The effects of fire and grazing management on unpalatable climax grasslands dominated by Cymbopogon validus and Hyparrhenia hirta

Abstract

Fire alone, and in combination with grazing, influence established individual plants, and their respective regeneration potential variously with consequences on species dynamics. This study gives attention to the effects of fire alone, and in combination with grazing in underutilized, problem rangeland communities. Firstly, the question exists whether annual fire in fire prone Hyparrhenia hirta dominated veld could be one of the factors that promote the persistence of this veld type in the Gauteng Highveld. Secondly, the impacts of fire and heavy grazing on species and associated changes in veld condition characteristics are documented in Cymbopogon validus dominated veld in the Eastern Cape. The grazing value of Hyparrhenia hirta dominated rangelands is often managed through frequent burning aimed at encouraging palatable re-growth. Despite this widely adopted practice, research on the effects of fire on H. hirta seed production (seed banks) and seed dynamics and therefore its regeneration potential, is lacking in South Africa. This study quantified and compared H. hirta soil seed bank density, and seedling emergence in the field in an annually burned site versus a > 2 year fire return interval site. It was hypothesized that H. hirta seed bank densities and seedling emergence would be greater in annually burned veld compared to > 2 year fire return interval veld. The seedling emergence method was used to determine seed bank densities of soil samples collected from the two sites. Seedling emergence was quantified and monitored under individual treatments within burn (clipping, soil disturbance, control) and unburned sites (soil disturbance, litter, clipping, and control) over a single growing season. Results revealed that H. hirta seed bank densities were nearly two fold higher in the annually burned site but not significantly so. In the seedling emergence trial, no consistent trend was identified in individual treatments but emergence was generally favored in control plots (associated with standing vegetation), and reduced in clipped plots (associated with a short sward). However, analysis of the pooled data (annually burned site versus > 2 year fire return interval site) revealed a significant increase (p<0.0001) in emergence in the annually burned site. It was concluded that frequent burning of H. hirta dominated stands may be one of the factors that facilitates its dominance, through increasing seed additions to the seed bank. This is likely the case where H. hirta is underutilized after burning, therefore allowing seed to set and encouraging its availability in the seed bank. Furthermore, underutilization inadvertently results in burning in subsequent years with accumulative effects on the soil seed bank, and subsequent emergence patterns. In the high rainfall, sourveld grasslands of the Eastern Cape, the eradication of underutilized stands of Cymbopogon validus has been directed through burning and intensive grazing on the farm Glen Gregor, outside Bedford. At this study site, species composition of standing vegetation, basal cover, tuft diameter and standing biomass were sampled in top and bottom slopes of five camps representing a gradient of fire and grazing management intensity. Additionally, the composition of the soil seed bank was determined through the seedling emergence method from soil collected from the same sites. Results showed that the success of C. validus eradication was not determined by burning alone, as burning without the required post fire grazing maintained a dominance of C. validus. Cymbopogon validus decreased along the grazing gradient according to the total number of seasons that a camp was grazed after burning. Heterogenous topography in camps with steep top slopes and gentle bottom slopes meant grazing was concentrated in bottom areas, reflected by significant reductions of C. validus compared to top slopes. In contrast, even utilization in camps of uniform topography reduced the prevalence of C. validus over the whole camp, reflected by similar frequencies in top and bottom slopes. Cymbopogon validus dominated sites were associated with large areas of bare ground between large tufted individuals. In contrast, C. validus eradication through fire and intensive grazing went hand in hand with the occupation of large bare spaces by other species, predominantly Sporobolus africanus, Cynodon dactylon and other subclimax species which lead to increases in basal cover. Cymbopogon validus was increasingly absent from the seed bank as its frequency was reduced in the sward. The presence of other species in the seed bank, regardless of C. validus dominance could indicate a highly functional role of the seed bank in this mesic environment. It is proposed that once established (as C. validus is suppressed) it is these individuals that will provide the source of additional seed for subsequent population establishment. Windows of opportunity for establishment are facilitated by cycles of intense grazing that prevent C. validus from growing out and dominating the canopy. After 17 years, the sward is increasingly composed of a variety of species with favorable grazing characteristics. Prior to targeting C. validus with fire and heavy grazing, areas occupied by C. validus dominated veld were in a poor condition and offered zero contribution to livestock production. Through burning and intense grazing, management was primarily able to make use of underutilized C. validus swards. Veld condition improved as the consequent species replacement that followed now caters for breeding stock, thus increasing livestock production in previously unavailable rangelands. Both H. hirta and C. validus are competitive Increaser I species (species increasing in underutilized veld) that dominate in end point or climax grassland states. That these species are prone to underutilization (low quality mature sward) means fire is a common and often necessary tool used to stimulate higher quality forage. However, this study has shown that annual burning of H. hirta (without grazing) increases seed banks and therefore maintains a consistent source of seed to maintain its dominance. The value of burning C. validus veld is only realized when combined with intense and efficient post fire grazing. A sustained cycle of heavy grazing and rest eliminates this unpalatable climax community and facilitates the development of a sub climax sward with improved veld condition characteristics favoring livestock production. In conclusion, both studies emphasize the importance of post fire grazing to manage and control unpalatable climax swards.