Abstract
Soil seed banks offer plants the possibility to disperse through time. This has implications for population and community dynamics, as recognised by ecological and evolutionary theory. In contrast, the conservation and restoration literature often find seed banks to be depauperate, weedy and without much conservation value or restoration potential. One explanation for these contrasting views might lie in a systematic bias in the sampling of seed banks versus established plant communities. We use the species–area relationship as a tool to assess and compare the per‐area species richness and spatial structuring of the diversity of the established plant community versus soil seed banks. To allow this direct comparison we extensively survey the species–area relationship of the vegetation and underlying seed bank of a grassland community across twelve sites spanning regional bioclimatic gradients. We also compile a global dataset of established vegetation and seed banks from published sources. We find that seed banks have consistently higher intercepts and slopes of the relationship, and hence higher diversity at any given spatial scale, than the vegetation both in the field and literature study. This is consistent across habitat types, climate gradients, and biomes. Similarity indices are commonly used to compare vegetation and seed bank, and we find that sampling effort (% of the vegetation area sampled for seed bank) was the strongest predictor of vegetation–seed bank similarity for both the Sørensen (R2 = 0.70) and the Raup–Crick (R2 = 0.25) index. Our study suggests that the perception that seed banks are intrinsically less diverse than established plant communities has been based more on inadequate sampling than on biological reality. Across a range of ecosystems and climatic settings, we find high diversity in seed banks relative to the established community, suggesting potentially important roles of seed banks in population dynamics and diversity maintenance.
Highlights
On evolutionary timescales, seed banks increase the mean generation times of populations, thereby affecting the potential rate and even direction of evolutionary change (Brown and Venable 1986, Evans and Cabin 1995, Evans and Dennehy 2005)
This has resulted in a systematic bias in the spatial scale of sampling of seed banks relative to the established plant community; the area sampled for seed banks is up to several orders of magnitude smaller than the areas sampled for the extant vegetation (Thompson et al 1997 report 20:1 as the average difference in sampling area between vegetation and seed bank in their database)
The only exception was our alpine field study sites, where fine-scale species richness was higher in the vegetation than in the seed bank, but even here, seed bank species richness surpassed that of the established plant community at spatial scales 100 cm2
Summary
On evolutionary timescales, seed banks increase the mean generation times of populations, thereby affecting the potential rate and even direction of evolutionary change (Brown and Venable 1986, Evans and Cabin 1995, Evans and Dennehy 2005). The established plant community, on the other hand, is typically sampled more extensively, by total census of species present in (relatively large) plots or other similar methods where the representativeness issue is considered for each sample and not for the amalgamated dataset This has resulted in a systematic bias in the spatial scale of sampling of seed banks relative to the established plant community; the area sampled for seed banks is up to several orders of magnitude smaller than the areas sampled for the extant vegetation (Thompson et al 1997 report 20:1 as the average difference in sampling area between vegetation and seed bank in their database). We complement this field study with a literature survey where we assess the diversity of seed banks versus established plant community across 89 published studies to allow an assessment of SAR relationships and patterns across habitats and along global-scale climatic gradients
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