Plant response to grazing: Observing changes in community structure and individual characteristics

Plants, like all living organisms, are capable of responding to their environment by adapting and evolving to better survive and reproduce. The phenomenon of this plant adaptation is known as phenotypic plasticity, when a genotype is capable of expressing different phenotypes in response to different environments (West-Eberhard 2008). Put more simply, phenotypic plasticity refers to the ability of a plant to change its characteristics in response to environmental variables, such as available nutrients, weather conditions, or disturbance. Among grasslands throughout British Columbia, grazing is a common disturbance that can affect the expression of plant characteristics and the structure of plant communities. A plant’s typical response to grazing includes changing its characteristics to be tolerant of, or resistant to, grazing (Wang et al. 2017). Characteristics known to show plasticity include size and number of leaves and tillers (Wang et al. 2017), as well as plant height (Olson and Wallander 1997). The direction and magnitude of which these characteristics will change depends on several variables, including grazing intensity and frequency (Wang et al. 2017), annual precipitation (Lohmann et al. 2017), and available light and nutrients (Hayashi et al. 2007). Typically, plants that have been subject to long term grazing will exhibit a lower number of shoots or tillers, smaller and fewer leaves, and will grow closer to the ground. This change in characteristics is an attempt to become grazing resistant or tolerant by lowering the likelihood that they will be grazed and reducing the amount of lost plant material when grazing does occur (Launchbaugh and Walker 2006). The structure of plant communities is also subject to change when grazing occurs. As grazing intensity changes, the functional groups and composition of the plant community also change (Zhang et al. 2018). There are mixed opinions on the ideal grazing intensity to manage the health of grasslands. Some studies argue that the occasional ‘resting’ of grasslands, meaning a period where no grazing occurs, can benefit grassland health and productivity by increasing the amount of palatable species, and reducing the amount of undesirable species (Zhang et al. 2018). Common between most papers, a high grazing intensity is thought to reduce grassland health and forage availability by increasing the amount of weeds present, and lowering the amount of grass species (Zhang et al. 2018, Pakeman RJ. 2004). However, it is also thought that grazing at a proper intensity can increase grassland health and biodiversity by reducing the amount of competitive and dominant species, allowing other species to better compete and successfully grow (Zhang et al. 2018). Managing the health of a grassland is not only important for the ecosystem, but should also be a concern of cattle ranchers who have grazing rights to the land. Biodiversity is important for an ecosystem to maintain function and productivity (Maestre et al. 2012), as well as increasing the resistance of a grassland to grazing (Lyons and Hansel 2001). As biodiversity and grazing resistance increases, so does the health and productivity of a grassland. With higher productivity comes more available forage for cattle, increasing the profitability of a rancher’s rangeland. This experiment explored how plant characteristics and community structures change in response to grazing. Plant characteristics focused on throughout this project were plant height, number of tillers produced per plant, and average leaf area. Available biomass and vegetation surveys were also examined. To compare the effects of grazing, three different treatments were performed. An exclusion cage treatment was constructed to examine plants which were not grazed for a single grazing event, but had been grazed in previous years. A grazed treatment consisted of area that has been continuously grazed, including throughout the study months. The final treatment involved a range reference area, where plants have not been subject to any cattle grazing since 1920 (Government of British Columbia 2003). It was hypothesized that grasses that have been exposed to long-term grazing, such as those in the cage and grazed treatments, would show signs of grazing resistance. As the literature suggests, it was thought that plants with higher grazing resistance would exhibit a lower plant height, fewer number of tillers, and reduced leaf area. It was also hypothesized that grazed sites would have higher measures of diversity as grazing reduces competition, allowing less-dominant species to succeed.