Demographic responses to a severe disturbance in an old-growth northern hardwood forest

Background/Question/Methods

Dynamics of old-growth temperate forests are poorly understood because processes structuring them occur over time-spans that aren't amenable to direct observation or experimentation. Long-held models invoking equilibrial processes are giving way to models suggesting that community pattern and diversity reflect transient but long-lasting effects of rare severe disturbances (in northern hardwood forests of the Upper Great Lakes region, primarily high winds). However, such events can have return times exceeding several centuries, and late-successional stands are few and small, so assessment of effects of major disturbance has generally depended on anecdote, indirect evidence, and modeling. The Dukes Research Natural Area (Hiawatha National Forest, MI) includes over 100 mapped permanent plots (originating in 1935) in old-growth northern hardwood forest. A 2002 storm caused severe but patchy disturbance. I use repeated plot measurements from before and after the storm to address several questions. In the decade following the storm: a) have recruitment patterns changed compared to pre-storm intervals? b) do surviving trees show size- and species-specific patterns in growth responses? and, c) how are growth responses related to magnitude of local canopy mortality?

Results/Conclusions

Recruitment of stems >1cm dbh has not increased overall or for any dominant species since the storm as compared to previous decades, although recent regeneration of Betula alleghaniensis is strongly associated with storm-caused mounds. Surviving trees of all species and sizes showed large increases in growth rates in the most disturbed areas. However: a) canopy trees (>30 cm dbh) with higher pre-storm growth rates, showed the smallest proportional increase, b) in all size classes, Fagus grandifolia showed greater increases in growth rates than other dominants (Acer saccharum and Tsuga canadensis), while Tsuga growth rates increased only at highest disturbance intensity; and c) permanent plots experiencing highest canopy mortality were associated with lowest pre-storm growth rates for canopy trees. The Intermediate Disturbance Hypothesis has been invoked to suggest that some species (e.g., Betula which has decreased substantially over several decades) depend on disturbance-initiated recruitment pulses to avoid competitive exclusion; patterns during the first decade following this disturbance do not show such phenomena, but may, instead, accelerate already-documented increasing dominance of Fagus. Recruitment effects may lag disturbance by >10 yr, but parallel studies suggest that LAI may recover rapidly in disturbed areas and re-establish severe suppression of smaller seedlings.