Urban Forest Health

Studies have consistently shown that diverse systems have exhibited greater resilience to extreme environmental conditions and greater potential to recover from disturbance than less diverse communities. This relationship makes less diverse communities inherently more susceptible to future changes and stressors. The diversity of potential responses of a system to environmental change (response diversity), is a critical component of ecosystem resilience. Response diversity is generally reduced in less diverse ecological systems.

Impact models agree that many temperate species will experience increasing suitable habitat and biomass across the assessment area, and that longer growing seasons and warmer temperatures will lead to productivity increases for temperate forest types. The list of species projected to increase includes American basswood, black cherry, bur oak, eastern white pine, red maple, white oak, and a variety of minor southern species. Models also indicate that deciduous forest types have the potential for large productivity increases across northern Minnesota.

Impact models agree that boreal or northern species will experience reduced suitable habitat and biomass across the assessment area, 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.

Large variation exists for projected changes in precipitation for Minnesota and the Upper Midwest. Although individual model projections may differ, there is general agreement that annual precipitation is expected to increase slightly (2 to 4 inches) during the 21st century. Models also tend to agree that precipitation patterns between seasons may shift substantially. Averages across multiple climate models indicate that winter and spring may experience 20-30% more precipitation by the end of the century, while summer precipitation is projected to decrease by less than 10%.

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 Minnesota.

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.

In the Upper Midwest, the duration of frozen ground conditions suitable for winter harvest operations has already shortened by 2 to 3 weeks in the past 70 years. 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 percent across northern Minnesota 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.