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Effects of canopy gaps on soil respiration and microbial activity in uneven-aged Norway spruce stands

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Title: Effects of canopy gaps on soil respiration and microbial activity in uneven-aged Norway spruce stands
Author(s): Roth, Eva-Maria
Contributor: University of Helsinki, Faculty of Agriculture and Forestry
Degree program: Master's Programme in Forest Sciences
Specialisation: Forest Ecology and Management
Language: English
Acceptance year: 2019
Uneven-aged forest management with selective cutting might hold the potential to limit carbon emissions from the soil after harvesting, compared to rotation forest management with clear-cutting, which is currently more common in boreal forests. The permanently remaining tree cover in uneven- aged forests protects the soil and the microclimate within the stand. It is still poorly understood how selective harvesting affects the carbon fluxes and pools of boreal forests. The aim of this research was to find patterns in the soil respiration and soil carbon content in uneven aged stands that are related to canopy gaps resulting from harvesting operations. I tried to assess the effects of the light, soil temperature and soil moisture conditions in canopy gaps on soil respiration, soil carbon content, microbial decomposition and microbial biomass carbon in order to draw conclusions how uneven aged forest management should be adjusted to avoid carbon losses from the soil. Soil respiration was measured during the summer 2018 on two uneven-aged Norway spruce (Picea abies Karst) stands in Lapinjärvi, southern Finland. Measurements were taken with a mobile closed chamber at fixed measuring points under canopy and in canopy gaps. Soil moisture and soil temperature were measured alongside the respiration measurements. Soil cores were collected at each point to a depth of 30 cm in order to determine carbon and nitrogen stocks. The microbial biomass carbon was analysed at each point with the chloroform fumigation extraction method and the microbial decomposition was determined with the teabag method. The Global Light Index (GLI) and the canopy openness were derived from hemispherical pictures to assess the gap size. I found that the soil respiration in canopy gaps was in average 0.0243 mg CO2 s-1 m-2 higher then under canopy. Soil temperature and soil moisture however did not differ between the points in canopy gaps and under canopy and neither did microbial biomass carbon or the microbial decomposition. Microbial decomposition and microbial biomass C differed strongly between the two study plots and were not correlated with the soil respiration. C and N stocks were higher in canopy gaps than under canopy, but the carbon-to-nitrogen ratio was not affected by the canopy gaps. A statistical relationship between GLI or canopy openness could not be confirmed with neither soil respiration, nor soil carbon or microbial decomposition. The main driver affecting soil respiration and C and N stocks in mineral soil was soil temperature. We modelled soil respiration with the collected data. The best fitting model was selected by comparing the second order Akaike Information Criterions. It included soil temperature as fixed effect variable and the collars and time of measurement as random effect variables. The results confirm that gaps have an influence on soil respiration; however, the gap size seems to be less important. Soil respiration is influenced by a number of factors and complex interactions. The increased soil respiration in the gaps is probably caused by a higher autotrophic respiration by roots and mycorrhiza.
Keyword(s): Uneven-aged Forest Structure Continuous Cover Forest Management Soil respiration Soil CO2 Efflux Gap Dynamics

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