The Influence of Multiple Stressors in Triggering Forest Understory Invasion by Native Plant Species

Alejandro A. Royo and Walter P. Carson

USDA Forest Service Northeastern Research Station and University of Pittsburgh Department of Biological Sciences

Exotic invasive plants are recognized as serious threats to forest ecosystems and have received considerable attention from the scientific community for decades.  Less emphasized are the many native forest understory plants that rapidly increase their spatial distribution following multiple disruptions to an ecosystem’s natural dynamics.  In many cases, these species expand and form persistent, monodominant thickets.  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. 3) Once this layer is formed it can resist displacement by other species and remain intact for decades.  In this paper we evaluate the processes that trigger the expansion of several plant species native to temperate and boreal forests across North America and review their ecological characteristics to provide general guidelines in assessing native invasion risk in forest stands.  

 We argue 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 majority of the cases (>60%) 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 herbivory strongly controls species richness and leads to depauperate understories dominated by one or a few species.  

We suggest 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 will help guide land managers in assessing the risk of native understory plant invasion within their stands.  We suggest vigilant monitoring of stand conditions to 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). We caution that decisions regarding partial or complete overstory removals should consider a site’s understory conditions including inadequate advance regeneration, presence of clonal understory plants, fire history, and high herbivore impact.  Finally, we suggest the implementation of management practices that more closely resemble natural disturbance levels.

Native Pests Session - Wednesday Afternoon

corresponding author:

Alejandro A. Royo
USDA Forest Service
Northern Research Station
P.O. Box 267
Irvine, PA  16329-0267
814-563-1040
aroyo@fs.fed.us