Carbon and Nutrient Pools and Fluxes in Unmanaged Mountain Norway Spruce Forests, and Losses after Natural Tree Dieback

Abstract

Forest areas infected by insects are increasing in Europe and North America due to accelerating climate change. A 2000-2020 mass budget study on major elements (C, N, P, Ca, Mg, K) in the atmosphere-vegetation-soil-water systems of two unmanaged catchments enabled us to evaluate changes in pools and fluxes related to tree dieback and long-term accumulation/losses during the post-glacial period. A bark-beetle outbreak killed >75% of all trees in a mature mountain spruce forest in one catchment and all dead biomass was left on site. A similar forest in a nearby catchment was impacted only negligibly. We observed that: (1) The long-term C and N accumulation in soils averaged 10-22 and 0.5-1.1 kg ha-1 yr-1, respectively, while losses of Ca, Mg, and K from soils ranged from 0.1-2.6 kg ha-1 yr-1. (2) Only <0.4 0.8% and 0.6-1.5% of the respective total C and N fluxes annually entering the soils from mature vegetation were permanently stored in the catchment soils. (3) The post-disturbance decomposition of the dead tree biomass decreased the elements pools in vegetation from 27% (C) to 73% (P) by 2019. (4) Tree dieback decreased net atmospheric element inputs to the impacted catchment, and increased the leaching of all elements and gaseous losses of C (~2.3 t ha-1 yr-1) and N (~14 kg ha-1 yr-1). The disturbed catchment became a net C source, but ~50% of N released from dead biomass still accumulated in the soils. (5) Despite the severe disturbance, respective leaching losses of Ca and Mg represented 52-58% of their rates from intact stands during peak atmospheric acidification from 1970-1990. (6) Disturbance-related net leaching of P, Ca, Mg, and K were 4, 69, 16, and 114 kg ha-1, respectively, and represented 7-38% of the losses potentially resulting from sanitary logging and removing all the aboveground tree biomass.

Keywords: Bark beetle, Carbon, Nitrogen, phosphorus, Base cations