Novel stressors, such as non-native insects and diseases, present a significant challenge to the long-term sustainable management of forest ecosystems around the globe [1
]. For example, recent estimates for the United States indicate that almost two thirds of all forestlands are susceptible to damage from non-native insects and diseases already present in the country [2
]. In many cases, these organisms are host-specific with the ability to functionally eliminate a given tree species from an ecosystem [3
]. As such, there is a significant need for the development of adaptive management strategies that increase forest resilience to the loss of a constituent tree species as a way to maintain key ecosystem functions after infestation by non-native species [4
]. This need becomes urgent when the host species is considered foundational to ecosystem functioning, such that its loss cannot readily be absorbed by other species in the ecosystem [5
Forest management guidelines developed in response to non-native insects and diseases have generally focused on altering host species structure and abundance to minimize spread and impacts of a given organism. These approaches may include removing all potential host species from forested areas within a set distance of known infestations [6
] or selective removal of larger host trees to reduce availability of brood trees [8
]. Although these strategies are warranted in areas proximal to introduction points and ongoing infestations, their application in areas not currently threatened may generate greater ecological and economic impacts than the non-native forest pest itself (e.g., [10
]). To date, less attention has been devoted to developing appropriate silvicultural strategies for increasing the resilience of these unimpacted areas to maintain long-term management options and ecological functions even after a pest’s establishment.
The introduced emerald ash borer (Agrilis planipennis
Fairmaire [EAB]) is one of the more significant non-native insects threatening North American forests given the importance of ash species across numerous forest types and regions [11
], and the ability of this insect to cause widespread mortality within 2–6 years of invasion [12
]. Most ash species exist as minor components of mixed species forests [11
] and loss of canopy ash to EAB will eliminate the unique ecological functions of ash, but forested conditions are expected to persist in these affected ecosystems. One exception are black ash (Fraxinus nigra
Marsh.)-dominated lowland forests in the western Great Lakes region, where black ash often forms almost pure stands, with up to 95% of stems composed of this species [5
]. As black ash is a foundational species in these forests, expectations regarding EAB impacts include not only a loss of canopy black ash, but also a concomitant shift in ecosystem structure and function to marsh-like conditions with little to no tree component [13
]. In particular, historic experience with clearcutting and recent field simulations of EAB impacts indicate that the loss of canopy black ash may cause the water table to rise and increase the duration of water at or near the soil surface [13
]. These changes represent not only a significant threat to organisms and processes associated with current black ash ecosystems [5
], but also a significant challenge to managers tasked with sustaining these ecosystems in the face of the threat of EAB invasion.
As with other forest types threatened by non-native insects and diseases, the management guidance for black ash forests was developed based on research and management experience developed prior to the introduction of EAB. This historical guidance largely focused on silvicultural strategies that encouraged the dominance of black ash over time [13
], a condition in contemporary contexts that is highly vulnerable to EAB impacts. Recent management recommendations for addressing EAB in these and other ash forest types have primarily focused on minimizing establishment and spread of the insect through reducing host abundance [8
]. Although these approaches, when applied in conjunction with insecticides, may reduce spread of EAB in localized, urban settings, they represent a reactive strategy to a current outbreak as opposed to a proactive approach that aims to increase resilience to impacts prior to invasion. Moreover, black ash can dominate extensive areas (1–40 ha) presenting significant logistical challenges for widespread application of insecticides to protect all black ash forests. Given the potential for significant ecosystem phase shifts following EAB mortality or pre-emptive liquidation harvests in lowland black ash forests (cf. [13
]), there is a great need for silvicultural strategies that reduce vulnerability of these ecosystems to EAB prior to its arrival. In addition, given the continued spread of this insect and the infeasibility of treating all black ash-dominated areas, there may also be a growing need for rehabilitation approaches to reintroduce forest cover to places where the insect or associated reactive management have resulted in a shift to non-forest conditions.
The overall objective of this paper is to review recent research and experience managing black ash ecosystems to provide general guidance for the adaptive management of these ecosystems in the face of EAB invasion. This includes describing the general ecological setting of these forests as a foundation for adaptive silviculture prescriptions, summarizing insights from long-term research trials evaluating the effectiveness of different silvicultural treatments at increasing the non-ash components in these forests, and discussing potential rehabilitation approaches for returning other species to these areas after EAB has eliminated black ash. To date, EAB has spread to 32 states in the US and three Canadian provinces since its discovery in Michigan in 2002, and has killed nearly 100% of ash stems >2.5 cm diameter at breast height (DBH; 1.3 m) in areas proximal to where it was first introduced [15
]. In most cases, the majority of ash trees are killed within 2–6 years of invasion; however, individual trees and stump sprouts have persisted for over a decade in some green ash (Fraxinus pennsylvanica
Marsh.)-dominated wetlands in southern Michigan, indicating areas may not always experience complete ash mortality following invasion [16
]. As such, the management recommendations reviewed in this work are not definitive given the great uncertainty around long-term EAB impacts and silvicultural outcomes, nor is it expected that they can be applied across all black ash-dominated areas and ownerships. Instead, they are meant provide a framework for approaching this threat to increase resilience and sustain a forested condition over the long term, particularly given the significant losses of ash observed in all ecosystem types impacted by EAB (>62–100% of basal area; [15
]). At the time of writing, EAB had not been detected in the areas serving as the focus for this review (northern MI, MN, and WI) and the experiments and recommendations largely assumed EAB arrival within the next 10–15 years based on location of known infestations (southern MN and WI, eastern Upper Peninsula of Michigan) relative to expected rates of spread [17
The invasive EAB has motivated considerable interest and research in understanding the ecological functioning of black ash forests, as well as suitable adaptive silviculture strategies for maintaining these ecological functions even after the arrival of EAB. This work has reinforced the complexity of ecological conditions characterizing black ash-dominated wetlands and the unique, foundational role black ash plays in affecting hydrology, patterns in biodiversity, and vegetation dynamics. Given its unique role, there is truly no replacement for black ash in these forests. Despite this, recent experience with adaptive planting approaches in conjunction with regeneration harvests that maintain canopy cover during the establishment phase (i.e., group selection and shelterwood approaches) suggest there may be opportunities to increase non-ash components to maintain some of the forested wetland functions historically provided by black ash. Results from these studies are still quite short-term; however, they provide a framework for addressing the threat of an introduced pest by focusing on building resilience as opposed to solely focusing on eliminating host tree populations.
Despite the progress made over the past two decades in understanding black ash forests and in developing strategies for increasing their resilience to EAB, there are several key knowledge gaps that remain in relation to our ability to sustain these areas as forested wetlands into the future. One key knowledge gap is the long-term suitability of potential replacement species that are already being evaluated, and a related gap surrounds the suitability of other potential replacements species in these forests, as well as how differences in stock type might affect within species performance. Moreover, general availability of many of the species that have proven successful in early trials is quite limited and a better understanding of nursery capacity for providing planting material necessary for reforesting these areas is crucial. Similarly, there has been little evaluation of other reforestation techniques, particularly aerial seeding of replacement species, which may be more cost-effective and logistically feasible relative to hand planting black ash swamps. The impacts and effectiveness of site preparation treatments in establishing non-ash species and controlling invasive species common in these ecosystems, particularly reed canary grass, also have not been evaluated. Finally, the new field trials in Wisconsin looking at restoration treatments after EAB invasion and tree mortality will provide critical information on approaches for restoring tree cover; however, there is a need for like evaluations across the great variety of habitat types on which black ash currently dominates and associated feedbacks with local site hydrology. Regardless of treatment or site evaluated, all adaptive management efforts should include retention of mature, seed-bearing black ash to maintain its unique ecological functions prior to EAB arrival and provide opportunities for natural resistance and reestablishment after invasion [54