A test of ecological succession hypotheses using 55-year time-series data for 361 boreal forest stands

Aim There has been much work on succession over many decades, but succession fundamentals are still debated because of the reliance on chronosequences and dendrochronological reconstruction, both of which are problematic approaches. Here we use time-series data to test four hypotheses that lie at the heart of successional theory: (1) the neighbourhood effect hypothesis tree species abundance is time dependent; (2) the density-dependence hypothesis a rare species is more favoured over time; (3) the resource ratio hypothesis species that can grow at the lowest resource level tend to dominate resource limited sites through succession; and (4) the intermediate disturbance hypothesis intermediate disturbances increase the abundance of rare species. Location Central boreal forest in Canada (47 degrees 50'-50 degrees 10' N; 80 degrees 10'-85 degrees 50' W). Methods We used repeated measurements from sequential aerial photography and ground surveys for 361 fire-origin stands that were measured over a c. 55-year period. Results Shade-intolerant Pinus banksiana decreased, tolerant Thuja occidentalis increased, intolerant Populus spp. and Betula papyrifera displayed a U-shaped trend, and intermediate-tolerant Picea spp. and tolerant Abies balsamea did not change with time since fire, showing evidence of negative, positive, or neutral neighbourhood effects. Species either persisted for longer or increased more in non-conspecific stands, and had higher increases in abundance when associated with species of contrasting shade tolerance, supporting the density-dependence hypothesis and indicating shade-tolerance complementarity as a mechanism for coexistence. Resource-poor soils favoured those species capable of tolerating limited resources, whereas rich soils permitted invasion and promoted mixtures, supporting the resource ratio hypothesis. Intermediate disturbances increased the invasibility of rare species. Main conclusions Contrary to previous studies where time since a stand-replacing disturbance is the sole predictor for succession, our study shows that time alone is either an insufficient predictor or is irrelevant to species dynamics in the boreal forest. Rather, density dependence, site resource and intermediate disturbances are key mechanisms in species dynamics and coexistence over time.