The Formation of Dense Understory Layers in Forests Worldwide: Consequences and Implications for Forest Dynamics, Biodiversity, and Succession

Alterations to natural herbivore and disturbance regimes often allow a select suite of forest understory plant species to dramatically spread and form persistent, mono-dominant thickets. Following their expansion, this newly established understory canopy can alter tree seedling recruitment rates and exert considerable control over the rate and direction of secondary forest succession. No matter where these native plant invasions occur, they are characterized by one or more of the following: (1) the understory layer typically has greater vegetation cover and lower diversity than was common in forest understories in the past; (2) this layer can delay stand renewal and alter species composition by inhibiting tree regeneration; and (3) once this layer is formed, it can resist displacement by other species and remain intact for decades. In this summary, the processes that trigger the expansion of several plant species native to forests are evaluated and their ecological characteristics reviewed to provide general guidelines in assessing native invasion risk in forest stands.

It is argued that major anthropogenic changes to disturbance and browsing regimes bring about the monopolization of the forest understory by native plants. In all cases reviewed, aggressive understory plant expansion followed alterations in overstory disturbance regimes. Although these disruptions included predictable and manageable impacts such as tree harvesting, other less predictable overstory disturbance agents including catastrophic fires, insect outbreaks, and pathogens were involved. Assessing and managing risk from these alternative threats is challenging as their occurrence is often erratic, hard to control, and not limited by land ownership and administrative boundaries. In many cases, the risk to forest understories was particularly acute if the effects of multiple stressors occurred in a stand, either in tandem or within a short period of time. Specifically, the synergy between overstory disturbance and uncharacteristic fire regimes or increased herbivore strongly controls species richness and leads to depauperate understories dominated by one or a few species.

It is suggested that aggressive expansion by native understory plant species can be explained by considering their ecological requirements in addition to their environmental context. Some plant species are particularly invasive by virtue of having life-history attributes that match one or more of the opportunities afforded by multiple disturbances. Increased overstory disturbance selects for shade-intolerant species with rapid rates of vegetative spread over slower growing, shade-tolerant herbs and shrubs. Altered fire regimes select for only those species that can survive the fire or resprout thereafter. Finally, overbrowsing selects for only those species that are well defended or tolerant to browsing. Ultimately, these processes create novel conditions that favor only a small subset of species that possess some combination of the following life-history characteristics: rapid vegetative growth, relatively shade-intolerant, fire-tolerant, and herbivore-tolerant. The result is a low diversity but dense understory that can persist for long periods of time even if the canopy closes.

The framework advanced by this review could aid land managers in implementing informed management policies and practices that both limit the spread of these plants and target control and remediation treatments directed at the precise mechanism of interference. Vigilant monitoring of stand conditions can ensure that alterations to the overstory and understory disturbance regimes do not operate concurrently, particularly when control over these factors falls under the purview of different management agencies; (e.g., wild game vs. forestry management agencies). Finally, caution is placed on decisions regarding partial or complete overstory removals and one should consider a site’s understory conditions including inadequate advance regeneration, presence of clonal understory plants, fire history, and high herbivore impact.