Trajectories and resilience of stand structure in response to variable disturbance severities in northern hardwoods

In late successional forests, stand development processes are often more easily monitored and are more closely related to key ecological parameters when using structural criteria rather than stand age or time since stand-replacing disturbance. In this paper, the effects of various disturbance regimes on long-term structural change and resilience in 70 primary northern hardwood stands were analyzed using the CANOPY individual-tree model. Compared to a 'dichotomous' disturbance regime of small gap dynamics with infrequent severe disturbance, the historic natural disturbance regime of periodic low- and moderate-severity disturbances produced a more complex landscape mosaic similar to the current landscape. Under the natural disturbance regime, mean residence time in each of 8 structural stages was short (8-35 years) and followed descending monotonic distributions. Mean residence time in old growth was 87 years. Old-growth stands with stable size distributions were much more frequent when defined by relevant structural criteria than when defined by their past history (timing and severity of disturbances). Repeated mild and moderate disturbances often caused stand structure to retrogress to mature or earlier old-growth stages without undergoing stand replacement. However, simulation experiments often demonstrated structural resilience to repeated mild disturbance, even at the 0.5 ha scale. In some cases, disturbances accelerated the development of more complex structure typical of late transition or steady-state stands. Recovery from heavy partial canopy removal was markedly faster than recovery after stand replacing disturbance. Stands recovered to an old-growth condition with a stable size distribution ('quasi-steady state') 175 years after disturbances removing 60% of the aggregate crown area, compared to 280 years after a disturbance with no residual trees. High dominance of these landscapes by old growth forest (78% of total area) is due not only to the low frequency of stand-replacing disturbance, but also to the structural resilience of old growth to periodic episodes of low- and moderate-severity disturbance. (C) 2016 Elsevier B.V. All rights reserved.