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US Forest Service: Weathering The Warmth - Early Spring's Impact On Trees

By Joseph M. Phillips, US Forest Service Northern Research Station

As the first official day of spring arrives [on March 19], many regions of the United States are experiencing above-average temperatures.

Across the Northeast and Midwest, the winter has been remarkably mild, with minimal snowfall.

Consequently, many deciduous trees are budding earlier than usual, causing concerns about the potential impacts of early warming on local trees and forests.

According to Kevin T. Smith, a plant physiologist with the USDA Forest Service Northern Research Station, this premature budding of leaves can pose risks to the health of trees.

“Early opening of buds exposes the new tender tissue to a late-season killing frost that could destroy or distort the new growth,” explains Smith. “Also, creatures that depend on that new tissue for their own nutrition will not find this food when they normally expect it. Early flowering can put the plant out of sync with pollinators.”

Beyond these more apparent effects, there can also be adverse physiological outcomes. During a normal growing season, trees utilize evapotranspiration, which is the normal evaporation of water from leaves and surfaces of the tree that draws sap up from the roots to branches and leaves.

Evapotranspiration also serves to help regulate leaf temperature which is important for photosynthesis.

However, if the ground remains frozen while the air has warmed, water is unable to enter the roots to replace moisture lost to evaporation through the leaves.

“So that lack of new water into the tree increases the tension (negative pressure) within the sap. Under extreme conditions, the tension may break the flow of sap and cause winter dieback of branches,” says Smith.

Maple syrup production, a hallmark of late winter and early spring, can also be impacted with the early arrival of warm weather.

“In early warming conditions, the first big flush of sap at a newly tapped tree might be all that the tree yields,” explains Smith. “Sap yield, in terms of both volume and sweetness, is favored by cold nights, deep snow to insulate roots where the sugar resides as starch, and sunny, slightly mild days.”

Without the cycle of cold nights and warm days, the sap yield from any given tree could be reduced.

While the beginning of springlike temperatures has been trending earlier for decades, efforts are underway to help trees adapt to the changing climate.

Long-term studies, such as the Forest Service’s DREAM project, aim to address these challenges through assisted migration of tree populations.

In the meantime, if warm weather has arrived early in your neck of the woods and you notice stressed trees in your yard, consider giving them an early-season watering once the soil has thawed at its base.

“Maintaining proper soil moisture is important, especially for nursery trees planted in the past few years,” says Smith.

Resource Links:

-- US Forest Service Northern Research Station Features

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