Warmer Temperatures, Less Snowpack May Limit Growth of Vermont Tree Species

Climate change and shifting temperature and precipitation patterns could have significant impacts on the future health and growth of tree species.

The northeastern United States and adjacent Canada have experienced unprecedented increases in precipitation over the last several decades, a pattern that is expected to continue or even increase as climate change progresses. Yet, while precipitation rates rose, this increase did not occur equally across all seasons. Winter and spring are projected to have the greatest rises in precipitation, but winter precipitation will likely fall increasingly as rain instead of snow. In contrast, summer precipitation may decrease, and drought events could jeopardize tree growth.

Along with a shift in seasonal moisture patterns, increases in temperature have not occurred uniformly across all seasons in the Northeast. Winter temperatures—in particular, minimum winter temperatures—have increased the most.

Vermont scientists studied the three major tree species in the northern hardwood forest: sugar maple, American beech, and yellow birch, along with a fourth species, red maple, which has increased in abundance in northeastern forests. They documented how climate change is associated with changes in growth of these four species by examining how each has responded to past environmental conditions. These data then help to determine how each tree species might respond to a future changing climate.

Using tree ring data, six researchers from the University of Vermont and the USDA Forest Service Northern Research Station looked at links between tree diameter growth from 1945 through 2014 and several environmental factors. They studied 690 trees on 45 plots at varying elevations throughout Vermont. On each plot, they chose large, mature trees. Using an increment borer, they removed two pencil-sized cores from the trunk of each tree to estimate tree age and growth patterns over the last several decades. They found that over the past 70 years all four tree species showed increasing growth followed by a slowing, or plateau, in growth, which indicates a naturally maturing forest.

The research team evaluated how each tree species’ growth was related to temperature and moisture from 1945 to 2014 and to pollutant deposition from 1980 to 2014. The researchers used climate data from the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center, pollution deposition data from the National Atmospheric Deposition Program, and moisture data from the Standardized Precipitation-Evapotranspiration Index.

Moisture and Tree Growth

All four species showed greater growth when there was more available moisture. Both current and previous summer moisture were important for growth of all four species, which shows a possible “legacy effect” of environmental conditions one year affecting growth the next.

“This contributes further evidence to recent broad-scale studies that highlight the importance of growing season moisture regimes in sustaining woody productivity,” said Rebecca Stern, lead author on the study which was part of her PhD research.

Growing season moisture is known to be important for growth of many tree species. But for species specifically adapted to the northern hardwood forests of northeastern North America, changing moisture conditions during the non-growing season may also have a substantial influence on growth. Trees of the northern hardwood forest are adapted to winters with consistent periods of snowpack. But in recent decades, with winter precipitation increasingly falling as rain, reduced winter snowpacks could detrimentally affect tree species’ health and productivity.

This study highlighted the importance of early winter snow in December for tree growth the following spring. Snowpack insulates forest soils from low air temperatures that can lead to soil freezing. Insufficient snowpack can increase the risk of root freezing injury and loss, which can disrupt sugar storage in the roots that is needed for aboveground growth in the spring.

For red maple, the relationship between growth and winter moisture was not as consistent, which suggests that this tree may be less susceptible to growth reductions as the climate changes. However, red maple, along with sugar maple and beech, grew better with an increasing number of days that snow depth was greater than one inch in March.

Temperature and Tree Growth

In this study, temperature was not associated with tree growth as much as moisture was. However, sugar maple, beech, and yellow birch showed increasing growth with lower previous summer and fall temperatures, while red maple grew better with higher summer and fall temperatures. Both sugar and red maple did not grow as well with warmer spring temperatures. Beech growth declined with warmer winter temperatures throughout the 70 years, while yellow birch exhibited this trend only in more recent decades.

The three classic northern hardwood species showed a slowing in growth with rising November and winter temperatures. In contrast, red maple showed increasing growth with warmer winter temperatures at higher elevations, suggesting that less extreme winter temperatures might benefit this species at some locations.

During the first half of the 70-year period, maple and yellow birch growth exhibited increased growth with warmer minimum summer temperatures. This response disappeared in more recent decades as those temperatures increased.

Pollution and Tree Growth

There were very few associations of pollution deposition with tree growth, especially for American beech, which has a documented tolerance to pollutants. Sugar maple growth showed very little association with pollutants, while red maple exhibited some growth decline related to sulfate deposition. Of all species, yellow birch exhibited the greatest decrease in growth associated with spring nitrate deposition and an increase in growth related to lower acid pollutant levels.

“Overall, pollutant levels are generally declining in the northeastern United States since implementation of the Clean Air Act in 1970 and should have less impact on tree growth,” said Stern.

Key Findings for Each Tree Species

• Sugar maple showed a slowed rate of growth over the 70 years but grew best with greater winter and summer moisture. Its growth was not as affected by temperature as it was by moisture, but warming temperatures limited growth to some extent.
• American beech trees selected for the study had little evidence of beech bark disease and showed moderate growth rates which have increased in recent years. Beech showed fewer associations with moisture compared to the other tree species and may be less vulnerable to summer drought than maple and yellow birch. Beech did show a reduction in growth related to warmer winter and summer temperatures.
• Yellow birch growth varied over time, possibly due to elevation and location. Growth increased with increasing previous year and current year summer moisture availability but showed declines with increasing temperatures in August and September.
• Red maple displayed the greatest growth rates of the four species, though its growth declined in recent years from maximum levels. This tree species grew best with greater summer moisture and higher October minimum temperature, while a higher March maximum temperature was associated with growth decline. Red maple growth did not slow with increasing winter temperatures and its correlation to snow depth was not as striking as for the other three species. Instead, red maple growth showed a positive relationship to warmer winter and even summer temperatures at some locations, suggesting that red maple may be less vulnerable to changes in climate than co-occurring sugar maple, beech, and yellow birch.

The team’s findings suggest that of all climate change variables, winter precipitation may be an important factor to consider in determining future habitat suitability for northern hardwood tree species adapted to cold climates.

“Understanding how these seasonal changes in moisture and temperature affect tree communities is crucial to managing the iconic northeastern forests for the current and future ecological, economic, and recreational services that they provide,” said Stern.