Managing for late-successional/old-growth characteristics in northern hardwood-conifer forests

Abstract

In the northern hardwood region of North America managing for late-successional forest habitats and functions is an important element of ecosystem management. This study tests the hypothesis that uneven-aged practices can be modified to accelerate rates of late-successional forest development. An approach, termed “structural complexity enhancement” (SCE), is compared against conventional uneven-aged systems modified to increase post-harvest structural retention. Experimental treatments, including controls, were applied to 2 ha units and replicated at two multi-aged northern hardwood forests in Vermont, USA. Structural objectives include vertically differentiated canopies, elevated large snag and downed log densities, variable horizontal density (including small gaps), and re-allocation of basal area to larger diameter classes. The latter objective is achieved, in part, by cutting to a rotated sigmoid diameter distribution. This is generated from a basal area (34 m 2 ha −1) and tree size (90 cm dbh) indicative of old-growth structure. Forest structure data have been collected over 2 years pre-treatment and 3 years post-treatment. Fifty-year simulations of stand development were run in NE-TWIGS and FVS comparing treatment and no treatment scenarios. Simulations also tested the sensitivity of large tree development to prescription parameters. Leaf area index retention was spatially variable but significantly ( P < 0.001) greater under SCE (91%) compared to conventional treatments (75%). Post-harvest aboveground biomass ( P = 0.041), total basal area ( P = 0.010), and stem density ( P = 0.025) were significantly different among treatments, with SCE generally retaining more structure than conventional treatments. SCE increased coarse woody debris volumes by 140%; there was a 30% increase under conventional treatments. SCE successfully achieved the rotated sigmoid diameter distributions, and sustained these 50 years into the future, resulting in reallocated basal area. Cumulative basal area increments are projected to increase by 3.7 and 5.0 m 2 ha −1 compared to no treatment scenarios for SCE and conventional treatments, respectively. Basal areas will be significantly ( P = 0.025) greater after 50 years in SCE units due to higher residual basal areas. Conventional treatments are projected to produce 10 fewer large trees per hectare (>50 cm dbh) than would have developed without treatment, whereas SCE is likely to recruit five more large trees per hectare than the no treatment scenario. Large tree recruitment rates were related primarily to the form of residual diameter distributions ( P = 0.006) and, possibly, to maximum diameter limits. Late-successional characteristics in northern-hardwood systems can be promoted through a variety of modified uneven-aged silvicultural approaches based on the results.