The stem exclusion bottleneck: Identifying the key drivers of plant diversity in temperate forests
Eastern forests are experiencing dramatic shifts in species composition as well as considerable declines in plant biodiversity. Interactions among fire suppression, smaller canopy gaps, and overabundant deer are likely central to these changes. Indeed, only a few years after restoring fire and canopy gaps to the landscape, we often observe increases in plant diversity but only when deer are at moderate densities. The impacts of these altered browsing, fire, and canopy gap regimes and their interactions on forest understory communities in later stages of forest succession are often overlooked. This is a critical oversight because the understory community contains more than 70% of vascular plant diversity. Moreover, we do not understand how fire, gaps, and herbivory interact to mediate forest communities especially as the forest transitions into the stem exclusion stage. This phase is characterized by a dense sapling layer that may be inimical to the herbaceous layer. We tested the hypothesis that the impacts of these processes and their interactions would persist for the woody community but would be transient for the herbaceous community as the forest transitions into the stem exclusion phase. To test this hypothesis, we restored fire, cut canopy gaps, and excluded deer using a split-plot, fully factorial experiment. We then measured the response of >21,000 stems representing >150 species after twelve years.
For saplings, we found that browsing caused a decline in species richness and diversity. Specifically, when we excluded deer, sapling diversity doubled under open canopies. For herbaceous species following fire, browsing increased species richness by approximately 40% likely mediated by reducing a dominant, fast-growing shrub (Rubus allegheniensis). We are the first to demonstrate that long-term interactions among disturbances and herbivory simultaneously cause contrasting responses in the herbaceous versus the sapling layer. We suggest that diverging understory responses to disturbances are driven by high stem density of the sapling layer that drives down both above- and belowground resource availability in the herbaceous layer.