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Responses of forest carbon and water coupling to juvenile thinning at the both leaf and individual tree levels in a 16-year old natural Pinus contorta stand Wang, Yi


Large-scale mountain pine beetle infestation (MPB) has generated overstocking lodgepole pine stands in the interior of British Columbia. A critical need is to determine sustainable management strategies to ensure their healthy growth and provision of various ecological functions. The objective of this study is to assess the effects of juvenile thinning on various carbon and water processes (tree growth, tree-level and stand-level transpiration, and leaf-level and tree-level water-use efficiency (WUE)) in a 16-year old, naturally regenerated lodgepole pine stand using an experimental approach. An experiment involving two thinning treatments (T1: 4,500 stems per ha; and T2: 1,100 stems per ha) and one control (C: 27,000 stems per ha) that were randomly assigned in three blocks, was established in 2016 and has been continuously monitored since then. Data on tree diameter at the breast height, sap flow and environmental variables have been collected since the first growing season (June to October), while additional leaf-level measurements on photosynthesis and transpiration were included in the second growing season. The results showed that juvenile thinning significantly increased tree radial growth and tree-level transpiration, but the significant difference between T1 and T2 only occurred under drought conditions (in the summer season of 2017), with T2 being more resilient to the drought effect. At the stand-level, transpiration of the thinned stands were not statistically different before and during the drought, while that of the unthinned stand was significantly reduced over the drought period. Thinning increased the tree-level WUE (the ratio of tree growth to tree transpiration) with greater enhancement in the more heavily thinned stands (T2), but it did not affect the leaf-level WUE (the ratio of leaf photosynthesis to leaf transpiration). The leaf-level iWUE (the ratio of leaf photosynthesis to stomatal conductance) was significantly higher in the unthinned stand than the thinned stands, with no statistical difference between the two thinned stands. We conclude that juvenile thinning had significant and positive effects on forest carbon and water processes at the individual tree level, and the heavier thinning was more effective for trees to cope with drought conditions.

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