulses are for regulating plant community compositional dynamics, which has significant implications for rangeland management and our ability to adapt and mitigate amplified climate influences in semi-arid ecosystems.