The recent 5-year drought and accompanying massive bark beetle outbreak in the Sierra Nevada region caused unprecedented conifer mortality. These are just two symptoms in a growing list of indicators that our climate is changing. California is also experiencing longer fire seasons, more extreme fires, life-threatening mudslides, species migration, and more.
To respond to these changing conditions, forest scientist Pascal Berrill (Humboldt State University) has been experimenting with new silvicultural techniques for forest restoration, such as variable-density retention (VDR). VDR
involves removing undesirable and unhealthy trees in a stand while keeping the healthy, desirable ones, regardless of their location, and then planting a new crop of trees among the healthy trees as well as in the open canopy gaps created by tree removals. At a Douglas-fir/tanoak forest (Humboldt County) study site, non-merchantable hardwoods and low-vigor, small conifers were harvested, creating a mosaic of different densities over the landscape (66 x 66 ft patches) as well as varying light levels and competition in the understory. Different environmental factors were found to affect growth of redwood seedlings planted at the site (outside their natural range) versus the native Douglas-fir seedlings planted, although both thrived. The VDR system met management goals of recreating a patchy forest mosaic, restoring conifer dominance, and enhancing structural complexity by introducing a new age class of trees growing at different rates.
In Sierra Nevada forests, including those affected by bark beetles and wildfires, the VDR system could be applied to remove unhealthy trees and regenerate new age classes of trees. Harvest efforts could mimic a mixed-severity fire regime in which fire kills smaller trees as well as random patches of trees, promoting tree regeneration. Additionally, instead of relying on natural regeneration from the local seed source, planting seedlings selected for resistance to disease or better adapted to the changing local climates of the future could be implemented.
Planting a new age class of trees in patches of high-severity burning, especially high-priority areas for restoration (i.e., adjacent to streams, steep slopes) will help stabilize these areas over the longer term. This could be done in tandem with short-term mitigation strategies such as felling some (unhealthy/dead) trees to create physical barriers to erosion, and establishing buffer strips of low-statured vegetation. Restoration of erosion-impacted sites is more difficult, and may require establishment of different species better adapted to degraded sites that will tolerate the poor conditions, prevent further degradation, rebuild soil, and possibly act as a nurse crop creating conditions favorable for additional species to become established over time and restore biodiversity.