Sustaining forest diversity amidst the interacting forces of northern temperate forests
Disruptions to historic disturbance and browsing regimes have altered plant species composition and diversity in forests worldwide. In deciduous forests of North America, three widespread changes appear central to this phenomenon: Fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. We conducted a large-scale experiment near the geographic center of eastern deciduous forests to test the hypothesis that once common disturbances and their interaction with the top-down influence of browsers can create conditions favorable for the maintenance of diverse woody and herbaceous communities in forest understories. We tested this hypothesis in two parallel experiments employing a factorial design wherein we manipulated canopy gaps (present/absent), low intensity understory fire (burned/unburned), and manipulations of white-tailed deer browsing (fenced/unfenced).
Fire, large canopy gaps, and browsing all dramatically affected understory composition and interactions among all factors were pervasive. Under gaps, recruitment of less shade-tolerant species to the community moderately increased seedling and sapling abundance, richness, and diversity under ambient browsing. However, when browsers were excluded from gaps, woody species density, richness and diversity more than doubled. Fire effect on seedling and sapling density, richness, or diversity depended entirely on its interaction with gap formation and browsing and created contrasting seedling and sapling communities. Prunus pensylvanica dominated in burned plots protected from browsers whereas Betula spp. and Acer pensylvanicum dominated in burned plots with browsers present. Effects of disturbance on the herbaceous community were generally comparable. Canopy gaps and fire each increased herbaceous richness and abundance and the response was amplified when these disturbances co-occurred. In stark contrast to woody species, herbaceous richness in disturbed areas with ambient browsing was nearly doubled relative to areas protected from browsing. Specifically, deer browsing indirectly increased forb and fern richness in disturbed areas by reducing the abundance of highly palatable and fast-growing shrubs that proliferated following disturbance. Our study demonstrates synergies between canopy gaps and understory fire can promote diverse understory plant assemblages and these effects are often modified by browsing. Although speculative, our results suggest harvest gaps may be an appropriate management strategy to sustain and enhance plant biodiversity. However, the efficacy and outcomes of this practice will often depend on additional co-occurring forces and potentially vary among plant growth forms.