Humans have altered many ecosystems via land use changes, such as agriculture, and these activities can have long-lasting ecological impacts. For example, following agricultural abandonment, plant species richness and community composition can differ from levels in remnant habitats for decades or centuries. Understanding the mechanisms maintaining this persistent legacy is critical for restoration and to understanding the primary forces (e.g., disturbance, dispersal, competition) structuring post-agricultural plant communities. Despite evidence that dispersal limitation (i.e., the arrival of viable plant propagules) is a primary determinant of plant distribution, the relative importance of dispersal limitation in preventing recovery from legacies of past land use remains unclear. Our objective was to determine if the importance of dispersal limitation in community assembly varies across communities that differ in their past land use, current management, and competitive environment. We conducted identical seed-addition experiments at 108 longleaf pine savanna sites that varied in agricultural and fire history across three locations in the southeastern United States. At each site, we factorially manipulated competitor presence via herbicide and dispersal limitation via seed addition and surveyed plant communities for the next three years. Specifically, to research dispersal limitation, we added seeds of indicator species typically absent in post agricultural areas.
Results/Conclusions
We found that seed addition increased indicator species richness by nearly 0.6, on average, for sites with agricultural history. Overall, sites with forested history had the highest indicator species richness regardless of treatment. Furthermore, seed addition in fire suppressed sites increased indicator species richness by nearly 0.7, on average, when herbicide was applied. Our results suggest that seed dispersal plays an important role in large-scale plant community structuring, which has several implications. First, past land use effects are maintained over the long term since seeds of several native plant species have not arrived in sites where agriculture was abandoned over 50 years ago. Second, these results illustrate the importance of current management techniques, such as use of herbicide to reduce competition in fire suppressed areas. Lastly, our results suggest that the ability of plant populations to colonize new areas (e.g., areas that have been recently restored or made favorable via climate change) will depend upon successful seed dispersal. As a result, factors that facilitate dispersal (e.g., habitat corridors, movement of biotic vectors, movement of seeds by humans) will play important roles in communities recovering from legacies of the past as well as the communities of the future.