How does the Kalahari vegetation response to seasonal climate and herbivory?

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The post provided by Ute Schmiedel

One of our permanent plots in the Kalahari, South Africa, at the peak of the growing season during an average rainfall season (2013) and after several years of very low rainfall (2015). The grazing herbivores removed most of the grass biomass and provided vegetation gaps for the new plants to grow. Due to the drought, there were very few newly established plants. Photo credit: Ute Schmiedel.

This post refers to the article Response of Kalahari vegetation to seasonal climate and herbivory: Results of 15 years of vegetation monitoring by Ute Schmiedel, Vasco Jacke, Berit Hachfeld, Jens Oldeland published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12927).

Annual vegetation monitoring of permanent plots are key for understanding vegetation dynamics (see also the post Why we still need permanent plots for vegetation science by de Bello et al.). By relating these changes to a number of environmental variables, we can draw conclusions about the main environmental drivers of the observed changes. This understanding is an important prerequisite for developing reliable projections of vegetation changes in the context of climate change and for making informed land management decisions to adapt to changing environmental conditions. The SASSCAL Observation Net, therefore, maintains a network of biodiversity observatories where plant species composition and their potential drivers are monitored annually in a standardised approach. Most of the observatories are monitored annually since 2001 and by now provide time-series data of two decades.

We analysed the vegetation change in response to climatic variables and herbivore density at one of the biodiversity observatories on a game farm in the South African Kalahari. We grouped the plant species into eight major plant strategy types (namely annual grasses, perennial grasses, annual forbs and perennial forbs, geophytes, dwarf shrubs, large shrubs and trees) and used counts of individual plants per strategy type, plot and year to see how the strategy types changed over time and how they responded to seasonal climatic conditions and density of herbivores. For climatic variables, we used the SPEI index for the four seasons (early and late summer and early and late winter). The index combines the total rainfall and the mean temperature per season. The herbivores on the farm were several antelope species, which we grouped into those that mainly depend on leaves of perennial herbs, woody shrubs and trees (browsers) and those that mainly utilise the grass biomass (grazers).

A flock of springbok antelopes (Antidorcas marsupialis) passing through a dune valley in the biodiversity observatory in the South African Kalahari. Springboks mainly feed on leaves and twigs of trees, shrubs and forbs and represent the “browsers” among the herbivores in the study area. Photo credit: Ute Schmiedel.

What did we find? As we expected, the browsers had a negative effect on the abundance of trees, dwarf shrubs and perennial forbs. However, we also found a negative effect of browsers on annual grasses. We explained this by the excessive trampling of these antelopes on patches dominated by annual grasses, which they chose for their mating behaviour. The grazers had a positive effect on the abundance of individuals of the annual forbs and annual and perennial grasses as well as on dwarf shrubs. This effect can be explained by the foraging behaviour of the grazers: by consuming the biomass of the grasses and forbs, they create gaps in the vegetation. These gaps provide an opportunity for new herbaceous plants and dwarf shrubs to germinate and establish and thus lead to an increase in abundance of these plants. However, this can only happen if the climatic conditions are suitable for them to establish.

Which climatic conditions did we find to be suitable for the various plant types to establish? Humid conditions during the main growing season in late summer, for instance, had strong positive effects on the abundance of annual grasses, forbs and dwarf shrubs by facilitating their germination and establishment. Dwarf shrubs were the only plant type that also positively responded to the humid conditions during the early summer season but of the previous summer. The humid conditions in the early summer season might be important for the biomass and seed production of the shrubs.

Brief personal summary: Ute Schmiedel is a vegetation ecologist at the Institute of Plant Science and Microbiology of the University of Hamburg in Germany. One of her main research interest is the vegetation dynamics and their environmental drivers in arid regions of South Africa. She annually monitors the vegetation on permanent plots in the Kalahari and the Succulent Karoo biome.