Forest Investment Account (FIA) - Forest Science Program
FIA Project Y071284

    Soil disturbance effects of hoe-forwarding on tree growth and site productivity
Project lead: Douglas, Mary-Jane
Author: Douglas, Mary-Jane
Imprint: Black Creek, BC : Foresol Consulting; KR Brown and Associates; Coast Forest Region, 2007
Subject: Forest Investment Account (FIA), Soils, British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
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 1990ís 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 along the tracks, between the tracks, along the flanks of the tracks, and on the untrafficked controls. Tree growth data collected through 9 years from the Holberg and Woss studies suggest that the use of puncheon is important to protect the soil by reducing disturbance, and subsequent compaction, even where soils were considered relatively 'robust' (Douglas and Courtin 2001, 2002). To further examine the benefits of using puncheon, a third experiment was established in 2001 east of Jordan River on southern Vancouver Island within the CWHxm2 biogeoclimatic variant. Soils were coarse to medium textured loamy sands to sandy loams, suggesting a hazard assessment rating of 'moderate to low' for soil compaction and puddling (BC Ministry of Forests 1999). Trees were felled using mechanical harvesters, and logs were removed from the site using hoe-forwarders. Three treatment blocks were established across the site during operational logging, each with replicates of 1, 2, or 3 passes by the hoe-forwarder. Half of each treatment line was trafficked with puncheon under the machine; the other half was trafficked without protection other than residual slash. A total of 18 randomly selected treatment lines (3 blocks by 6 treatment lines) were established on this site. Trafficking by the mechanical harvester precluded establishment of a 'control' treatment. Douglas-fir was planted along the trafficked lines at 2 m spacing. An additional experiment was also established on this site to assess the effect of specific soil disturbance types (Forest Practices Code Soil Conservation Surveys Guidebook; BC Min of For 2001) on growth of planted seedlings. Utilizing the tracked lines, specific locations with ruts of three depth classes (< 5 cm, 5 to 30 cm, and >30 cm) were noted, in addition to the occurrence of no measurable compression by the hoe-forwarder. Areas where the forest floor only had been removed (scalped) and where both forest floor and mineral soil had been removed (gouged) were also identified. Areas where both the forest floor and mineral soil appeared undisturbed within a 1.5 m radius of the point were identified as 'control' points. Each point was planted with Douglas-fir seedlings in February 2002. Approximately 50 scalped and 50 'undisturbed' points were identified in each block; only a few 'gouged' locations were noted in two of the blocks. Three areas with 'repeated trafficking' were also identified, and planted with 6 to 10 trees in each at a minimum of 2 m spacing. We propose to re-measure tree growth in both Jordan River experiments as part of this project. Soil water content was measured at the time of trafficking on all three sites; soil physical data was collected only on the Holberg and Jordan River sites. Soil disturbance data were collected from all 3 sites. Tree growth has been measured at Holberg and Woss after 1, 3, 5, and 9 growing seasons (Douglas and Courtin 2001, 2002); seedlings at Jordan River have been measured after 1 and 3 years (Douglas and Courtin 2004). Research results have been extended to field foresters through Extension Notes published online, field tours, soils training courses, and conferences. Two more recent presentations included the FORREX conference in Nanaimo in March 2004, and the Northwest Forest Soils Council field tour in July 2005.
Related projects:  FSP_Y082284FSP_Y093284
Contact: Douglas, Mary-Jane, (250) 337-1834,


Executive Summary (29Kb)

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Updated August 16, 2010 

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