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Soil disturbance effects of hoe-forwarding on tree growth and site productivity Douglas, Mary-Jane

Abstract: This project will assess the long-term effects of soil disturbance on site productivity through measurement of tree growth and nutritional status in three existing field experiments established 5, 14, and 15 years ago. For more than a decade, it has been recognized that ground-based harvesting systems disturb forest soils. Compaction and rutting of soils may increase soil density, potentially reducing root growth, and decreasing soil aeration and rooting volume (Heilman 1981; Minore et. al 1969). These effects may lead to short-and long-term reductions in tree and forest productivity. Compaction and rutting vary with soil texture, forest floor thickness, and soil water content at time of harvest (Millar and Sirois 1986, Corns 1988). In addition, machine type, size and operating practices will directly influence the level of disturbance. Concerns about soil disturbance levels from ground-based harvesting systems lead to development of the 'Site Degradation Guidelines for the Vancouver Forest Region' (BC Ministry of Forests 1991). Long-term data from properly designed experiments, which can be used to validate soil disturbance guidelines are lacking. Attempts have been made to assess longer-term effects on tree productivity through retrospective studies on previously logged sites (Thompson 1989, 1990; Douglas and Schwab 1991). Data from such studies are difficult to interpret, due to lack of proper experimental design, limited knowledge of site conditions at the time of harvest, and, typically, insufficient numbers of sample trees (Douglas and Schwab 1991). Proper assessments of site conditions and treatments at the time of harvesting, and in subsequent years is essential. To address the question of how soil disturbance affects tree growth over the short- and long-term, this project will measure tree growth and nutrition at 3 existing research sites established 5-15 years ago following trafficking with varying number of passes by a hoe-forwarder. Hoe-forwarding was introduced to coastal B.C in the early 1990s for use on gently sloping terrain. The system was expected to reduce soil disturbance by reducing ground pressure, particularly when puncheon (logs and debris placed under the tracks of the machine) was used under the machine. However, the level of soil disturbance, and the actual effects of the hoe-forwarding machines on site productivity were unknown at the time. Two benchmark studies were therefore established on Vancouver Island in the early 1990?s to record levels of soil disturbance created by this machine, and to measure tree growth over the longer-term as an indication of site productivity. Periodic results from these two trials to Year 9 have provided site-specific growth response data in relation to these varying levels of soil disturbance. A description of these two benchmark trials is presented below. The first trial was established in 1991, in the CWHvh1 biogeoclimatic variant, west of Holberg. Soils were fine-textured silt loams with thick (45 cm) forest floors. Three replicate blocks were established, consisting of 1, 2, 4 and no passes (control) with the hoe-forwarder (12 treatment lines). The site was trafficked during the summer under moist soil conditions, and protective puncheon was used under the hoe-forwarder at all times. Western hemlock seedlings were planted along the tracks, between the tracks, and on the flank of the tracks. A row of trees was also planted on the untrafficked control. The second trial was established in 1992 in the CWHmm1 biogeoclimatic variant, south of Woss. Soils were coarse to medium textured loamy sands to sandy loams. The site was operationally logged during the winter, without any protective puncheon. Fifteen treatment lines were established along access trails trafficked twice by the hoe-forwarder. An additional fifteen lines were established in undisturbed areas immediately adjacent to the tracked lines (total of 30 treatment lines). Douglas-fir seedlings were planted ...
Douglas, Mary-Jane. 2007. Soil disturbance effects of hoe-forwarding on tree growth and site productivity. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2007MR316
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), Soils, British, Columbia
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