Seedling establishment shapes the distribution of shade‐adapted forest herbs across a topographical moisture gradient

Abstract

Summary1. 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. 2. 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). 3. 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. 4. 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. 5. 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. 6. 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. 7. 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.

Long-term consequences of mechanical fuel management for the conservation of Mediterranean forest herb communities

“…These results are in line with studies showing that recovery after major disturbances of typical forest herbs, mainly perennials, may take several decades or even centuries (Bellemare et al 2002;Flinn and Vellend 2005), probably because they require particular regeneration niches that may take a long time for developing (Albrecht and McCarthy 2009), they often lack a persistent seed bank (Decocq et al 2004b;Dölle and Schmidt 2009) and they have low colonization rates from external sources due to limited dispersal capacity (Bellemare et al 2002;Brunet 2007).…”

The putative niche requirements and landscape dynamics of Microstegium vimineum: an invasive Asian grass

“…The tight correlation means that growth and reproduction respond similarly to light (Cheplick 2010;Claridge and Franklin 2002;Horton and Neufeld 1998;Winter et al 1982). Increased seed production has been measured across sun vs. shade habitat (Huebner 2010a;Schramm and Ehrenfeld 2010), but canopy openness is a poor niche proxy as soil moisture, temperature and leaf litter also vary with canopy heterogeneity (Albrecht and McCarthy 2009;Warren 2010), making the direct measurement of multiple environmental variables along putative resource gradients or experimental manipulations necessary for conclusive associations. The correlation between M. vimineum growth and increased light (Claridge and Franklin 2002;Droste et al 2010;Flory 2010;Horton and Neufeld 1998;Winter et al 1982) suggests that reproduction increases with solar irradiation, and Cheplick (2010) demonstrated that seed production increased with direct sunlight and not soil moisture.…”

The shape of things to come: woodland herb niche contraction begins during recruitment in mesic forest microhabitat

“…recruitment, survival) contribute unequally to plant population viability [19,20], and each stage may respond differentially to abiotic conditions [21 -23] and biotic interactions [24,25]. Most plant mortality occurs during the transition from seed to juvenile [26,27], making recruitment success (the 'regeneration niche') a critical phase in structuring the spatial distribution of plant populations [20,21,28]. Warren et al [13] found that ant-mediated seed dispersal often fails where soil moisture is relatively high in deciduous forests so that seedlings may be absent from habitat where they would otherwise thrive.…”