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Some ecological effects of operations used to convert densely stocked lodgepole pine stands into young pine plantations in west central British Columbia Blackwell, Bruce Alan


Large areas of the interior of B.C. are covered with densely stocked lodgepole pine {Pinus contorta) forests, 40-60 years old. These forests are growing very slowly and some are not considered to be capable of contributing to future timber supplies. When stands have densities around 20,000 stems/ha or greater they are unlikely to produce merchantable trees within a reasonable period of time without treatment. To bring the more repressed stands back into timber production it is necessary to clear them and regenerate new ones with a more desirable stocking level (rehabilitation). A study of the ecological effects of rehabilitation treatments on such stands in the Sub-Boreal Spruce Zone in the Lakes Forest District, west central B.C. was begun in July, 1985. This study quantified the effects of treatments on fuels, ecosystem nutrient status, associated vegetation, mineral soil and planted lodgepole pine seedlings to help assess the most economically and ecologically desirable method of treatment. The study investigated the effects of two treatments - mechanical knocking down of trees followed by broadcast slashburning and mechanical knocking down followed by windrowing then burning - and 4 sets of burning conditions designed to give 4 different types of burning severities based on fuel consumption. Each treatment/burning severity was replicated 2-3 times on plots approximately 1 ha in size. The knocking down commenced in late 1985 and 3 of 4 sets of burns were carried out during spring and summer 1986. The remaining set of burns was carried out during spring, 1987. It was found that over the range of burning conditions used, windrow burns consumed similar amounts of slash fuels, unlike broadcast burns which consumed greater amounts of slash fuel as the fuel moisture codes of the Canadian FWI system increased. Forest floor consumption was generally only a small proportion of the total fuel consumption. Total nutrient losses decreased in the order N > S > P > Mg > Ca > K > Na. For the windrow burns, generally > 90% of all nutrients present in organic matter were lost during the burning. Nutrient quantities lost from windrow burns were significantly greater than quantities lost from broadcast burns. Windrow burn nutrient losses were generally greater than those lost from operational broadcast slashburns and were more similar to reported losses from whole tree harvesting operations. Some increases and some decreases in nutrient quantities were observed in the surface mineral soil in both the treatments. The greatest changes in soil nutrients were generally observed for Ca and N. Fire severity significantly influenced the loss of both total and mineralizable N, while both site preparation treatment and fire severity influenced the loss of S. Relatively large increases in mineral soil nutrient quantities were observed beneath windrows. The mineral soil inter-windrow areas, however were found not to be greatly affected by the treatment. The large increases beneath windrows were attributed to the burning of large fuel accumulations. Lodgepole pine seedling survival after two years was highest on areas between windrows and lowest on areas beneath windrows. The better survival in areas between windrows was attributed to improved soil temperature, while poorer survival in windrows was attributed to moisture stress caused by the creation of a hydrophobic layer or seedlings being planted in ash rather than mineral soil. Total height, height increment and basal diameter of lodgepole pine seedlings were greater on areas between windrows and least on broadcast burned areas. Biomass of understory vegetation during the first two post-treatment growing seasons decreased in the order: herbs > shrubs > mosses. Neither site preparation treatment nor fire severity appeared to have a significant effect on herb, shrub, or moss biomass during the first two post-treatment growing seasons. For individual species, biomass decreased in the order: Epilobium > Cornus > Spirea > Rosa > Linnaea.

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