Historical reindeer corrals in northern boreal forests reveal divergent post-disturbance reorganization by forest type

Current theoretical frameworks on alternative ecosystem states in boreal forests do not contain a clear prediction on how the sensitivity for state transitions depends on site productivity, which forms a major gap in understanding how disturbances impact these ecosystems. Here, we compared 26 historical reindeer (Rangifer tarandus L.) corrals used in animal husbandry between the late 1800s and early 1960s with reference forests in the northern boreal forest zone in Finland. We found that mesic forests experienced strong forest reorganization that involved shifts in both tree species composition and density. Sub-xeric forests were most commonly on a trajectory toward the pre-disturbance state due to strong self-thinning. No state transitions in understory vegetation occurred. There was little difference in understory vegetation recovery between mesic and sub-xeric forests; however, disturbance increased spatial heterogeneity in understory vegetation. These findings from historical sites support a high resilience of northern boreal forests to disturbances. They also indicate that disturbances may more likely induce reorganization in more productive mesic sites. Although stronger environmental constraints in less productive sub-xeric sites may slow down recovery, they also direct post-disturbance trajectories toward the pre-disturbance state.