In post-wildfire landscapes in the western Sierra Nevada, the availability of live, reproductive trees is a strong predictor of conifer regeneration. Planting small “founder stands” has been proposed as a means of establishing future conifer seed sources in high-mortality areas where reforestation is difficult at scale. However, certain post-fire successional processes (e.g., the growth of competing vegetation) are also known to inhibit tree establishment and growth, and these processes may become dominant before planted trees become reproductive. Thus, it is unclear whether a planted founder stand that produces viable seed could plausibly result in seedling establishment and forest expansion in this system. We investigated this by conducting an observational field study testing the contribution of now-reproductive planted trees relative to site characteristics in driving conifer seedling recruitment in unplanted areas. At three locations in the western Sierra Nevada that were planted 22-25 years ago, we surveyed conifer regeneration in transects that crossed plantation boundaries, characterizing tree recruitment, competing vegetation, and proximity to seed sources.
Results/Conclusions
We modeled seedling recruitment as a function of distance to the nearest plantation, distance to other seed sources, competitive environment, and other environmental characteristics that could affect seedling establishment, and we focused analysis on recruitment of trees that were 10 years old or younger (i.e., those that established after plantation trees became reproductive). Seedling recruitment of this younger cohort was negatively correlated with the distance to plantation edges (p < 0.01), which suggests that plantations do contribute to tree establishment outside of the planted area. We did not find a significant effect of shrub cover nor overstory cover on recruitment, leading us to reject the hypothesis that shrub cover limits recruitment even when seeds are present.
We had focused the study on areas that had high post-fire tree mortality in an effort to distill the specific contribution of planted trees rather than trees that had survived the wildfire; however, we still found that distance to surviving trees was a significant predictor of recruitment in our plots (p < 0.05). Thus, the influence of plantations on long-term recruitment and forest expansion is happening alongside natural regeneration from large, surviving trees. Collectively, these results suggest that plantations may be a viable option for catalyzing tree recruitment in unplanted areas, but this strategy may be most relevant in areas where large, surviving trees are not available as seed sources.