Boreal forests are frequently subjected to disturbances, including wildfire and clear‐cutting. While these disturbances can cause soil carbon (C) losses, the long‐term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear‐cutting are similar (0.15 and 0.20 Mg ha⁻¹ year⁻¹, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid‐succession (e.g. 15–80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build‐up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal‐saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N₂‐fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N₂‐fixation with the functioning of vascular plants. Neglecting nonvascular N₂‐fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N₂‐fixing plants are more common. Adequately accounting for nonvascular N₂‐fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N₂‐fixation responses to global change factors (elevated CO₂, N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N₂‐fixation using nine Earth system models. The majority of empirical studies found positive effects of CO₂, warming, precipitation, or light on nonvascular N₂‐fixation, but N deposition strongly downregulated N₂‐fixation in most empirical studies. Furthermore, we found that the responses of N₂‐fixation to elevated CO₂ were generally consistent between models and very limited empirical data. In contrast, empirical‐model comparisons suggest that all models we assessed, and particularly those that scale N₂‐fixation with net primary productivity or evapotranspiration, may be overestimating N₂‐fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N₂‐fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N₂‐fixation and that field studies include more targeted measurements to improve model structures and parameterization.

Anthropogenic nitrogen (N) pollution is a cause of eutrophication globally ¹ . However, recent datasets indicate that some ecosystems may be experiencing widespread oligotrophication—declining N availability—which is suggested to be a response to elevated atmospheric carbon dioxide (CO ₂ ) ² . Plant N isotope (δ ¹⁵ N) chronologies have served as primary evidence for oligotrophication, but there is wide disagreement whether rising CO ₂ or temporal changes in N deposition explain these patterns ^3–6 . Here we construct δ ¹⁵ N tree-ring chronologies using archived samples from Sweden’s 23.5-million-hectare forest area from 1961 to 2018. The study area spans a 1,500-km latitudinal distance where N deposition varies fourfold, but where rising CO ₂ is spatially uniform. Our data show declining δ ¹⁵ N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed-effects models showed that rising CO ₂ is the strongest predictor of δ ¹⁵ N values, whereas N deposition variables, temperature and forest basal area had lower explanatory power. Our findings suggest that elevated atmospheric CO ₂ is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle ^7–9 .

Most plant diversity in temperate deciduous forests is found in the ground layer, but nearly all studies comparing plant community assembly using taxonomic, trait, and phylogenetic diversity indices are limited to woody plants. To examine the relationship between short-term ground-layer plant community assembly and disturbance severity, we leveraged a silvicultural experiment that applied a combination of harvest and site preparation treatments in a northern hardwood forest in Michigan, USA. We predicted that after two growing seasons, plant communities would be less sensitive to harvest treatments when compared with site preparation treatments that disturb the rhizosphere and modify rooting substrate. We also predicted that an increase in taxonomic diversity would accompany a decline in trait diversity and phylogenetic diversity. Instead, plant species composition responded similarly to harvest treatment and site preparation treatment. However, our measure of disturbance severity was positively correlated with both trait diversity and taxonomic diversity but negatively correlated with phylogenetic diversity, indicating that increasingly diverse traits and taxonomies along this disturbance severity gradient were comprised of more phylogenetically simple plant communities. Informed management decisions should therefore consider the underlying value of each diversity measure, as taxonomic diversity alone may not be the best metric for assessing plant community assembly.