A project undertaken at the School of Life and Environmental Sciences, The University of Sydney, and supervised by Aaron Greenville
Grasslands are important but undervalued ecosystems. They cover a large part of land on Earth and support services that we rely on such as our food supply. Environmental change, including altered rainfall from climate change, nutrient supply, and grazing practices put pressure on grasslands. We need to understand how plants respond to these disturbances and if there is potential for them to recover to protect food supply and biodiversity as our world continues to rapidly change. This project had three main objectives: Understand how changes to extreme rainfall events (droughts and flooding rains), nutrients, and cattle grazing work together to affect which plants grow and by how much. Use remote cameras to study how plants grow and green up over the year and determine if extreme rainfall events, nutrients, and cattle grazing affect these growing patterns. Identify the types of plants responsible for changes in the amount of plant growth and track this over time.
Our study site was on the University of Sydney’s farm, Llara, in Narrabri, northwest NSW. Llara is a mixed cropping and cattle farm with patches of remnant native vegetation. The study site was within a native grassland paddock used for pasture, with a mix of native and exotic species. We simulated a one in one hundred drought using six rain-out shelters that stopped half of the rain reaching the plants below. Any water caught by the shelters was irrigated onto a neighbouring plot to simulate an extreme increase in rainfall. We also had six control plots where rainfall was not manipulated. These plots were divided into four subplots and each subplot received one of four nutrient and cattle grazing treatments: Nutrients only, grazing only, nutrients and grazing, control (no nutrients or grazing). In the nutrient addition plots, nutrients that are essential for plant growth were added every year. Cattle were excluded from the nutrient only and control plots using fences but were otherwise allowed to roam freely. Figure 1 is a drone picture showing the shelters, water tanks for irrigation, cattle fences, and dark green areas where nutrients were added. Figure 2 shows cattle grazing the site.
We performed vegetation surveys before the treatments were applied in January 2021, then again in 2022, 2023, and 2024. We counted the number of species present and how much biomass each species produced. There were no differences after one year of applying treatments, but in July 2022 and January 2023 there were changes. Under drought conditions, grazing reduced the amount of plant biomass. We also found that adding nutrients increased plant growth with extreme and ambient rainfall, but this was not the case under drought conditions. By looking at the amount of nutrients in the soil we found that when plants didn’t get enough water, they couldn’t use the extra nutrients for growth. This meant the extra nutrients were building up in the soil. While adding nutrients increased the amount of plant biomass, it reduced the number of species we found. This could be because only a few species grow very quickly and take the space of others. We also found that drought increased the number of species we found, so the reduced plant growth under these conditions might make space for more species. We also set up cameras that took pictures every day to watch how plant greenness, which indicates plant growth, changed. We found that plant greenness overall was very variable and closely linked to rainfall, but that there was a spring/summer growing season between September–March where plants were greenest.
Adding nutrients increased plant greenness and increased the length of the growing season. We also found that drought reduced plant greenness and delayed the start of the growing season by about two months. Figure 3 shows reduced greenness under the drought shelters. The extra water added to extreme rainfall plots reduced the amount of variation in greenness during the growing season, meaning plants did not die off as much in the absence of rain. In this project we have shown how grasslands could respond to a combination of farming management practices and environmental stresses from climate change. This information will be very useful for land managers to maintain productivity in a fast-changing environment.
