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 permanen t plots in old-growth mesic (hemlock-northern hardwood) forests in northern Michigan, USA. Multiple remeasurements allow assessment of stability of la te-successional forests and analysis of spatial patterns and environmental linkages. 2 This forest is not compositionally stable. Betula alleghaniensis has decl ined with little regeneration, suggesting dependence on episodic canopy dis ruption for persistence. Tsuga canadensis and Fagus grandifolia have increa sed in dominance in all size classes independently of major disturbances. A cer saccharum populations have remained little changed overall. Dynamics ap pear to be successional in nature, even though there has been no major dist urbance for at least 400 years. Different areas in the stand may have exper ienced 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 patt ern. Fagus grandifolia has invaded only on relatively fine-textured soils w ith impeded drainage. 5 Trends suggest that Fagus, Tsuga, and perhaps Acer saccharum would, in di fferent parts of the stand, achieve near-total dominance in the absence of large-scale disturbance, but only after elapsed time of a millennium or mor e. Estimated return times for major disturbance in this region are of simil ar magnitude.