Low thinning and crown thinning of varying intensities as restoration tools at Redwood National Park

Teraoka, J.; van Mantgem, Phillip; Keyes, C.

Approximately half of Redwood National Park is comprised of young, second-growth forest. Managers at Redwood National Park are using silvicultural treatments to adjust species composition, increase tree growth, and improve stand vigor. Low thinning (the removal of trees in the lower crown classes to benefit trees in the upper crown classes) has been utilized by the Park as the primary silvilutural method, but other silvicultural treatments may be effective in meeting restoration objectives. This study assessed the effectiveness of four silvicultural treatments as forest restoration tools in terms of stand structure and tree growth response at Redwood National Park’s A-972 Forest Restoration Study. Prior to treatment, the 48-year-old stand averaged 1,600 trees ha-1 and 70.0 m2 ha-1 basal area and was dominated by Douglas-fir with lesser amounts of coast redwood. In 2008, four treatments were used: low thinning and crown thinning (the removal of trees in the upper crown classes to benefit trees in the same crown classes) each with two basal area retention treatments of 80% and 45%. The study area included 20 blocks (0.25 ha each): four replicates of each treatment and four untreated controls. All trees greater than 10 cm dbh were inventoried in monitoring plots (0.08 ha) located at the center of each block. Plots were surveyed before and after treatment in 2008 and again in 2013. Results indicated that stand density (number of trees and basal area) were reduced immediately following treatment in 2008 for all thinned plots compared with untreated controls (F≥20.0, df=4, P<0.001), with differences still apparent five years after thinning. Reductions in stand density were more pronounced in the 45% retention treatments of both the low and crown thinning. Five years after thinning, growth was enhanced for all treatments with net basal area gains of 30% for both low and crown thinnings with 45% basal area retention, 24% for crown thinning with 80% basal area retention, 20% for low thinning with 80% basal area retention, and 14% for the control. Comparing individual treatments showed that growth improvement was significant for the 45% retention treatments (P<0.001), while improvement in growth in the 80% retention treatments was marginal or non-significant (P≥0.06). Coast redwood appeared to respond more positively to thinning compared to Douglas-fir (P<0.001) across all treatments. We conclude that all the treatments improved stand conditions, but the 45% retention treatments were more effective in accomplishing restoration objectives. These results lend support to utilizing higher intensity thinning treatments that incorporate crown thinning for greater restoration potential. Higher intensity thinning may also prove to be more effective in the future, as funding constraints and removal of roads makes second-entry restoration treatments unfeasible.

Low thinning and crown thinning of varying intensities as restoration tools at Redwood National Park

Collections