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Green-tree-retention harvesting as a tool to maintain soil microbial diversity and function in harvested sites

Churchland, C. T.. 2013. Ph.D. thesis, UBC

Abstract

Green-tree or variable-retention harvesting is increasingly being used in the Pacific Northwest due to the perceived benefits to aboveground biodiversity. Little research has been conducted on the value of this harvesting practice for soil organisms, though retained live trees on harvested sites are thought to benefit belowground biodiversity by acting as "hubs" of both species-specific and symbiotic microbial communities. Access to these communities may be necessary for seedling growth and forest regeneration after harvest. Live trees support microbial communities by maintaining a constant source of labile carbon through litter and root exudates. The aim of this thesis dissertation project was to trace the flow of carbon from live trees retained on clear-cut sites in different variable-retention harvesting regimes into the soil microbial community to determine which variable-harvesting regime best maintained pre-harvest soil microbial communities and soil microbial function. Two variable-retention strategies were compared: aggregate-retention, where intact forest patches are retained, and dispersed-retention, where individual trees are retained across the site. Both harvesting strategies decreased the fungal to bacterial ratio although the dispersed-retention harvesting treatment mitigated the effects of harvesting on soil nutrient availability. Aggregate-retention harvesting, even within 9 meters of the retention patch, did not appear to influence nutrient availability, but evidence suggested the microbial community within this area was supported by recent plant-carbon. Through analysis of stable-isotope natural abundance and application of a novel stable-isotope labeling stem-injection technique, I was able to discern that individual trees support the fungal community up to 20 meters into a clear-cut. However, the lack of recently-derived labile plant carbon in clear-cuts resulted in changes to soil carbon-cycling. The microbial community in clear-cut sites appeared to rely on tightly recycled labile microbial-derived carbon that was probably released during microbial turnover, rather than dissolved organic carbon. In the highly disturbed clear-cut areas, the microbial communities may have lost some of their ability to break down recalcitrant soil organic matter. Both variable-retention strategies investigated affected soil microbial community composition; though it appeared that dispersed-retention best maintained microbial community function on harvested sites.