Resource quantity and heterogeneity drive successional plant diversity in managed and unmanaged boreal forests

The understory vegetation of boreal forests plays a crucial role in maintaining biodiversity by creating habitats, supplying food resources, and regulating microclimate and soil conditions. This essential layer is frequently affected by disturbances such as forest fires and clear-cutting, which significantly alter understory communities and the ecosystem resource availability and heterogeneity. This study aimed to understand how these disturbances influence the spatial and temporal dynamics of key ecosystem resources, and subsequently the patterns of understory diversity. We analyzed and compared understory vegetation diversity in a rotational management chronosequence and an unmanaged fire chronosequence of Scots pine Pinus sylvestris forests across northern Sweden. We assessed the relationship of above- and belowground resource availability and heterogeneity with alpha and beta diversity using generalized additive models and multivariate analyses. We found that belowground resource availability (especially inorganic nitrogen) and aboveground resource heterogeneity (especially variation in forest structural complexity) were most strongly positively correlated with alpha and beta diversity, varying across successional stages. In early stages (0–60 years), high availability of belowground resources and aboveground heterogeneity was associated with high alpha and beta diversity. In mid-stages (100–200 years), reduced belowground resource availability and aboveground heterogeneity was linked to lower diversity. In late stages (> 250 years, which only exists in the unmanaged fire chronosequence), increased aboveground heterogeneity associated with tree mortality was linked to a resurgence in alpha and beta diversity. These results highlight the necessity of maintaining a mosaic of stands with different disturbance regimes and successional stages, particularly early post-fire stands and late successional stands, which are currently much rarer on the landscape, to support biodiversity at the landscape level.