Seed limitation and microsite limitation at Tenalquot Praire, in Western Washington

Background/Question/Methods The Garry Oak Ecosystem is in rapid decline with some estimates stating that less than 5% of the original system remains intact. Anthropogenic pressures such as fire exclusion, urban development, and fragmentation have all contributed to the ecosystem’s degradation and has facilitated invasion by many non-native species, placing further pressure on native plant populations. Invasive plant species impact ecosystems by lowering native diversity, changing the vegetation structure, impacting soil chemistry, altering fire regimes, and affecting native fauna. As land managers struggle to make appropriate and timely restoration choices, decisions are often made based on assumptions and anecdotal information. When working to increase native diversity and decrease the number of invasive species, it is difficult to decide between adding the maximum amount of costly native seed or allocating resources toward the preparation of the restoration site before seed addition.  It would behoove us to know whether or not native plant populations are more constrained by seed limitation or by microsite limitation or if it is some gradient of the two. In order to answer this question, I set up a fully randomized 2-way factorial experiment with four levels of seed augmentation (no, low, medium, or high levels of seed added) and four levels of microsite manipulation (no, low, medium, and high levels of burning) in a Western Washington Prairie in the summer of 2008. Using 5 x 5 meter plots, I established four burning treatments replicated four times for a total of 16 plots.  Within each plot, I placed four 1m² seed augmentation treatments so that each plot contained all four levels of seed addition. I surveyed for seedlings in the spring of 2009.

Results/Conclusions Higher levels of seed addition always resulted in greater numbers of native seedlings. Microsite manipulation also had a significant effect, suggesting that native plant populations are strongly constrained by both seed limitation and microsite limitation, though the greatest number of seedlings occurred in the medium level burning treatment, suggesting that factors other than the simple exposure of bare soil may significantly impact the ability of seedlings to germinate and establish.