Boreal tree species change as a climate mitigation strategy: impact on ecosystem C and N stocks and soil nutrient levels

Abstract

To increase the annual uptake of CO 2 as well as the long-term storage of carbon (C) in forests, the Norwegian government consider large-scale replacements of native, deciduous forests with fastergrowing species like Norway spruce. To assess the effects of tree species change on ecosystem C and nitrogen (N) stocks and soil chemistry, we used a paired plot approach including stands of native downy birch and planted 45-to 60-yr-old Norway spruce. The birch stands were used as reference for the assessment of differences following the tree species change. We found significantly higher C and N stocks in living tree biomass in the spruce stands, whereas no significant differences were found for dead wood. The cover of understory species groups, and the C and N stocks of the aboveground understory vegetation were significantly higher in the birch stands. The tree species change did not affect the soil organic carbon (SOC) stock down to 1 m soil depth; however, the significantly higher stock in the forest floor of the spruce stands suggested a re-distribution of SOC within the profile. There was a significant positive correlation between the SOC stock down to 30 cm soil depth and the total ecosystem C stock for the birch stands, and a negative correlation for the spruce stands. Significant effects of tree species change were found for C and N concentrations, C/N, exchangeable acidity, base saturation, and exchangeable Ca, K, Mg, Na, S, and Fe in the organic horizon or the upper mineral soil layer. The total ecosystem C stock ranged between 197 and 277 Mg/ha for the birch stands, and 297 and 387 Mg/ha for the spruce stands. The ecosystem C accumulation varied between 32 and 142 Mg/ha over the past 45-60 yr, whereas the net ecosystem C capture was considerably lower and potentially negative. Our results suggest that the potential to meet the governments' targets to increase C sequestration depend on the C debt incurred from the removed birch stands, the rotation length, and potentially also the susceptibility of the different stand types to future risk factors related to climate change.

Shift in tree species changes the belowground biota of boreal forests

“…Historically, the birch stands have been subjected to rough grazing and selective harvesting. The spruce stands were relatively dense compared with the birch, with a total age of c. 45-60 yr (Kjønaas et al, 2021). The understorey vegetation in all birch stands consisted of a mix of dwarf shrubs (mainly Vaccinium myrtillus L.), herbs, ferns, graminoids and bryophytes.…”

“…The understorey vegetation in the spruce stands was dominated by bryophytes that varied in density and distribution. Detailed site descriptions, sampling design, the impact of tree species change on tree biomass, understorey vegetation cover, soil chemistry and ecosystem SOC and N stocks are reported in the Methods S1 and by Kjønaas et al (2021).…”

“…We measured concentrations of ergosterol (as a proxy for fungal biomass) in freeze-dried and finely ground soil fractions, following the protocol of Ransedokken et al (2019). Methods on soil chemistry (exchangeable elements, pH, organic matter and C and N concentrations), soil SOC and N stocks, understory vegetation cover and biomass and aboveground C and N stocks are given in Kjønaas et al (2021).…”

“…New Phytologist (2022) 234: 2073-2087 www.newphytologist.com Ó 2022 The Authors New Phytologist Ó 2022 New Phytologist Foundation a large extent located in the LFH layer (Puhe, 2003), and the fine root production and biomass of spruce trees may potentially be higher relative to birch (Hansson et al, 2013b). Due to the significantly higher understorey vegetation biomass in birch relative to spruce stands (Kjønaas et al, 2021), the total flux of aboveground and belowground litter may be similar between the tree species, as found by Hansson et al (2013a). However, the chemical composition of the litter is expected to differ between the stand types.…”

“…Change in tree species from native birch to faster-growing spruce has been proposed in Norway to increase annual CO 2 uptake and long-term carbon (C) storage in forests (Norwegian Ministry of Environment, 2012). In western Norway, the consequences of these changes include shifts in the structure and biomass of understorey vegetation, changes in soil C stocks and chemistry, and a substantial increase in the total ecosystem C in spruce (357 AE 22 Mg ha À1 ) compared to birch stands (208 AE 7 Mg ha À1 ) (Kjønaas et al, 2021).…”