Urban Forest Health

Impact models agree that boreal or northern species will experience reduced suitable habitat and biomass across the Upper Midwest, and that they may be less able to take advantage of longer growing seasons and warmer temperatures than temperate forest communities. Across northern latitudes, it is generally expected that warmer temperatures will be more favorable to species that are located at the northern extent of their range and less favorable to those at the southern extent.

Evidence indicates that an increase in temperature and greater moisture stress will lead to increases in these kinds of stressors, but research to date has examined only a few species. Invasive species are already a persistent and growing challenge across much of the United States. Changes may exacerbate this problem, as warmer temperatures may allow some invasive plant species, insect pests, and pathogens to expand their ranges farther north. Northern Minnesota may lose some of the protection offered by a traditionally cold climate and short growing season.

At a global scale, the scientific consensus is that fire risk will increase by 10 to 30 percent due to higher summer temperatures. For the early part of the 21st century, there is low agreement in this trend across climate models. By the end of the century, however, most models project an increase in wildfire probability, particularly for boreal forests, temperate coniferous forests, and temperate broadleaf forests. Studies from southern Canada also project more active wildfire regimes in the future.

Heavy precipitation events have been increasing in number and severity in the upper Midwest in general and for Minnesota in particular, and many models agree that this trend will continue over the next century. For example, storms in the 99th-percentile category incrased by 42% from 1958 to 2016 across the Midwest. Large storms are also expected to deliver more rainfall as well. In the Midwest, 20-year return storms are projected to deliver 11 to 20% more rainfall by the end of the century. Most heavy precipitation events occur during summer in the Upper Midwest.

Evidence at both global and local scales indicates that growing seasons have been getting longer, and this trend is projected to become even more pronounced over the next century. As seasons shift so that spring arrives earlier and fall extends later into the year, phenology may shift for plant species that rely on temperature as a cue for the timing of leaf-out, reproductive maturation, and other developmental processes. Longer growing seasons could also result in greater growth and productivity of trees and other vegetation, but only if balanced by available water and nutrients.

Cold-season soil temperatures are projected to increase between 1.8 and 5.4 °F by the end of the century, and total frost depth is projected to deline by 40 to 80 percent across northern Wisconsin and western Upper Michigan by the end of the century. These conditions could increase water infiltration into the soil and reduce runoff, but they may also lead to greater soil water losses through increased evapotranspiration.

A variety of models project that across the Upper Midwest, more winter precipitation will be delivered as rain, more snow will melt between snowfall events, and the snowpack will not be as deep or consistent. Lake-effect snowfall may increase in the short-term, but these events may convert to rain as temperatures increase.

Soil moisture is expected to change in response to warmer temperatures and seasonal changes in precipitation. Changes are likely to vary seasonally as well as geographically. More intense and prolonged precipitation events would be expected to create wetter soil conditions, while increased temperatures and less frequent rainfall events would lead to drier soils .

Model results project that species currently near their northern range limits in the region may become more abundant and more widespread under a range of climate futures.Results from forest impact models suggest that species such as black cherry, chestnut oak, and yellow-poplar may have increases in both suitable habitat and biomass, and some deciduous forest types have the potential for productivity increases across the assessment area.

Across northern latitudes, warmer temperatures are expected to be more favorable to individuals near the northern extent of their species’ range and less favorable to those near the southern extent.Results from climate impact models project a decline in suitable habitat and landscape-level biomass for northern species such as black spruce, red spruce, tamarack, and paper birch, as well as spruce-fir forest communities. These northern species may persist in the region throughout the 21st century, although with declining vigor.