On the causes and consequences of changes in the dynamics and disturbance regimes across the eastern deciduous forest
Eastern deciduous forests are undergoing major changes in species composition and diversity and we are not sure why. Changes in fire regimes, canopy gap regimes, and browsing regimes likely underlie many of these changes. However, which of these processes is most responsible is a matter of great debate. We conducted a large-scale experiment near the geographic center of the Eastern Deciduous Forest Biome to test three hypotheses. 1. The Fire Hypothesis: fire suppression limits diversity because a few shade-tolerant, fire-intolerant species replace and suppress a variety of fire-tolerant species. 2. The gap hypothesis: small gaps typical of today’s forests promote dominance of a few shade-tolerant species. 3. The browsing hypothesis: over-browsing by deer limits diversity to a few unpalatable species. We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size ~255 m2), and browsing by deer. We followed fates of more than 28,000 seedlings and saplings for five years.
Understory tree communities in control plots were dominated (up to 90%) by Fagus grandifolia, averaging a little more than two species, whereas the overstory was diverse with 10-15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity of eastern deciduous forest trees: browsers reduced the benefit of gaps and created depauperate understory communities following fire. Consequently, two major disturbances that once promoted tree species diversity no longer do so because of browsing. Our findings also appear to reconcile equivocal views on significance and magnitude of impacts of fire and gaps on diversity. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances each promoted diversity (3- to 5-fold) and created distinct understory communities. Our results are applicable to large portions of the eastern deciduous forest because deer are over-abundant throughout much of eastern North America. Overall, our results provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre-European settlement forests were capable of maintaining high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.