Forest development 20 years after simulated hurricane in central New England

Background/Question/Methods

Major hurricanes affect central New England every 50-100 years and can cause catastrophic forest disturbance, but few studies have addressed the long term legacies of wind disturbance on forest vegetation change.   In 1990, the Harvard Forest began its Long-Term Ecological Research (LTER) program and initiated a large (0.8 hectare) hurricane simulation experiment in a maturing red oak-red maple forest  Long term vegetation plots were established at the site, and in a nearby unmanipulated control site. In the experimental plot, 250 trees were mechanically toppled in a northwesterly direction, using a winch and a skidder. Eighty percent of the canopy trees, and two-thirds of all trees >5cm dbh, were damaged resulting from direct and indirect effects of the manipulation. In addition, pits and mounds covered 8% of the site and 13% was covered by uprooted tree stems after the pulldown.   The objective of this presentation is to demonstrate the magnitude and persistence of changes in forest vegetation structure and composition, 20 years after the simulated hurricane. 

Results/Conclusions

Vegetation response in the disturbed plot included sprouting of damaged trees, retention of large legacy oaks, and rapid growth of advance regeneration. Forest structure divergence between the control and disturbed plots is seen in forest biomass – the control site will continue to have higher basal area for decades – but not litterfall productivity, which converged in less than 10 years. Whereas the control site continues to develop as an even-aged, stratified stand with increasing importance of dominant red oak over time, the experimental site is three-tiered, with a novel black birch component and a large pool of woody debris. Herbaceous plant composition shifted, but this was a transient change that peaked around year five.  In contrast, the sapling cohort is suppressing shrub cover in the disturbed site, whereas shrub cover has increased in the control site over time.

Early interpretations of the experiment emphasized tree seedling establishment and sprouting by damaged trees.  Long-term, survival and growth of the surviving 20% of the canopy, and recruitment of black birch that were established seedlings prior to the disturbance, are driving forest development. Explicit contrasts with a control plot allow an assessment of the divergence of developmental trajectories in the damaged and undisturbed forests.