Long-term change and spatial pattern in a late-successional hemlock-northern hardwood forest

1 Tree populations were monitored for six decades on a grid of 140 permanent plots in old-growth mesic (hemlock-northern hardwood) forests in northern Michigan, USA. Multiple remeasurements allow assessment of stability of late-successional forests and analysis of spatial patterns and environmental linkages. 2 This forest is not compositionally stable. Betula alleghaniensis has declined with little regeneration, suggesting dependence on episodic canopy disruption for persistence. Tsuga canadensis and Fagus grandifolia have increased in dominance in all size classes independently of major disturbances. Acer saccharum populations have remained little changed overall. Dynamics appear to be successional in nature, even though there has been no major disturbance for at least 400 years. Different areas in the stand may have experienced different histories. 3 Fagus grandifolia has invaded the stand recently through range expansion, and has come to dominate regeneration in some parts of the stand. 4 Canonical correspondence analysis suggests that soil texture, chemistry, and drainage influence successional dynamics, producing strong spatial pattern. Fagus grandifolia has invaded only on relatively fine-textured soils with impeded drainage. 5 Trends suggest that Fagus, Tsuga, and perhaps Acer saccharum would, in different parts of the stand, achieve near-total dominance in the absence of large-scale disturbance, but only after elapsed time of a millennium or more. Estimated return times for major disturbance in this region are of similar magnitude.