Living, dead, and absent trees-How do moth outbreaks shape small-scale patterns of soil organic matter stocks and dynamics at the Subarctic mountain birch treeline?

Nele Meyer,Kristiina Karhu,Outi‐Maaria Sietiö,Angela Martin,Otso Suominen,Lona Van Delden,Katri Karjalainen,Sylwia Adamczyk,Kevin Mganga,Kristiina Myller,Yi Xu,Christina Biasi

doi:10.1111/gcb.15951

Abstract

Mountain birch forests (Betula pubescens Ehrh. ssp. czerepanovii) at the subarctic treeline not only benefit from global warming, but are also increasingly affected by caterpillar outbreaks from foliage-feeding geometrid moths. Both of these factors have unknown consequences on soil organic carbon (SOC) stocks and biogeochemical cycles. We measured SOC stocks down to the bedrock under living trees and under two stages of dead trees (12 and 55years since moth outbreak) and treeless tundra in northern Finland. We also measured in-situ soil respiration, potential SOC decomposability, biological (enzyme activities and microbial biomass), and chemical (N, mineral N, and pH) soil properties. SOC stocks were significantly higher under living trees (4.1±2.1kg m²) than in the treeless tundra (2.4±0.6kg m²), and remained at an elevated level even 12 (3.7±1.7kg m²) and 55years (4.9±3.0kg m²) after tree death. Effects of tree status on SOC stocks decreased with increasing distance from the tree and with increasing depth, that is, a significant effect of tree status was found in the organic layer, but not in mineral soil. Soil under living trees was characterized by higher mineral N contents, microbial biomass, microbial activity, and soil respiration compared with the treeless tundra; soils under dead trees were intermediate between these two. The results suggest accelerated organic matter turnover under living trees but a positive net effect on SOC stocks. Slowed organic matter turnover and continuous supply of deadwood may explain why SOC stocks remained elevated under dead trees, despite the heavy decrease in aboveground C stocks. We conclude that the increased occurrence of moth damage with climate change would have minor effects on SOC stocks, but ultimately decrease ecosystem C stocks (49% within 55years in this area), if the mountain birch forests will not be able to recover from the outbreaks.

Highlights

Mountain birch (Betula pubescens Ehrh. ssp. czerepanovii) is the dominant tree species at the subarctic treeline of Fennoscandia, and significantly contributes to the CO2 sink capacity of subarctic landscapes (Christensen et al, 2007)
We found that soil organic carbon (SOC) stocks were on average significantly higher under living trees compared with treeless tundra (Figure 3c)
We found lower NO3 contents and SIRcum under dead trees compared with living trees, which points to smaller amounts of available nutrients

Summary

Introduction

Mountain birch (Betula pubescens Ehrh. ssp. czerepanovii) is the dominant tree species at the subarctic treeline of Fennoscandia, and significantly contributes to the CO2 sink capacity of subarctic landscapes (Christensen et al, 2007). While mountain birch can compensate defoliation to some extent (Huttunen et al, 2012), intense damage in such nutrient-poor soils and in cold climatic conditions often leads to forest dieback, leaving large areas of deadwood behind (Tenow, 1996). Considering that 5,000 km of mountain birch forest were affected in the 1960s in northern Finland alone (Nuorteva, 1963), and 10,600 km during the 2000s in northern Fennoscandia (Jepsen et al, 2009), this issue is of quantitative relevance. The combination of grazing and moth damage induces a conversion of former forests into treeless tundra vegetation, resulting in immense reductions of tree primary production (Olsson et al, 2017), and CO2 sequestration potential

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