Vegetation pattern divergence between dry and wet season in a semiarid savanna e Spatio-temporal dynamics of plant diversity in northwest Namibia S.K. Hassler a, * , J. Kreyling b , C. Beierkuhnlein b , J. Eisold c , C. Samimi d , H. Wagenseil e , A. Jentsch f a Institute of Earth and Environmental Sciences, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany b Biogeography, University of Bayreuth, 95440 Bayreuth, Germany c Institute for Social-Ecological Research (ISOE), Hamburger Allee 45, 60486 Frankfurt, Germany d Department of Geography and Regional Research, University of Vienna, Althanstr. 14, 1090 Vienna, Austria e Nuremberg, Germany f Geoecology/Physical Geography, Fortstrasse7, 76829 Landau, Germany article info Article history: Received 2 July 2009 Received in revised form 26 April 2010 Accepted 24 May 2010 Available online 19 June 2010 Keywords: Beta diversity Distance decay Precipitation gradient Species richness Systematic sampling Tree islands abstract African savannas are primarily used as pastures and are subject to changes in climate and management strategies. For sustainable management of these landscapes ecological knowledge on seasonal and long- term variability in plant community composition and the availability of green biomass is essential. In this study, we assessed the effects of dry and wet season on species richness and beta diversity for three sites along a gradient of increasing vegetation cover and precipitation in northwest Namibia. A hexagonal systematic sampling design was used to record floristic data. The Simple Matching, Soerensen, and multi- plot similarity coefficient and distance decay analyses were applied for examining beta diversity. Anal- yses were repeated while separating the plots according to the presence of woody vegetation. Species richness nearly doubled from dry to wet season; compositional similarity increased from dry to wet season and with increasing aridity of the study sites; distance decay was more pronounced in the dry season without any link to the precipitation gradient. Woody elements in the landscape, which occur along drainage lines or as tree islands, govern spatial and seasonal plant diversity fluctuations. Moni- toring them is important for conservation strategies and for establishing grazing rules that ensure a sustainable use of savanna ecosystems. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Two fifths of Africa’s land area is covered by savannas (Scholes and Walker, 1993), characterised by low and highly variable precip- itation and by the coexistence of trees and grasses. The utilization of grass biomass in livestock farming provides the livelihood for a large part of the local populations. However, several studies suggest that savannas under pastoral land use have become increasingly threat- ened by degradation and desertification due to a poor adaptation of management, non-adapted grazing strategies and over-utilization (e.g. Dean and Macdonald, 1994; Moorsom, 1995; Mistry, 2000; Quaas et al., 2007; Walker and Noy-Meir, 1982). In other cases, where local management strategies did not lead to a general degradation of the productivity of rangelands, a decrease in species diversity could be observed (e.g. Bollig and Schulte, 1999; Müller et al., 2007; Sullivan, 1999). Currently, in the light of climate change and shifting management ideologies, it is essential to adopt management techniques to changing environmental conditions as the livelihoods of local inhabitants depend on the possibility to continually use the green biomass of semiarid environments. Particular challenges in adopting land use strategies to the natural dynamics of semiarid ecosystems over larger scales include three major aspects; (1) intra-annual seasonality in precipitation patterns, (2) spatial gradients in savannavegetation, precipitation amount and variability and (3) spatio-temporal patterns of plant species diversity and their implications for savanna stability. First, the strong seasonality between the wet and the dry season repeatedly leads to periodic restrictions in green biomass avail- ability over large spatial areas. Balance in the tree-grass coexis- tence, ecological site conditions, and spatial organisation of vegetation are affected by these annual fluctuations in water availability (Guttal and Jayaprakash, 2007; Ludwig et al., 2001; Scanlon et al., 2005). Strong variability in biomass production is occurring from year to year (Wagenseil and Samimi, 2006). In order * Corresponding author. E-mail addresses: sibylle.hassler@uni-potsdam.de (S.K. Hassler), juergen. kreyling@uni-bayreuth.de (J. Kreyling), eisold@isoe.de (J. Eisold), cyrus.samimi@ univie.ac.at (C. Samimi), Hendrik.Wagenseil@web.de (H. Wagenseil), jentsch@uni- landau.de (A. Jentsch). Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv 0140-1963/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2010.05.021 Journal of Arid Environments 74 (2010) 1516e1524