Tree recruitment and forest expansion following reforestation
Principal Investigator: Tara Ursell
Project Partners: Hugh Safford, Ph.D.
Institution: University of California, Davis
Project Type: Graduate Student
Grant Award #8GG18807
Amount awarded: $61,250
Award Date: September 2018
Post-wildfire regeneration in many fire-adapted forests in the Western United States is dependent on the availability of seed from nearby surviving or unburned trees (“seed trees”). In the context of increasingly frequent, high-severity wildfires that result in vast areas without surviving seed trees, planting can enable regeneration by alleviating seed availability constraints. Theoretically, planted trees can also provide a seed source for further seedling establishment once the planted trees become reproductive. In fact, the strategic planting of trees to achieve broader landscape restoration has been investigated in other ecological systems, such as in deforested tropical areas and in abandoned agricultural land. Planting small “founder stands” also has been proposed for California forest types as a strategy to promote forest recovery in areas where seed trees are scarce and where reforestation is difficult to achieve at large scales. However, it is unclear whether changing site quality over time (for example, through the growth of competitors like woody shrubs) limits tree establishment in the long term, even if seed availability increases. Much of what is known about seedling establishment in California’s pine-dominated and mixed-conifer forests comes from studies performed soon after wildfires, when shrubs are small or not present and when resources like soil, water, and light are readily available. Changes in vegetation that occur in the years following wildfire present unique challenges to tree establishment and growth – for example, shrub competition is known to reduce the growth and survival of trees, particularly among shade-intolerant pine species. This makes the long-term, landscape-level impact of reforestation difficult to predict.
This project is designed to address questions about whether planted trees contribute to seedling establishment and forest expansion away from the planted area, given the many other factors that may also influence tree establishment. To achieve this, we survey seedlings and site characteristics within and in varying distances from planted stands that are newly reproductive. These surveys allow us to investigate the relative impact of seed availability (proximity to reproductive planted trees) compared with site conditions (competing vegetation, substrates, etc.). This information should be useful for more precisely predicting the expected long-term effect of reforestation – both across space and over time. We hope that this study will help guide efforts for post-wildfire forest management.