Climate and Minnesota’s woodlands: Reflections from a February 2012 event

What can foresters and landowners do to maintain the health and productivity of the state’s woodlands during a period of rapid change and uncertain future conditions? In February 2012, the University of Minnesota’s Sustainable Forests Education Cooperative (SFEC) convened researchers and natural resource professionals to answer this question. This article offers a short summary of the presentations and ideas discussed at the symposium.

With increasing storm frequency and an increasingly clear trend of increasing temperatures, particularly in winter, in Minnesota, a great deal of recent attention has been focused on the future of Minnesota’s forests. First, what’s really changing? University of Minnesota climatologist Mark Seeley discussed recent observations of Minnesota’s climate. Generally avoiding discussion of predictive models, Seeley focused on data observed already in Minnesota, emphasizing a few prominent trends: Milder winters, rising dewpoints, and “the changing character” of precipitation toward a smaller number of more intense rain events. This last trend, the shift from a large number of low-intensity rain events to a smaller number of major storms means that more rainfall runs off the surface and less is retained in the soil, increasing the frequency and intensity of drought events even as total precipitation increases.

Lee Frelich, also of the University of Minnesota, discussed the effects of the changes in drought frequency on biome boundaries. Frelich emphasized that as with most ecosystem attributes, biome boundaries have shifted in the past and will shift again, likely driven primarily in the coming decades by increasing drought frequency, coupled with increasing impacts from wildfire and intense wind events like the one that affected the Boundary Waters Wilderness in July 1999. Frelich emphasized the important role that landowners can play to help forests adapt to these changes.

Emerging bur oak leaves. Click for a larger version.

Ecosystems will respond to these changes in a variety of ways. Can trees themselves change their “behavior”? Describing a major Cloquet study of tree physiology and phenology changes in response to warmer conditions and higher temperatures, Rebecca Montgomery shared some interesting insights. A large study manipulating soil and air temperatures as well as ambient carbon dioxide concentrations (the B4WARMED study) found that all ten of the Minnesota tree species studied have some flexibility in their response to higher temperatures, but each species responds in its own way. For example, the timing of budbreak in sugar maple increased in a linear fashion with increasing temperature, but paper birch had a more complex response. Noting the comments already made about the changing character of Minnesota’s summer precipitation, Montgomery noted that the future moisture regime would be important to how Minnesota’s trees responded. In general though, her research suggests that boreal conifers such as black spruce and balsam fir appear to be less able to adapt, while temperate species such as red maple, along with some woody invasives, seem well positioned to thrive.

The morning presentations suggest that A) conditions have already begun to change in important ways; B) these changes are likely to continue, although there’s a great deal of uncertainty in exactly what future conditions will be like; C) these changes will affect the composition and health of Minnesota forests and woodlands; and D) different species and ecosystems will respond differently.

Taking action: How to maintain healthy, productive family woodlands?

So what can Minnesota’s woodland owners, who manage 1/3 of the state’s forest land, do to maintain the health and productivity of their woods? This was the topic of the afternoon presentations. Family woodland owners have a great opportunity to be part of the solution in Minnesota.

Drawing on detailed data collected on a 125-year old red pine stand in the Superior National Forest, University of Minnesota silviculturist Tony D’Amato discussed the complex relationship between thinning and drought response. The data suggest that for the same reasons that you thin the plants in your garden, thinning is one of the most effective strategies to improve vigor and health. (For a variety of reasons, this is less true for aspen than for most other forest types.) But D’Amato also found that thinning too heavily can reduce the stand’s ability later in life to grow through major drought events, particularly as the stand ages. Young stands were more “plastic” and able to grow through droughts than old stands in which trees carry a higher physiological load–it takes more energy to maintain a large, spreading crown. Thinning remains one of the most important and effective strategies to improve the health and productivity of many of Minnesota’s important forest types. (Even stands thinned too heavily are in much better condition than stagnant, overstocked stands.) But historic data suggest that when it comes to drought, a series of light thinning treatments may produce a more resilient stand than one or two heavy thinning.

Mark White of The Nature Conservancy in Minnesota and Brian Palik, a researcher with the US Forest Service, both emphasized that landowners and natural resource managers will will need to integrate a higher level of uncertainty into their plans now than they have before. This is particularly true given the long rotation lengths of many Minnesota tree species–at best, growing aspen in Minnesota, foresters are planning for 35-40 year horizon. With red pine or other, longer-rotation species, rotation lengths are more than twice that time.

Palik discussed current factors considered by natural resource managers in developing silvicultural plans. Some of the more important factors are the following:

  • Regional forest and landscape priorities: The Minnesota Forest Resources Council’s regional landscape plans are a good local example.
  • Stand-scale considerations such as implementation of Minnesota’s voluntary site-level guidelines, riparian zones, the wildland-urban interface, visual quality, ecological classifications, soils, site index, management history, etc.
  • Current stand structural characteristics and species composition.
  • Stand health and risk factors
  • Landowner objectives
  • Fish and wildlife habitats
  • Cultural heritage considerations
  • Current and future markets for harvested products

In considering climate change, the same planning framework remains valid. The major change is that uncertainty is much higher now than in the past. Given Minnesota’s relatively short growing season, our consequent long rotation lengths, and the pace of observed climatic changes discussed above, uncertainty becomes an important consideration that may impact stand health and productivity to a greater degree than in the past. Specific considerations include projected future habitat suitability of component species due to expected warmer conditions, more frequent storms, and more frequent drought events. Novel native pest behavior, introductions of new non-native insect, disease, and plant species, increasing wildfire risk, and greater plant stress are also potential sources of uncertainty.

A red pine thinning treatment. Click for a larger version.

Palik discussed a case study of managing pine systems around the Chippewa National Forest in north central Minnesota. A century from now, habitat conditions are expected to be less well suited than they are now for most of the important tree species in this system. What can land managers do now to prepare stands to remain viable and productive as conditions change? Palik emphasized three broad strategies:

  • Density management: Frequent thinning is perhaps the most important strategy available. Palik suggests maintaing stands closer to the lower end of traditional stocking recommendations to emphasize tree vigor, health, and resistance to bark beetle, which may become a more important pest in Minnesota in the coming decades.
  • Composition management: A key factor here is maintaining complex stands with more native species represented and a broader suite of age classes across the landscape, favoring species and genotypes with a high level of adaptability to changing conditions, and adding new species and genotypes likely to perform well under expected conditions.
  • Fuels treatments: Wildfire may be more widespread in the future, particularly given the possibility of more common drought conditions. Prescribed burning is risky, particularly in areas near human inhabitation. Other kinds of fuels reduction treatments and strategies like reducing the density of understory hazel brush will facilitate regeneration of desired trees and increase soil moisture.
Mixedwoods release treatment, Grand Rapids. Click for a larger version.

White discussed similar practical strategies to manage forest stands to increase resilience and adaptive capacity in the face of uncertain future climate conditions. White described a series of alternative silvicultural strategies. His talk focused on conifer restoration in northeastern Minnesota. In both case studies, White emphasized climate adaptive strategies involving increasing the mix of even- and uneven-aged management, increasing species and life history diversity, and increasing mid-tolerant species. Assisted migration of southern species into these systems may be important as well considering the relatively slow pace of natural migration of important species.


Several of the presentations at the SFEC event emphasized a consistent theme: Managing for an uncertain future by increasing forests’ resilience to changing conditions. We heard repeatedly about the relatively new concept of increasing response diversity: recognizing that different species and age classes will respond differently to changing conditions, managing to increase both species and age-class diversity. Increasing stand- and landscape-level diversity promotes an inherent capacity for diverse responses to change. This in turn increases the chance that at least one or a few will respond favorably to the changes that emerge.

Thinning has long been recognized as an essential tool in increasing forest productivity and vigor. Looking to the future, thinning is likely to become even more important, not only to simply increase crown size and growth of existing dominant trees but also as a silvicultural strategy to accelerate compositional change toward a diverse mix of species more resilient to uncertain future conditions.

More information on the February 21, 2012 SFEC Adaptive Management in the Face of Climate Change symposium, including PDF versions of the presentations, is at SFEC’s Summaries of Past Workshops page.

Eli 's work addresses Minnesota forest ecology & management. He's based in St Paul.

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