The relative tolerance of mesic grassland species to defoliation and competition.

Abstract

Effective grazing management is dependent on understanding grass species responses to herbivory. These responses to herbivory can be broadly grouped into 3 categories, namely decreaser (plants that decline in abundance) and increasers (plants that increase in abundance). Tolerance is defined as the capacity of a plant to withstand herbivory, while suffering little loss in growth or its ability to reproduce and it can be equated to the ability of the plant to compensate. The relevance of this strategy to rangeland management has become increasingly apparent and has allowed researchers to investigate more questions and test long-standing ideas within the Grassland Science discipline. Therefore, the general aim of this research was to determine how Increaser and Decreaser grass species common in mesic grasslands tolerate defoliation and competition. Two experiments were conducted as controlled pot trial experiments under shade cloth at the NM Tainton Arboretum. Simulated herbivory in the form of clipping was used for both experiments. Categorising species into four grazing response groups (Increaser I, II,III and Decreaser) has led to generalisations made across and between species in terms of responses to herbivory. To determine if these generalisations are appropriate, the growth response of two Decreaser (Themeda triandra and Tristachya leucothrix) and two Increaser (Eragrostis curvula and Eragrostis plana) grass species was investigated. Results indicate that defoliation tolerance is not necessarily explained by response groups and differences can be observed between species, within response groups. Grouping species into response groups may be an over simplification. This implies that species identity may be more important in understanding species composition changes within natural communities than originally thought. Plant traits, such as shoot biomass, roots biomass , tuft height and root to shoot biomass ratios also responded differently across species highlighting the importance of further research on specific species as generalisations may not be entirely useful. To provide more insight into this, the response of T. triandra to defoliation and competition with E. curvula was investigated. Results show that veld dominated by T. triandra and few E. curvula tufts should be leniently grazed every other year with rest applied following a growing season, to allow T. triandra tufts to regrow. A non-selective grazing system should be adopted by veld managers during the growing season to lower the competitive pressure exerted on T. triandra tufts by other species and to enhance growth. These results need to be considered as a basis to understand how T. triandra swards respond on a small scale and further investigations are necessary to validate impact on natural communities. Overall, defoliation tolerance is species specific and depends on the combined effects of defoliation and competition as these affect the cumulative and morphological 2 responses of important mesic grassland species.