SEED DISPERSAL AND RECRUITMENT LIMITATION ACROSS SPATIAL SCALES IN TEMPERATE FOREST FRAGMENTS

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Despite increasing evidence of seed limitation in forest ecosystems, data remain sparse on spatial patterns of seed rain at large (>1 ha) spatial scales. We monitored seed rain (28.5 m2) throughout five northern hardwood forest fragments (27 ha sampled across 14-km2 area) in southeastern Michigan over two years. Four fragments were nearest neighbors (300–700 m), yet varied in species composition, providing the opportunity to detect landscape-scale seed exchange. Of the 37 species of woody plants present in the seed rain (98 032 mature seeds), only three (Betula papyrifera, Ostrya virginiana, and Ulmus americana) had widespread seed dispersions within all fragments containing resident sources (seed in >70% of traps in each fragment). Seed dispersions, measured as the percentage of traps within a fragment receiving seed, differed among species using different dispersal vectors with animal-dispersed species arriving in a lower percentage of seed traps than wind-dispersed seeds. At a given source density, seed dispersions increased with decreasing seed mass. Light-seeded, fecund species such as Betula or Tsuga required lower source densities to saturate fragments with seed compared to heavy-seeded species (Acer, Fraxinus, Tilia). Heavy-seeded wind- and animal-dispersed species also displayed the strongest evidence of seed limitation, with seedling presence significantly associated with presence of seed for Carpinus caroliniana, Fagus grandifolia, Prunus avium, and Tilia americana. Of 17 species, landscape-scale seed exchange was detected for only four disturbance-associated species (Acer negundo, Betula papyrifera, Celastrus scandens, Eleaganus umbellata). No exchange was detected for Acer rubrum, Betula alleghaniensis, or Tsuga canadensis, despite broad seed dispersions (>50%) in fragments with resident sources, suggesting the potential for seed limitation for these species at larger spatial scales. Seed encounter probabilities suggest that potential seed competitors often fail to simultaneously colonize microsites. We suggest that all dominant species in northern hardwood forests can be seed limited at some spatial scale and that results are consistent with “winning by forfeit” scenarios of diversity maintenance in forest ecosystems.