Herbaceous species diversity patterns across various treatments of herbivory and fire along the sodic zone of the Nkuhlu exclosures, Kruger National Park

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Date
2013Author
Van Coller, Helga
Siebert, Frances
Siebert, Stefan J.
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Understanding relationships between large herbivores and plant species diversity in dynamic
riparian zones is critical to biodiversity conservation. The Nkuhlu exclosures in the Kruger
National Park (KNP) provided opportunity to investigate spatial heterogeneity patterns within
riparian zones, as well as how these patterns are affected by fire and herbivory. A monitoring
project was initiated to answer questions about the dynamics of the herbaceous layer and
was aimed at determining, (1) whether there exists meaningful variance in herbaceous plant
species richness and diversity across different treatments in the ecologically sensitive sodic
zone and (2) whether an increase in herbaceous biomass, an artefact of herbivory and fire
exclusion, suppresses herbaceous plant species diversity and richness. Herbaceous vegetation
was sampled in two 1 m2 circular sub-plots in the eastern and western corners of 81 fixed
plots. The biomass of each plot was estimated with a disc pasture meter (DPM) diagonally
with the plot. DPM-readings were converted to kg/ha, according to the latest conversions
for the Lowveld Savanna. Species richness and biomass showed significant variance across
treatments, whereas no significant variation in herbaceous species diversity was perceived.
Combined treatment of fire absence and herbivore presence contributed to higher forb
species richness in the sodic zone. Biomass is significantly higher in fully fenced areas where
herbivores are excluded, as opposed to the open and partially fenced areas. Although no
significant variation was recorded for diversity across treatments, lowest diversity was
recorded in the absence of all herbivores, especially in combination with fire treatment.
Therefore herbivores are essential in sustaining herbaceous plant species richness in the sodic
zone, whilst no significant results were found with regard to their effect on species diversity.
Although statistically non-significant, fire seems to suppress species richness.