Savanna woody regeneration in response to different treatments of herbivory and fire
Abstract
Woody regeneration is one of the fundamental processes responsible for structuring savanna systems. Herbivory and fire are two primary drivers of woody structure, and therefore also woody regeneration. Regeneration responses of woody species to savanna controlling factors can be examined and explained by using the combined competition-based and demographic bottleneck model. Woody encroachment has received increasing attention in savanna system dynamics since savannas are considered vulnerable to encroachment. Woody species can broadly be categorised as encroachers, which are considered as species with the ability to outcompete other life forms when disturbances occur, and non-encroachers, which are considered more desirable. The savanna demographic bottleneck model has not yet been applied to these two woody functional groups separately. This could bring insight into woody encroachment and to what extent it can be controlled by herbivory and fire without negatively affecting non-encroacher species.
The main aim of this study was to test the effect of herbivory and fire (presence and release thereof) on woody regeneration of both encroacher and non-encroacher woody species across a small-scale (139 ha) heterogeneous landscape in a riparian semi-arid savanna ecosystem. The specific objectives were to (i) describe and compare dominant woody families and species and basic PFtraits of the woody layer across different treatments of herbivory and fire (presence and exclusion), (ii) evaluate the effects of herbivore exclusion on woody species assemblages, (iii) evaluate the effect of herbivory and fire on woody species abundances, and (iv) evaluate the effect of herbivore and fire presence and exclusion on woody community and population demography and stability. Woody species assemblages referred to the statistically tested woody species composition in the specific treatment and woody communities is the group of woody species within the boarders of the herbivore or fire treatment. The broad hypothesis states that the exclusion of both herbivory and fire from a semi-arid savanna ecosystem will enhance regeneration and recruitment of woody species.
The study was conducted at the Nkuhlu long-term exclosures situated in the southern parts of Kruger National Park, South Africa. The exclosures are divided into different treatments of herbivory and fire. Herbivore treatments consist of a fully fenced exclosure (designed to exclude all mammalian herbivores larger than a hare); a partial exclosure (designed to exclude elephant, but also excludes giraffe due to their body height) and a control site (exposure to all large mammalian herbivores). Each herbivore treatment was devided into a fire-exposed and fire-excluded area by means of a fire break. Woody individuals were sampled inside permanently marked plots located on transects initiating in the riparian vegetation zone (close to the Sabie River), extending across the sodic midslopes to the crest (uplands). Results from the floristic analyses indicated that 13 years of excluding herbivory and fire was long enough to initiate changes in abundance of dominant families and plant functional types, and excluding herbivores also changed species composition. Herbivores played an important role in structuring the woody layer, although fire had much less effects on woody regeneration than was expected in this savanna type. Woody species abundance results indicated that herbivore activity negatively impacted recruitment of both encroacher and non-encroacher species, with effects differing between the two groups. Herbivore effects were also evident in community and population demography. Herbivores managed to suppress regeneration of key encroacher species, except for Dichrostachys cinerea. Key non-encroacher species differed in their response to herbivore activity, with some indicating demographic resistance to herbivore pressure