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CAPITOL LAND TRUST
Increasing forest health through young-stand thinning & seedling release
The Capitol Land Trust (CLT) owns a 52-acre forested property along the eastern shoreline of Hendersen Inlet north of Olympia, WA. The property was purchased by CLT from the Stillman Tree Farm in 2016 in order to conserve ecologically significant shorelines and upland woodland habitat and demonstrate ecologically-based forest management practices. The Stillman property hosts several forest types, including a shoreline zone along Hendersen Inlet that contains a mix of ages and species of hardwoods and conifers, a small plantation of 35 year old Douglas-fir, and a 20 year old plantation of Douglas-fir that comprises the majority of the property. The fir plantations were established following clearcutting of the prior forest by the previous landowner, and since that time, numerous hardwood and conifer species have naturally regenerated throughout, in particular, the 20 year old fir plantation. One management challenge facing CLT is overstocking and increasing competition between trees in the 20 year old stand. Red alder and big leaf maple have colonized many areas of the stand and are directly competing with the planted fir.
Pre-commercial thinning completed September 2020. Project monitoring underway.
This project treated a 20-year-old dense, mixed hardwood conifer stand with the following two prescriptions:
This stand comprises a Douglas-fir plantation that was established approximately 20 years ago. Although the stand was planted to Douglas-fir, a wide range of other tree species naturalized throughout the area. Red alder is the most common, and occurs both as scattered individuals and in small, dense clumps. In many cases alder has colonized small gaps in the forest where the Douglas-fir did not survive. Big leaf maple is the second most common tree species, and primarily occurs either as multi-stemmed stump sprouts, or as wildly sprouting, old, decadent trees. Naturally regenerating western red cedar, noble fir, grand fir, and western hemlock also occur throughout the forest.
Prior to treatment, stocking densities varied significantly across the forest, ranging from as low as 150 trees-per-acre (TPA) to over 600 TPA, but averaging approximately 400 TPA. Douglas-fir averaged 50’ tall, with diameters ranging from 4” – 14” and averaging 7” at breast height (DBH). Alder averaged 46’ tall, with diameters ranging from 4” – 10” and averaging 4.8” at breast height. The composition of the forest also varied significantly. Although the stand was planted to Douglas-fir, due to seedling mortality and competition, the stocking of the fir varied considerably, with some areas dominated by hardwoods. Further, large maple clumps, in particular when they occurred in groups of more than two, had effectively suppressed most of the fir and nearly every other tree or shrub species within the reach of their broad crowns. Approximately 25% of the Douglas-fir had some form of storm-related damage in their crowns, namely broken or forked tops.
Comparing a treated site to a non-treated site allowed observations of the growth differences between manually released trees with lower stocking densities, and higher stocking densities where trees endure a greater degree of competition.
USDA stand density guidelines for Douglas-fir within this diameter range suggest a residual thinning density of 250 – 360 TPA (average 11’ – 13’ between trees). For alder, a residual thinning density of 200 – 260 TPA (average 13’ – 15’) is recommended. As a climate adaptation strategy to improve forest resilience during predicted longer and drier summers, this project proposed to thin the forest to the lower end of the stand density guidelines to reduce competition for late summer moisture.
Additional thinning prescriptions included:
This project included two 1.25-acre treatment sites. Within each treatment site, four 1/20th-acre (0.05 acres) permanent monitoring plots/acre were installed prior to implementation of prescribed treatment. Each plot was permanently monumented with a rebar stake and 4’ white PVC pipe with bright orange ribbon tied to the top. For each plot, the following data were recorded: GPS plot location, Average stand age, Soil type, Site index & class, Elevation, Rainfall, Aspect, Slope. Immediately following the thinning treatment the plots were remeasured, then measured again 24 months later.