Silviculture to facilitate marbled murrelet nesting structure

Background/Question/Methods

In Washington State, marbled murrelets (Brachyramphus marmoratus) typically nest on large branches in the upper canopy of old-growth trees within 52 miles of marine foraging habitat.  They are federally listed as threatened primarily because of the loss of nesting habitat to timber harvest.  Given the temporal scale of old-growth structural development, the recovery of the species will likely require decades, if not centuries, of preserving current nesting habitat while restoring degraded habitat.  In forests where growth is limited by light, we established an experiment to investigate if silvicultural treatments could help to reallocate tree growth resources to increase branch size and nesting structure in the upper canopy of second-growth western hemlock and Douglas fir trees.  Two treatments were randomly assigned to co-dominant trees at spatial intervals of at least 180’: 1) eliminating inter-tree competition for light to stimulate overall tree growth including upper canopy branches (e.g., creating 40’-radius “gaps” around trees), and 2) removing “leader” branches to stimulate within tree growth of branching structures immediately below the leader (e.g., “topping” upper 20’ of trees).  The diameters of upper canopy branches were measured to the nearest tenth of an inch. 

Results/Conclusions

The experiment was installed in a 75-acre stand of 65-year-old forest in October, 2010, at an elevation of 2,000’ above mean sea level.  The eight block design incorporated two tree species, two treatments, and controls.  Budget limitations restricted the sample size to 48 trees (six replicates per block).  Initial measurements indicate that co-dominant western hemlock trees grow (both in height and diameter) slower than Douglas fir, and there is some evidence that upper canopy branches in western hemlock are slightly smaller than in Douglas fir (p=0.078).  There is strong evidence of larger variation in upper canopy branch sizes for western hemlock than for Douglas fir (p=0.012).  Treatment effects on branch growth will be evaluated with remeasurements beginning in 2021.  Treatments were relatively cost-effective and replication will likely occur at other locations as forest restoration work progresses.