Rainfall pulse matters in Arizona rangelands

Rainfall provides an essential resource for plant germination and growth, and thereby, the outcomes of ecological restoration. Identifying the underlying mechanisms of rainfall regulation is crucial for predicting long-term plant community dynamics in a water-limited ecosystem, such as semi-arid rangeland coupled grazing effects in Arizona.

The Santa Rita Experiment Range (SRER), in the southwestern US, is approximately 40 km south of Tucson, AZ. The SRER has been dedicated to long-term ecological research for land-use management and decision making. The vegetation contains shrubs, herbaceous, and cacti plants. We classified 80 recorded plant species from the SRER dataset (https://cals.arizona.edu/srer/data.html) into three main synusiae based on life-form: woody plants, herbaceous, and cacti. The site has been experienced livestock grazing at a relatively low intensity, thus, grazing effects on plant community were not profound.

However, plant community cover showed a decreasing trend that started the 1990s, likely because of a substantial reduction of rainfall intensity during the dry season (October to May). In particular, the intensity of relatively small rainfall pulses (e.g., 5.1 ~ 15 mm·day^-1) dropped more than others in the last three decades. Based on the statistical analyses, these small rainfall pulses had a positive correlation with the plant cover of all subgroups of woody and herbaceous species, thereby increasing plant community cover. Here, rainfall pulse denotes the size of rainfall.

Furthermore, the rainfall pulses during the wet season had negative effects on herbaceous species and positive effects on woody plants, suggesting a phenological niche partitioning between woody plants and herbaceous in this dryland. This possibly explains a positive correlation between woody plants and herbaceous (e.g., introduced grasses). These results highlight the critical rainfall regulations for regulating plant community compositional dynamics, which has significant implications for rangeland restoration and our ability to adapt and mitigate amplified climate influences in semi-arid ecosystems.

The study has been published in the professional peer-reviewed journal recently (Liang, M., Feng, X. and Gornish, E.S. (2020), Rainfall pulses mediate long‐term plant community compositional dynamics in a semi‐arid rangeland. Journal of Applied Ecology. Accepted. doi:10.1111/1365-2664.13780)