Increasing Evergreen Trees Reduces Soil Nitrogen and Aboveground Biomass Accumulation in Canadian Forest Plots

Abstract

Forests mitigate global warming by sequestering 45% of the world’s terrestrial carbon (C), keeping it out of the atmosphere where it amplifies global warming as a greenhouse gas. Carbon is stored in forests within woody tissues and soil organic matter (SOM). Nitrogen availability can limit plant growth and SOM stocks. Understanding which variables increase soil C, N, and ecosystem productivity is of interest in climate change mitigation efforts. Using data from the Canadian National Forest Inventory, we built Linear Mixed Effect Models to investigate the effects of symbiont affiliation, wood carbon: nitrogen ratio (C:N), and evergreen/ deciduous status on changes in soil C and N and aboveground biomass over a 10-year period. We found that decadal change in plot biomass decreased with evergreen dominance and increased with higher mean annual temperature. In the organic and mineral soil horizon, soil N decreased in plots with higher initial soil C: N ratios. C accrual in the mineral horizon decreased in plots with higher initial soil N stocks. N accrual in the organic horizon decreased with evergreen dominance as well (p <= 0.1). Interestingly, symbiont affiliation or wood C:N did not have a significant effect on soil C or N accumulation in either soil horizon. Forest stands with higher proportions of evergreen trees were associated with less biomass and soil N accrual over a decadal timeframe, possibly due to litter quality and microbial decomposition dynamics. In the face of a rapidly changing climate, maintaining forest productivity and C storage is highly pertinent. Understanding the factors that impact aboveground biomass and soil C and N accrual can help inform proper forest management, build accurate terrestrial biosphere models, and ensure that Canadian forests can continue to sustain growth and sequester C. Keywords: Canadian Forests, Soil Carbon Stocks, Soil Nitrogen, Symbionts, Evergreen and Deciduous Trees, Wood Nutrients