How Warming Winters Affect Maine’s Forests

On September 27, 2022, the CREA community learned how Maine’s warming winters are affecting some of our forests’ elegant adaptations to extreme cold from UMO Professor of Soil Science Ivan Fernandez and UMO Assistant Professor of Forest Ecosystem Physiology Jay Wason III. You can watch the video here and at the end of this summary.

Professor Fernandez, who co-chairs the Scientific and Technical Subcommittee of the Maine Climate Council, recommended several resources for those who want more information on how climate change is affecting Maine. He mentioned Maine’s Climate Future (2020 update), Scientific Assessment of Climate Change and Its Effects in Maine, and Maine Won’t Wait: Maine Climate Science Update 2021.

He cited some sobering data on how Maine’s climate has changed in the last 100 years:

  • Air temp has increased by 3.2°F
  • Warm seasons have extended by 2 weeks. Winter is warming faster than other seasons, particularly Jan and Feb.
  • High heat days/yr have increased
  • Precipitation has increased by 15% with more coming in intense downpours
  • Snow has decreased by 7%, and will decrease by an additional 20-40% by 2050, particularly in coastal regions
  • Ocean temp has increased by 0.01°F/yr
  • Sea level has risen by 0.62 ft

Then he addressed the question: Does a changing winter matter? As expected, it does – to people, communities, and the natural world. A changing winter impacts outdoor recreation, water quality, public health (expanding ranges of pests and pathogens), biodiversity, habitat, the forest products industry (harder to harvest when ground isn’t frozen), and more.

Scientists are using a range of metrics to evaluate how winter is changing, including the number of thaw days, ice days, frost days, extreme cold days, snowmaking days, bare ground days, and more. Analysis of the data reveals that cold days are decreasing and thaw days are increasing.

A 2015/2016 snow removal study revealed that heavy snowpack largely prevented underlying soil from freezing and delayed the snow-out date by almost two months. By comparison, a light snow cover did not protect underlying soil from freezing multiple times throughout the winter. And, it exposed soil to cycles of rain and thaw, followed by freezing temperatures, leading to formation of ‘concrete frost’- rain soaks into light snowpack, saturates the soil, fills pore spaces in the soil, then freezes hard, damaging soil microbes, mycorrhizae, and tree roots.

The changing climate is causing ‘winter whiplash’ – rapid changes that organisms are unaccustomed to (e.g. cold temps on spring leaves, winter heat waves). Professor Fernandez cited Feb. 24, 2022, in which Maine experienced a 50° temperature change overnight. Scientists have traced a yellow cedar dieback in Alaska to climate changes – the cumulative impact of less spring snow (less protection for soil), early dehardening by trees, and root damage caused by concrete frost.

He closed by noting the elegant synchrony of ecosystem components, in which organism functions and life cycles are carefully timed (e.g. white snowshoe hares are sitting ducks on bare ground in winter). We still have much to learn about how ecosystems will respond to climate changes that are happening rapidly.

Professor Wason followed with a closer look at trees and how they sense and respond to cold temperatures. Trees and plants detect the coming of winter principally by changes in daylength, with some secondary signals from temperature and moisture. They need to prepare – to protect tissues from freezing (to avoid damage caused by ice formation), and to resorb sugars/nutrients from leaves so they’ll be available to fuel spring leaf out.

Our brilliant fall leaf colors signal tree preparation for winter. Green chlorophyll production declines, revealing yellow carotenoids in leaves. As chlorophyll declines, red anthocyanins are produced to protect leaves from excess light energy (principally in maples) as sugars are resorbed. The most brilliant colors require warm days (to produce anthocyanins) and cool nights (resorption of chlorophyll). We can expect slightly more color and possibly one extra week of color duration due to our changing climate, although heat stress and drought may reduce or delay color. If you like brilliant colors, make sure maples remain in our landscapes.

Professor Wason then moved to differences between evergreen vs. deciduous cold tolerance strategies. Evergreens retain their needles through winter, so needles need to tolerate cold, while deciduous trees drop their leaves to avoid cold temps.

Different species have different cold tolerances. Red spruce is less cold tolerant than white pine, hemlock, and white spruce. Red spruce increases cold tolerance by adding salts to its tissues.  The timing of cold temperatures makes a difference (e.g. extreme cold is tolerated best in December – February).

In the tree world, it appears that spring activity is moving forward more than fall activity is moving back. Wisconsin experienced an extreme warming event in February of 2017. It had six days above 60 degrees, 2 months earlier than usual. Observations indicated that shade-intolerant species had weaker dormancy and started to leaf out.

In general, warming temperatures in the summer allow warm-adapted species to thrive, but growth of boreal trees declines. However, drought, which is associated with climatic changes, causes declines in most species. The growth of Maine’s signature white pine is particularly affected by warming combined with drought.

Professor Wason closed by highlighting new research that is evaluating the impact of extreme climate events on tree regeneration in the northern forest. Stay tuned for reports from that work!     

Professor Fernandez powerpoint

Professor Wason powerpoint