Constructing alternate states: Invasive seed bank dominance in restored versus reference vernal pools

Background/Question/Methods

The soil seed bank can be a viable metric in understanding the potential future of a community as well as its history.  Vernal pools, or ephemeral wetlands, are a system that builds up substantial seed banks through their annually dominated communities and highly variable climates.  Our primary objectives in this study were to determine seed bank densities within vernal pools and their germination responses to different inundation treatments. We then used these results as a proxy for the potentialof the aboveground community to answer our primary question: Are constructed vernal pools in alternate states compared to naturally occurring pools?  To answer this we used a long term vernal pool restoration experiment divided up into three pool types: 1) Constructed for restoration 2) Naturally occurring pools dominated by invasive species, 3) Naturally occurring pools dominated by native species. We then collected soil seed bank samples within each of these pool types at three distinct gradient zones within the pool (bottom, transition, and edge).  These soil samples were then split into three equal parts and germinated in the greenhouse under conditions that mimicked field conditions (always inundated, intermittently inundated and never inundated).

Results/Conclusions

The sampled gradient zone was the driving factor for total seed densities; the highest seed density among group types and inundation treatments was found in the pool bottom zone.  Pool group type and inundation treatment had limited importance on the total seed densities counted.  These results differed however when the total seed counts were broken up into invasive and native categories.  The sampled gradient zones again drove the abundance of native seeds found, yet the pool type determined invasive species densities.  Native species may be more influenced within pool conditions where invasive species presence may be determined at the pool level.  Though the constructed pool type did not have less total seeds present they did have a higher instance of invasive species.  These proportions, though higher, were similar to the naturally occurring invasive dominated pool type.  Constructed and invasive dominated pools appear to be in alternate states where the seed bank is inadequate to return native species to the vegetation community.  Finally, because the greenhouse inundation treatments had little impact on the total observed seed bank germinants it may take more than an environmental shift to return the degraded pools to those of the naturally occurring native dominated pools.