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Adjacent to uplands, forested wetlands found in valley-bottoms of the British Columbia Northern Interior receive nutrient-rich groundwater seepage, and thus have significant timber value. However, because of the poor bearing capacity of the ground and natural surface drainage patterns, these transitional sites to peatlands often become waterlogged after forest practices. Canopy removal typically increases water-table levels by reduced evapotranspiration and equipment disturbance impede surface drainage due to rutting and puddling. These sites are considered to be of concerns for Industry in term of sustaining productivity. A management approach being proposed is that we use prescription drainage with mounding, which is also known as ditch-mounding. A ditch that runs with the main surface water gradient removes excess water from surface substrate while mounding creates raised plantable spot such that edaphic conditions in the rooting zone are improved and frost risk is reduced. Ditch-mounding uses a low density of ditches and can be effective at managing high water table conditions caused by forest activities on small forested wetlands with medium to fine-textured soils. Depending on the proximity to the uplands, a lateral water table gradient may be present confounding the hydrological and tree growth response to ditch-mounding, therefore affecting how these valley-bottoms should be managed. Without the hydrological influence of the adjacent uplands, changes to water table level was apparent on 2 mineral soils and 1 organic soil after ditching. Height growth of hybrid spruce seedlings improved significantly closer to the ditch edge on the wettest mineral soil 6 years since planting. The absence of regeneration response despite an apparent drop in water table was attributed to insufficient time since drainage and weak drainage effect respectively, on the organic soil and the drier mineral soil. On the remaining 3 sites, the closer proximity to adjacent uplands created a spatial gradient of water table. Under these conditions, we believe that drainage may lower the moisture gradient and potentially, improve tree performance but the design of our experimental study prevented us to answer this question. Overall, ditch-mounding significantly increased seedling survival on these wet sites by creating favourable plantable spots for seedlings. Improved soil aerobic and moisture status were found in the mounds formed of either mineral or organic substrate during late summer conditions. However, ditch mounding did not reduce the risk of frost damage to seedlings. The information presented here is based on operational drainage experiments; however our retrospective study has shown apparent improvement in growing conditions as a result of ditch-mounding and increased awareness about management of wetter sites. A more rigorous and sound experimental approach would clearly answer our hypothesis that ditch-mounding systems are effective in rehabilitating disrupted natural drainage of degraded wet sites in Northern BC.
Stephane Dube.
Dubé, Stephane. 2004. Management approaches for rehabilitation of degraded wet sites in BC northern interior: FII forest research program 2003/04 annual progress report. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2004MR184
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