Seedling establishment shapes the distribution of shade-adapted forest herbs across a topographical moisture gradient
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In deciduous forests, herb distribution patterns can shift dramatically across topographical gradients, yet it remains unclear whether topographical associations reflect regeneration niche differences that arise during early life-history stages. We examined: (i) whether seedling recruitment patterns were consistent with topographical distributions of established populations and (ii) how environmental heterogeneity at two spatial scales influences spatial patterns of seedling recruitment in four shade-adapted forest herbs (Actaea racemosa, Hydrastis canadensis, Panax quinquefolius and Sanguinaria canadensis), which are harvested from the wild for their medicinal properties but differ in life histories and seed mass. Topographical distributions were quantified in transect surveys of forest stands, and then seed was experimentally transplanted into litter microenvironments (bare, shallow and deep) on opposing topographical positions (NE-facing cove forest and SW-facing oak forest). Forest herbs that were more frequent in moist NE-facing cove forests (A. racemosa, H. canadensis and S. canadensis) suffered higher mortality when their seeds were dispersed into the drier SW-facing oak forest, although the stages that limited recruitment differed among species. For A. racemosa and S. canadensis, the selectivity of the slope topographical filter varied in strength among years that differed in soil moisture. Seedling distributions expanded across the topographical gradient during a 'wet' year but contracted during a 'dry' year. Litter effects were often context-dependent. Litter-removing disturbance increased seedling recruitment of H. canadensis, but only in the NE-facing cove forest. When soil moisture was limiting in either space or time, microenvironments where litter was present tended to enhance emergence and/or survival relative to litter-free microenvironments. For P. quinquefolius, which has been harvested from the wild for over 200 years, seed limitation is a fundamental constraint on its distribution along a topographical moisture gradient. Across all microenvironments, the net recruitment rate of P. quinquefolius, the largest-seeded and least abundant species, was an order of magnitude greater than that of A. racemosa, the smallest-seeded and most abundant species. Synthesis. Many shade-adapted forest herbs are declining in abundance due to anthropogenic factors. Conservation efforts must consider dispersal limitation in the spatial context of environmental filters that can vary in strength and quality over time. © 2009 British Ecological Society.
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