Spatio-temporal variation in soil seed banks under contrasting grazing regimes following low and high seasonal rainfall in arid Namibia

Abstract

a b s t r a c tIn arid rangelands, soil seed bank characteristics vary with rainfall patterns, livestock pressure and habitat heterogeneity. We investigated this issue by assessing spatio-temporal patterns in soil seed bank density and richness in a degraded (heavy grazing) and non-degraded (light grazing) Nama Karoo ecosystem. To account for variation in seed distribution, different microsites were compared, and the effect of diaspore size on the patterning analyzed. Long-term heavy grazing increased seed densities and species richness of the soil seed bank by favoring small-seeded and tiny-seeded annual species. The spatial patterning of the soil seed bank was, however, similar to that found under sustainable grazing. The overall seed distribution was dictated by the small-scale environmental heterogeneity, which interacted with diaspore size. Our results suggest that the degraded environment is still efficient in controlling secondary dispersal and retaining seeds. This is through low stature barriers like surface rocks, which replace the function of grazing-sensitive perennial grasses in trapping and storing small diaspores. Soil seed banks were persistent and accumulated readilygerminable seeds over time. Observed temporal pattern across the study sites indicates that both condition and sequence of preceding rainfall seasons are likely to have a profound effect on soil seed banks of subsequent years.

Species, functional groups and community structure in seed banks of the arid Nama Karoo: Grazing impacts and implications for rangeland restoration

“…The inter-shrub matrix consists almost entirely of annual vegetation with a very few remnants of lean tussocks of perennial grasses. Most of the species, including the larger-seeded shrubs, possess at least short-term persistent seed banks (Dreber and Esler, 2011). According to Esler (1999), seed is the major regeneration strategy among Karoo species with very little reproduction by vegetative means.…”

An ecology of difference: fence-line contrast photographs as scientific models in ecology

“…The two studies analyzed are not alone in assuming these links and reinforcing the perception of communal areas as permanently degraded and in need of profound changes (see for instance Dreber and Esler 2011;Powrie 2010, 2011;Scholes and Biggs 2005). As the main topic of these articles is not necessarily the link between grazing, tenure and degradation as such, but for instance between grazing and biodiversity, the links appear in introductions or other sections that are, most likely, considered peripheral.…”

Understanding relations between pastoralism and its changing natural environment

“…Studies of effects of grazing on species diversity and plant abundance in soil seed banks show disparate results. A high grazing intensity may decrease (Tessema et al, 2012) or increase species diversity in soil seed banks (Dreber and Esler, 2011) or have no effect (Meissner and Facelli, 1999). A high grazing intensity may decrease (Tessema et al, 2012) or increase seed abundance (Dreber and Esler, 2011) in the soil seed banks.…”

“…A high grazing intensity may decrease (Tessema et al, 2012) or increase species diversity in soil seed banks (Dreber and Esler, 2011) or have no effect (Meissner and Facelli, 1999). A high grazing intensity may decrease (Tessema et al, 2012) or increase seed abundance (Dreber and Esler, 2011) in the soil seed banks. Moreover, in Sub-Sahara African rangelands, studies to elucidate the role of soil seed banks in the restoration of the grazing-degraded vegetation are scant (Dreber and Esler, 2011;Hopfensperger, 2007).…”

“…A high grazing intensity may decrease (Tessema et al, 2012) or increase seed abundance (Dreber and Esler, 2011) in the soil seed banks. Moreover, in Sub-Sahara African rangelands, studies to elucidate the role of soil seed banks in the restoration of the grazing-degraded vegetation are scant (Dreber and Esler, 2011;Hopfensperger, 2007).…”