Experimental evidence for dispersal-limitation of multiple vernal herbaceous species in mesic hardwood forests

Background/Question/Methods

The flora of secondary hardwood forests contains significantly fewer vernal herbaceous species than otherwise similar primary forests. Vernal herbs may play a key role in forest dynamics, yet little is known about how to conserve or restore these herbs in secondary forests. Many vernal herbs are ant- or gravity-dispersed and are believed to be dispersal-limited. Dispersal-limitation may therefore be a key characteristic to consider in the conservation and restoration of vernal herbs. However, few studies experimentally investigate dispersal-limitation of vernal herbs, and none of those that focus on vernal herbs have replicated experimental sites, quantified survival across all life cycle stages, and approximated population growth rates. We tested whether five vernal, perennial species were dispersal-limited by artificially dispersing adults and seeds into four replicate forests. We followed survival across all life cycle stages for four years, and we approximated growth rates for all five species at each replicate site. 

Results/Conclusions

All species germinated, flowered, and recruited seedlings in all sites. Within three years, three species with the shortest generation times produced seeds that matured and flowered across the sites, and growth rates for most of the introduced populations were near or above one. Our results indicate that vernal species are a suite of dispersal-limited herbs that will readily establish in formerly disturbed forests. Furthermore, our experimental method was a rapid and cost-effective way to assess habitat suitability for long-lived species. Due to the rarity of successful long-distance dispersal events, increasing habitat fragmentation, and increasing pressures of climate change, artificial dispersal of vernal herbs and other slow-dispersing species may be an effective tool in preventing their extinction.