title: Are there consistent grazing indicators in Drylands? Testing plant functional types of various complexity in South Africa's Grassland and Savanna Biomes.
abstract: Despite our growing knowledge on plants' functional responses to grazing, there is no consensus if an optimum level of functional aggregation exists for detecting grazing effects in drylands. With a comparative approach we searched for plant functional types (PFTs) with a consistent response to grazing across two areas differing in climatic aridity, situated in South Africa's grassland and savanna biomes. We aggregated herbaceous species into PFTs, using hierarchical combinations of traits (from single- to three-trait PFTs). Traits relate to life history, growth form and leaf width. We first confirmed that soil and grazing gradients were largely independent from each other, and then searched in each biome for PFTs with a sensitive response to grazing, avoiding confounding with soil conditions. We found no response consistency, but biome-specific optimum aggregation levels. Three-trait PFTs (e.g. broad-leaved perennial grasses) and two-trait PFTs (e.g. perennial grasses) performed best as indicators of grazing effects in the semi-arid grassland and in the arid savanna biome, respectively. Some PFTs increased with grazing pressure in the grassland, but decreased in the savanna. We applied biome-specific grazing indicators to evaluate if differences in grazing management related to land tenure (communal versus freehold) had effects on vegetation. Tenure effects were small, which we mainly attributed to large variability in grazing pressure across farms. We conclude that the striking lack of generalizable PFT responses to grazing is due to a convergence of aridity and grazing effects, and unlikely to be overcome by more refined classification approaches. Hence, PFTs with an opposite response to grazing in the two biomes rather have a unimodal response along a gradient of additive forces of aridity and grazing. The study advocates for hierarchical trait combinations to identify localized indicator sets for grazing effects. Its methodological approach may also be useful for identifying ecological indicators in other ecosystems.
authors.affiliation: Range Ecology and Range Management Group, Botanical Institute, University of Cologne, Cologne, Germany; Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Linstädter
authors.affiliation: Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Schellberg
authors.affiliation: Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Brüser
authors.affiliation: Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Cristian A
authors.initials: CA
authors.lastname: Moreno García
authors.affiliation: Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Roelof J
authors.initials: RJ
authors.lastname: Oomen
authors.affiliation: Department of Soil, Crop and Climate Sciences, University of the Free State, Bloemfontein, Republic of South Africa.
authors.forename: Chris C
authors.initials: CC
authors.lastname: du Preez
authors.affiliation: Range Ecology and Range Management Group, Botanical Institute, University of Cologne, Cologne, Germany; Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Jan C
authors.initials: JC
authors.lastname: Ruppert
authors.affiliation: Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Ewert
mesh_terms: D000818:Animals; D018533:Biomass; D055864:Droughts; D004435:Eating; D065948:Grassland; D010944:Plants / Q000145:classification*; D013019:South Africa
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