Using Ecological Restoration to Aid Native Forests Afflicted by Invasive Pests

Introduced, tree-afflicting pests such as insects, fungal pathogens, and various microorganisms are a major agent of change in forests. In the United States, for example, over 400 species of forest insects are estimated to have been introduced (Aukema et al. 2010). Some, like the emerald ash borer (Agrilus planipennis), have eliminated or nearly eliminated formerly dominant tree species from forest overstories. Through their effects on trees, introduced pests can alter many ecosystem attributes such as habitat structure and food for fauna, soil nutrient cycling, carbon storage, microclimate, and aquatic systems (Liebhold et al. 2017).

As efforts under invasion biology to understand and limit invasions of undesired pests continue, there is a major role for ecological restoration in sustaining forests in the face of ongoing pest invasions. This role encompasses three broad categories: propagation or resistance breeding programs of the affected tree species, habitat restoration after an introduced pest has damaged a forest, and proactive improvement of degraded forests to potentially enhance their resistance and resilience to invasive pests.

Once resistant trees are available or a pest invasion has been remediated to the point that the target tree species can be re-introduced to the landscape, many ecological restoration questions arise. For example, if other native tree species have expanded into the vacancy left by the afflicted tree species, can or should the target tree species be interplanted back into the forest or only be reintroduced into new disturbances lacking existing forest? Addressing this question requires knowledge of the tree species’ life history traits (e.g., shade tolerance) as well as reference conditions of species-environment relationships.

For forests damaged by an introduced pest, many restoration strategies could facilitate forest adaptation in scenarios of continued pest presence or reduced pest presence. For example, replacement trees of other species could be seeded, outplanted, or facilitated via assisted natural regeneration. Restoration research and adaptive management experiments can contribute to this effort by developing or refining candidate techniques tailored to the variety of species, site conditions, management goals, and budgets that practitioners may encounter. Treating non-native plants or facilitating recruitment of native shrubs and herbaceous understories could also form strategic and comprehensive approaches for restoration in pest-afflicted forests.

Although proactive restoration in degraded forests may or may not reduce susceptibility to pest invasion or influence tree resistance, in some cases it can at least slow or limit severity of pest effects and strengthen native forest resilience. For instance, restoration that diversifies forest composition to reference levels, reduces tree density if outside the range of variability, or that curtails other stressors all could increase forest resistance to pests in some cases and forest resilience in other cases (Pellegrini et al. 2021).

As a large body of literature continues accumulating on the challenges that introduced pests pose to forest conservation, some of my own ongoing research has reinforced my thinking on linkages of fields such as forest pathology, insect ecology, and integrated pest management with ecological restoration. My collaborative research in forests affected by the introduced insect hemlock woolly adelgid (Adelges tsugae), emerald ash borer, and invasive fungal pathogen oak wilt (Bretziella fagacearum) has underscored to me the damage that invasive pests can inflict upon forests (e.g., Abella 2014, Abella et al. 2019, 2021). For example, in resurveying long-term oak forest plots in summer 2024 in partnership with Metroparks Toledo in northwestern Ohio, we have found continued trends for oak tree mortality associated with oak wilt and replacement of oaks by other species. Owing to oaks’ unique features for wildlife and uncertainty with the long-term effects of oak wilt, this situation exemplifies the possible role that ecological restoration techniques could play in potentially enhancing the resistance, resilience, or adaptation of forests to this poorly understood fungal pathogen (Figure 1).

Disturbances created by invasive forest pests are of course just one of many types of disturbance or degradation that can trigger a need for restoration. With legacy and ongoing invasions by forest pests continuing to expand, numerous opportunities are likely for ecological restoration to contribute to sustaining native forests invaded by or susceptible to invasive forest pests.

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Figure 1. Mortality of oak trees associated with the fungal pathogen oak wilt in Wildwood Preserve Metropark, northwestern Ohio. Canopy gaps below dead oak trees (left photo) have not exhibited oak tree recruitment, likely primarily because the individual gaps are too small (right photo). Image credit S.R. Abella.

Quercus velutina (black oak). N.L. Britton and A. Brown, 1913. An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 vols. Provided by Kentucky Native Plant Society, New York. Scanned By Omnitek Inc. USDA-NRC PLANTS Database.