Natural disturbance and stand structure of old-growth northern white-cedar (Thuja occidentalis) forests, northern Maine, USA

Abstract Natural disturbance histories and stand structures derived from old-growth forests are increasingly used to guide forest management prescriptions. Although such information is readily available for a number of forest types, it is lacking for others, such as northern white-cedar (Thuja occidentalis) forests, despite this forest type’s wide distribution, ecological value, and economic importance in northeastern North America. We applied standard dendrochronological methods to six old-growth northern white-cedar stands within the Big Reed Forest Reserve of northern Maine, USA, to reconstruct the frequency and severity of past natural disturbances. The prevalence of internal decay (well-known for this species) precluded the construction of robust age-class distributions. Overall, 63% of cedar trees contained internal decay, and the probability of rot increased with increasing diameter. Evidence from growth releases reveals sporadic pulses of low- to moderate-severity canopy disturbances. The mean disturbance rate was 6.4% of canopy area disturbed per decade, and pulses rarely exceeded 25% per decade. Based on the subset of complete cores (i.e., those without internal decay), 44% of current canopy trees showed one growth release before achieving canopy status, 26% showed two releases, and 7% showed three releases. Of the 23% that showed no release, most showed persistent slow growth that eventually placed them in the canopy. However, an apparent hiatus in cedar recruitment in recent decades (albeit based on a subset of complete cores), as well as low cedar abundance in the sapling layer, suggest that cedar may not maintain dominance in the future overstory. Current structure in these stands is similar to that reported from other old-growth conifer forests in the region: mean living tree basal area was 44.3 m2 ha−1, density of large (>40 cm dbh) living trees was 130 trees ha−1, and coarse woody debris volume was 183 m3 ha−1. Taken together, these findings suggest that multi-aged silvicultural treatments incorporating periodic harvests of low to moderate intensity, retention trees or patches, and protection of coarse woody debris would be appropriate for sustaining or restoring lowland northern white-cedar forests.