Do mycorrhizal symbionts drive latitudinal trends in photosynthetic carbon use efficiency and carbon sequestration in boreal forests?

• Published in: Forest ecology and management 2022-09, Vol.520, p.120355, Article 120355
• Main Authors: Mäkelä, Annikki, Tian, Xianglin, Repo, Anna, Ilvesniemi, Hannu, Marshall, John, Minunno, Francesco, Näsholm, Torgny, Schiestl-Aalto, Pauliina, Lehtonen, Aleksi
• Format: Article
• Language: English
• Subjects: Carbon allocation; Carbon residence time; Carbon use efficiency; Forest Science; Mycorrhiza; Net ecosystem production; Skogsvetenskap; Soil C balance; Soil C:N ratio
• ISSN: 0378-1127; 1872-7042; 1872-7042
• DOI: 10.1016/j.foreco.2022.120355

•Empirical C balance implies a latitudinal trend in forest carbon use efficiency.•The trend is attributable to the carbon use of symbiotic fungi.•A proxy is developed to quantify fungal impacts on forest C balance. There is evidence that carbon fluxes and stocks decrease with increasing latitude in boreal forests, suggesting a reduction in carbon use efficiency. While vegetation and soil carbon dynamics have been widely studied, the empirical finding that ectomycorrhizal fungi (ECM) become more abundant towards the north has not been quantitatively linked to carbon use efficiency. We formulated a conceptual model of combined fine-root and ECM carbon use efficiency (CUE) as NPP/GPP (net primary production/gross primary production). For this, we included the mycorrhiza as gains in plant NPP but considered the extramatrical hyphae as well as exudates as losses. We quantified the carbon processes across a latitudinal gradient using published eco-physiological and morphological measurements from boreal coniferous forests. In parallel, we developed two CUE models using large-scale empirical measurements amended with established models. All models predicted similar latitudinal trends in vegetation CUE and net ecosystem production (NEP). CUE in the ECM model declined on average by 0.1 from latitude 60 to 70 with overall mean 0.390 ± 0.037. NEP declined by 200 g m−2 yr−1 with mean 171 ± 79.4 g m−2 yr−1. ECM had no significant effect on predicted soil carbon. Our findings suggest that ECM can use a significant proportion of the carbon assimilated by vegetation and hence be an important driver of the decline in CUE at higher latitudes. Our model suggests the quantitative contribution of ECM to soil carbon to be less important but any possible implications through litter quality remain to be assessed. The approach provides a simple proxy of ECM processes for regional C budget models and estimates.