Are There Consistent Grazing Indicators in Drylands? Testing Plant Functional Types of Various Complexity in South Africa’s Grassland and Savanna Biomes Anja Linsta ¨ dter 1,2 *, Ju ¨ rgen Schellberg 2 , Katharina Bru ¨ ser 2 , Cristian A. Moreno Garcı´a 2 , Roelof J. Oomen 2 , Chris C. du Preez 3 , Jan C. Ruppert 1,2 , Frank Ewert 2 1 Range Ecology and Range Management Group, Botanical Institute, University of Cologne, Cologne, Germany, 2 Institute of Crop Science and Resource Conservation, University of Bonn, Bonn, Germany, 3 Department of Soil, Crop and Climate Sciences, University of the Free State, Bloemfontein, Republic of South Africa 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. Citation: Linsta ¨dter A, Schellberg J, Bru ¨ ser K, Moreno Garcı ´a CA, Oomen RJ, et al. (2014) Are There Consistent Grazing Indicators in Drylands? Testing Plant Functional Types of Various Complexity in South Africa’s Grassland and Savanna Biomes. PLoS ONE 9(8): e104672. doi:10.1371/journal.pone.0104672 Editor: John F. Valentine, Dauphin Island Sea Lab, United States of America Received November 18, 2013; Accepted July 16, 2014; Published August 11, 2014 Copyright: ß 2014 Linsta ¨dter et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was funded through the research project FOR 1501 by the German Science Foundation (http://www.dfg.de/en/). AL, JR and JS also acknowledge support by the German Federal Ministry of Education and Research (http://www.bmbf.de/en/) via the ‘Limpopo Living Landscapes’ project within the SPACES programme (grant numbers 01LL1304-C and -D). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: anja.linstaedter@uni-koeln.de Introduction Resource availability and disturbances are widely recognized as key drivers of plant community structure and composition [1]. Rangeland vegetation is shaped by recent grazing pressure, together with multiple combinations of land use history and available resources [2,3]. In the case of dryland rangelands, management effects are particularly difficult to detect against the background of a high environmental variability [4,5]. There is a growing need for ecological indicators that provide aggregated information for assessing states and trends of vegeta- tion dynamics [6]. For this purpose, it is crucial to decide on an optimum type and level of aggregation of vegetation characteristics [7]. Plant functional types (PFTs) are a typical example for an aggregation of plant species, and PFTs are frequently applied as indicators for the state of dryland rangelands [8]. PFT classifica- tion is a widely supported method in data analysis to aggregate species and to reveal a consistent response of ecosystems, irrespective of species identities [9]. The underlying rationale is that different species within a PFT share traits that show a similar response to grazing disturbance [10]. Hence, PFTs can be powerful indicators for rangeland condition due to their functional relation to ecosystem processes [11]. A-priori classifications of PFTs have been criticised for neglecting the specific environmental settings and the evolutionary history of the study area [12]. However, individual plant traits are typically highly interrelated, and thus pairs or groups of traits usually co-vary [13]. This complexity of relations among traits can usually and effectively be summarized by aggregating individual traits to one or few key traits that capture a large proportion of variation in vegetation responses to grazing [14]. Life forms [15] and growth forms are early and well-known expression of such correlation among traits [13,16]. They are extensively used to describe functional plant responses to grazing [17,18]. However, despite the vast literature on plant trait and PFT responses to grazing in drylands, there is still no consensus if an PLOS ONE | www.plosone.org 1 August 2014 | Volume 9 | Issue 8 | e104672