Seed bank spatial structure in semi-arid environments: beyond the patch-bare area dichotomy

Abstract

The prevalence of patchy structures in vegetation is a common feature in semi-arid ecosystems. Although the effect of patches on seed density is widely known, we still lack information on how patch features affect seed bank density and composition. Our aim was to answer two basic questions:(1) How do seed bank density and composition vary within and outside patch aboveground physical limits? and (2) Do patch characteristics affect soil seed bank density and composition? We sampled 50 shrub patches in a semi-arid gypsum system in Central Spain, measuring patch size, composition and structure, and seed bank at three locations per shrub (centre, edge and outside). We calculated the effect of interior patch location, patch composition and structure on seed density and composition. Patches acted both as seed sources, increasing seed density in neighbouring areas and as seed sinks by trapping seeds from bare areas. Patch structure (erect perennial cover) had the greatest effect on seed bank density, whereas patch size and microslope had the greatest influence on bare area density. Patch structure, composition and interior location explained the variation in seed bank composition. Patch effect extends to the surrounding bare matrix creating a seed bank gradient in density and composition. This effect is modulated by patch structure and composition and affects seed bank composition. Our results suggest that the spatial structure of gypsum community seed banks may act as a mechanism for a spatial storage effect contributing to the maintenance of high levels of diversity in semi-arid environments

Nitrogen fertilization and water supply affect germination and plant establishment of the soil seed bank present in a semi-arid Mediterranean scrubland

“…Consequently, plant recruitment was not limited in the field by seed availability, as the establishment limitation paradigm poses (Méndez et al 2008). The high density recorded here could be related to a well developed persistent soil seed bank consisting of dormant seeds accumulated in the soil profile, combined with a previous growing season favourable for seed production and viability (transient seed bank) (Caballero et al 2008).…”

How best to quantify soil seed banks in arid rangelands of the Nama Karoo?

“…However, spatial patterns in seed distribution also include small-scale seed bank gradients in density and composition that are beyond the simple comparison between undercanopy and open-matrix seed banks (Caballero et al 2008). Therefore, a larger number of samples should be taken, considering also the smaller-scale spatial patterning of seed deposition (compare Reichman 1984) in order to capture the actual species diversity in the seed bank.…”

“…This is particularly true for (semi)arid ecosystems, whose often patchy vegetation and variable climate account for much spatiotemporal variation in seed banks (Kemp 1989). Patch-scale heterogeneity is a determining factor in the spatial patterning of soil seed banks with physical barriers acting as seed traps (Caballero et al 2008;Reichman 1984). Several seed adaptations have evolved that buffer the environmental heterogeneity of microsites and irregular rainfall patterns, such as those that cope with the uncertainty of favorable conditions for germination and establishment (Venable and Brown 1988).…”

Seed rain alteration related to fire and grazing history in a semiarid shrubland

“…These patches are conformed of phytogenic mounds dominated by shrubs, with grasses, subshrubs, herbs and biological crusts growing under their canopy (Bisigato and Bertiller, 1999;Cecchi et al, 2001). Several mechanisms have been proposed to explain the maintenance of these structures, including a complex balance between competition and facilitation (Caballero et al, 2008). Recently, it has been highlighted that the formation of vegetation patches is a product of grazing in the semiarid regions of the world (Allington and Valone, 2013).…”

“…Under grazing conditions, key resources, such as water and nitrogen, are more available in the shrub patches (Allington and Valone, 2013). Also, the importance of the shrub patches as a seed source and trap forms a spatial pattern of the soil seed bank that reinforces this heterogeneous distribution (Caballero et al, 2008). Concentration of defoliation and trampling in the interpatches reduces the cover of the grasses and biological crusts that protect the soil, leading to edaphic changes that decrease nutrient concentration and infiltration, and increase the loss of vegetation cover in these spaces (Allington and Valone, 2013;Fuls, 1992).…”