Growth and yield, structure, composition, and soil compaction in a Western Oregon Douglas-fir forest after 35 years of modified selection thinning Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1c18dk708

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  • Modified selection thinning has been utilized by some non-industrial foresters in Douglas-fir forests of Western Oregon and Washington for at least 35 years. This silvicultural strategy has not been tested, but has often been associated with reduced volume production and other undesirable effects. It continues to be used on many small woodland properties. This case study evaluated a management system that has utilized modified selection thinning and natural regeneration since 1964, on a 250 acre private forest tract on the east slopes of the Oregon Coast Range. Six conditions (Types) were identified and mapped on the property based on the number of thinning entries and stand vegetation conditions. Current field data along with past cruise information and aerial photos were used to assess past and current volume production, stand structure, species composition arid soil compaction. Stand growth and yield was projected for 100 years using a simulation model to compare selection thinning and short rotation evenage management. At the Tract level, inventory records showed standing conifer volume from 1964-95 increased from 1,939 MBF (thousand board feet) to 2,150 MBF, while 5,606 MBF of volume was removed. Harvests occurred every 1-4 years with an average volume of 715 bd. ft./acre/year removed. Stands in early stages of conversion with modified selection thinning were growing at 80-100% of potential based on regional growth and yield tables for stands of similar site quality and stocking. Volume growth in stands in intermediate stages of conversion was 35-70% of potential compared to the same yield tables adjusted for stocking. Under a similar management regime, simulated total Tract standing volume increased to 4,250 MBF after a 100-year projection with harvests of 600 bd. ft./acre/year. Tract average periodic annual increment after 100 years was similar to current levels. Simulation studies showed that selection thinning yielded 9% less total Tract board foot volume (standing + harvest) than 50-year rotation evenage management after a 100- year simulation. Two of six stand Types simulated with selection thinning produced yields within 2% of the 50-year rotation evenage system after 100 years. The other four Types produced from 8-20% less volume. Under modified selection thinning the Tract contained 3 times more standing volume, and produced 2.8 times more volume in logs greater than 16 inches (scaling diameter) after 100 years. Management using modified selection thinning maintained a per acre average of at least 10 MBF/acre standing timber over 90% of simulation period vs. 40% for the evenage regime. Modified selection thinning between 1964-1995 led to changes in stand structure and composition. Both conifer reproduction and shrub cover increased in stands with more thinning. Conifer basal area decreased with thinning but the range of tree diameters increased. Composition of natural regeneration in the understory of stands thinned 9-12 times was dominated by Douglas-fir with 47% stocking and 290 trees per acre. Past and current cruise reports along with aerial photographs indicate hardwoods increased during the management period between 1964-97. Soil compaction was significantly higher in areas thinned 9-12 times vs. those thinned 1-4 times (p-value =.04), and was significantly higher in areas with less than 35% slope (p-value =.0O1). Across the Tract 60% of sampling points fell in a low compaction class, 33% in moderate, and 4% in high. Uneven-aged stand structure in a group selection sense was observed at the Tract level. Individual stands or plots did not contain functional individual tree selection uneven-aged structure. However, stands thinned repeatedly were two-storied in places. Based on comparison of different stand Types it appears modified selection thinning in Douglas-fir stands could be used to perpetuate even-aged structure, or to convert stands towards an uneven-aged condition. In either scenario, management is recommended to address increased hardwood stocking, inconsistent natural regeneration of conifer species, and understory growth losses caused by reserve "legacy" trees.
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