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The nitrogen nutrition of lodgepole pine (Pinus contorta var. contorta) and Sitka spruce (Picea sitchensis (Bong.) Carr.) on a cedar-hemlock cutover on northern Vancouver Island Bothwell, Karen Sheila

Abstract

Nursing mixtures with pine or larch have been widely used in Britain and Ireland to aid the establishment of Sitka spruce (Picea sitchensis (Bong.) Carr.) on low-nitrogen, Calluna dominated heathlands and peatlands. The overall result is increased height and diameter growth and increased foliar N concentrations of the spruce and increased N mineralization rates in the forest floor in mixed stands, about 8-10 years after planting. Cedar-hemlock cutovers are ecologically analogous to Calluna sites, both being dominated by ericaceous shrubs and having extremely low nitrogen availability. Growth check symptoms, including reduced leader growth and chlorosis after 5 to 8 years are akin to symptoms observed in British and Irish plantations. A nurse trial was established on a cedar-hemlock cutover on northern Vancouver Island with the objective of testing the efficacy of lodgepole pine (Pinus contorta Dougl. var. contorta) and western white pine (Pinus monticola Dougl. ex D. Don) as nurse species for Sitka spruce regeneration. There were three treatments (pure spruce, spruce in mixture with lodgepole pine and spruce in mixture with white pine), replicated three times. The treatments were established at three different spacings. The trial was remeasured at ten years, however there was no evidence of increased height or diameter growth or increased foliar nitrogen concentrations of the spruce in mixture. Possible reasons for this are suggested. What was evident was the superior growth and nutrition of lodgepole pine on this nitrogen-poor site. The second part of the research focused on examining possible mechanisms leading to the superior growth of lodgepole pine on sites with low nitrogen availability. A field biomass harvest of pine and spruce trees from a cedar-hemlock cutover and a greenhouse bioassay were conducted to examine whether pine accessed more nitrogen, distributed more biomass and nitrogen above ground, and/or was more efficient in the use of the nitrogen taken up through greater production of biomass per unit of nitrogen taken up and/or through greater nitrogen retranslocation at time of senescence. Rooting distribution patterns were examined to determine if pine might have access to different nitrogen pools. Some mycorrhizae, including some species specific to pine, have access to organic forms of nitrogen, therefore mycorrhizal assessments of the communities infecting the pine and spruce were conducted. The abilities of pine and spruce to access different nitrogen sources, both inorganic and organic were also tested. Lodgepole pine took up more nitrogen and had much larger aboveground biomass production. The greater growth and nitrogen status of pine were attributed to greater foliage efficiency. Pine had a deeper root system and the root tips were heavily infected with mycorrhizal fungi, which have high proteolytic activity such as Suillus.

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