Regional Example - Western Great Lakes

Regional examples of silvicultural adaptation strategies: Western Great Lakes Mixed-Pine Ecosystem

Preparers

Paul Anderson, Pacific Northwest Research Station; Brian Palik, Northern Research Station

Potential Vulnerabilities

The Great Lakes mixed pine ecosystem occurs on sand to loamy sand soils derived from glacial outwash and ice-contact topography (1). The soil moisture regime is dry-mesic to xeric and as such, a decrease in growing season soil moisture under a warmer, drier climate could negatively impact the system in several ways. Higher growing season temperatures, combined with reduced summer precipitation, will result in greater plant water deficits, if evapotransporation demand exceeds moisture availability more frequently and for longer duration. Reduced photosynthetic capacity, growth, and tree vigor are likely consequences. Increasing atmospheric CO₂ concentration may increase growth in the short term, but the long-term response is not clear at present. For example, increasing ground-level ozone concentrations, which can damage forest trees, as well as soil moisture limitations, may offset the positive effect of CO₂ fertilization on growth.

Odocoileus virginianus) is a significant problem in many parts of the region (2). In Minnesota, for example, it is virtually impossible to regenerate eastern white pine, and often jack and red pines, without browse protection of some type (3). It is difficult to predict how white tail deer will respond to a climate that is expected to be warmer, with potentially deeper snowpack in winter.

Dryer conditions during the growing season may result in reduced moisture of woody fuels more frequently in space and time, with a concurrent increased risk of wildfire. While the Great Lakes mixed-pine ecosystem is fire dependent, fuels build up due to long-term fire suppression poses a significant risk of catastrophic wildfire in many locations. This may be particularly true if fuels are drier. Soil moisture deficits during the growing season will also exacerbate the potential for wildfire. More frequent heavy rainstorms accompanied by high winds may result in increased blowdown, particularly of older pines, as both red and eastern white pine can reach super-dominant crown positions, with high wind exposure.

Warmer, drier growing season conditions, along with milder winter temperatures, may shift competitive advantage towards more generalist species, and those currently at the northern edge of their distribution in the region. For instance, habitat suitability for several currently minor oak (Quercus) species is projected in increase (4), as is that for species that currently do not occur in the region, such as eastern red cedar (Juniperus virginiana). In contrast, predictions suggests a moderate reduction in habitat suitability for eastern white pine and substantial reductions for red pine throughout much of the region, with both low and high emission scenarios (4). Other species that occur in the mixed-pine ecosystem, including trembling aspen, paper birch and balsam fir, are projected to have large declines in habitat suitability as well.

Potential compositional changes may lead to long-term effects on fuels and fire risk that are complex and unpredictable. If CO₂ fertilization and milder temperatures increase productivity, fuel loadings may increase and potentially fire risk along with this. Alternatively, drier growing season conditions could reduce productivity and thus the standing crop of fuels. Additionally, shifts in composition to species producing less flammable fine fuels, such as maples, could lead to decreased fire risk.

Insect and disease pests of red and eastern white pine may respond to climate change in ways that increase tree mortality. Changes in pest populations could result directly from changes in climatic factors. For example more frequent drought could increase bark beetle outbreaks, a potentially serious threat to eastern white and red pines. Climate change may result in more complex interaction with other environmental changes. For example, populations of Ips bark beetles are greater on red pine with bole scorch from surface fire, resulting in increased mortality (5). If fire is more frequent with climate change, bark beetle-induced loss of vigor and mortality may increase as well.

Management Options

Several of the general strategies outlined in ‘Silviculture for climate change’ may have relevance for managing Great Lakes mixed pine ecosystems in the face of climate change. These strategies can be broadly segregated into resistance, resilience, and response or facilitation strategies (6), although there is some ambiguity in assigning specific approaches to a broad strategy.

Resistance

Resistance strategies may include activities that, at least in the short-term, greatly increase regeneration of species that are expected to have reduced habitat suitability in the future, such as red pine and eastern white pine. Increased use of prescribed surface fire to create appropriate seedbeds and reduce competing understory vegetation may be one such strategy.

Corylus Americana, Corylus cornuta). This response may negate potential enhancement of soil moisture availability to overstory trees. As such understory control may need to be more widely practiced than it currently is. Greater use of growing season prescribed fire would help reduce hazel populations (7). As with other forest types, thinning may also help to reduce the potential outbreaks of density dependent pest species, such as bark beetles.

Resilience

A primary resilience strategy involves restoring and sustaining the native species and structural conditions currently associated with reference conditions for the Great Lakes mixed-pine ecosystem. The specific magnitudes of climate changes, especially for precipitation, are still uncertain and the exact response of various tree species is not well understood. As such, hedging one’s bet and increasing options for the future by maintaining the full suite of native species in the ecosystem is appropriate. There is tremendous opportunity to restore native species composition in the Great Lakes mixed pine ecosystem (8), as much of the current resource occurs in near mono-specific stands (often plantations) of either red pine or eastern white pine. Generalist species, such as red maple (Acer rubrum), and those with a more southerly distribution such as some oaks (Quercus spp.), that currently are a minor components of a stand may be favored by a resilience strategy, under the premise that conditions for their survival, growth, and fecundity may be enhanced in the future. Density management may be a tool to promote resilence of these species if they already occur in a stand. Specifically, thinning during the stem exclusion stage of stand development may prevent their competitive exclusion and allow them to recruit to the overstory.

The structural condition of many pine stands in the region is greatly simplified, relative to reference conditions. The later is inclusive of multi-cohort stands (8), as well as "even-aged" stands composed of one dominant cohort, along with a smaller population of older individuals. Concurrent with these age structures is greater complexity and heterogeneity in vertical and horizontal canopy distribution, greater amounts of large dead wood, and greater ranges of tree sizes, including very large individuals. The greater diversity of tree age and sizes may allow more flexibility of a species to persist in the face of climate change, if certain age or size classes are less susceptible to drought or other negative influences. As such, a resilience strategy may involve restoration of more complex age and size structures in forest of the region.

Response

Managing native species composition, as a strategy to adapt the system to climate change may meet with limited success in the long-term. Red pine, along with most of the associated "boreal" species, are predicted to face habitat conditions that are not suitable for long-term sustainability of populations. Thus, as a response strategy to facilitate the transition of forests to a new state, the silviculturist may also begin considering compositional manipulation that is currently outside the norm of practice. Initially, this may include the introduction of more southerly genotypes of contemporarily occurring species. This should be fairly straightforward when the species under consideration is widely regenerated artificially, as are both eastern white and red pines and some oaks. However, this approach may require relaxing seed source regulations, so as to allow material from outside of currently acceptable ranges.

A more controversial approach, but one that is gaining attention, is assisted migration of species not currently part of the contemporary ecosystem. In the Great Lakes mixed pine ecosystem, this might include, for example, eastern red cedar and short leaf pine. Assisted migration of species not currently part of the ecosystem should only be "experimented" with on a limited basis in the short-term. The problem with habitat suitability projections for these species is that the occurrence and magnitude of extreme climatic events, particularly extreme low temperatures, may not be well predicted in models. While average temperature or precipitation may begin to coincide with a species range requirements, the occurrence of even one deep freeze could decimate populations of these "formerly" southern species.