Some Surprising Aspects of Climate Change on Eastern US Forests

During my 40-year career as a forest ecologist and tree physiologist, I have seen climate change grow from a fringe idea to the dominant topic of environmental studies. Indeed, much of the world has warmed since the 1980s, a trend that started about 1880 (Figure 1). During this time, the average global temperature has increased by about 1°C or 2°F. This increase has been attributed to a dramatic increase in greenhouse gases, most notably CO₂, which has increased from 280 ppm to about 410 ppm.

Ice cores from glaciers and icecaps around the world have provided long-term climate records. Over the last 10,000 years, since soon after the last ice age ended, we have seen dramatic changes in temperature (Figure 1). This includes a huge decline about 8,200 years ago, which is attributed to glacial melting and a change in ocean currents from being inundated with huge amounts of fresh water. Then, there was a series of rapid rises and falls during the Holocene Thermal Maximum, which produced the highest temperatures during this era, and then a steady decline in temperature during the period called Neoglacial Cooling.

It is important to understand that the benchmark used for the current rise in temperature is among the lowest temperatures seen in the last ten thousand years, at the end of the Little Ice Age (Figure 1). It seems to me that the 1°C warming that has occurred is moving us closer to some middle ground rather than some calamitously high temperature. This is especially true for the eastern US, where temperature increases have been subtle and coupled with increasing precipitation, on average (Figure 2). However, some regions have had warming well above the 1°C average, which has been problematic, particularly when coupled with severe drought conditions.

My research has shown that, in general, trees in the eastern US have fared quite well during climate change, which is not surprising given the subtle variations. Unfortunately, eastern forests have been greatly impacted by other factors, such as fire suppression, exotic insects, diseases, and invasive plants. It is important to understand that while CO₂ is a dominant greenhouse gas associated with global warming, it is also the gas that fuels photosynthesis. When CO₂ is combined with water, the result is a molecule of sugar (glucose) in the photosynthetic pathway. Thus, more CO₂, more photosynthesis, more plant-tree growth.

We now have longer growing seasons, by about 7–10 days, due to global warming, which gives plants more time to grow. This has been highly evident in my tree ring research, where we routinely see trees putting on much more diameter growth now than in the past. This is particularly evident in young trees, but we see it in old trees as well. This is remarkable; old trees should be slowing down, not speeding up. This increase has even been noted in 4000-year-old bristlecone pines, the oldest trees in the world.

Forest composition in the eastern US has been changing quite dramatically during the last century. The main change has been an increase in shade-tolerant and mesophytic (middle moisture) trees, such as red maple (the #1 increaser), followed by black birch, tulip poplar, blackgum, and others. Species like hemlock and beech would also be increasing if not for problems with hemlock woolly adelgid and beech bark disease. These increases have been to the detriment of oak, hickory, and pine trees. My research has found that the main driver of this change is not climate change but the suppression of fire, starting with Smokey the Bear legislation in the 1930s. If climate were the major driver of forest change in the east, we would be seeing an increase in heat- and drought-adapted oak, hickory, and pines rather than the increase in trees adapted to mesic, cool, and/or shady sites, counter to global warming.

Greenhouse gases and climate change have impacted fall colors in some positive and some negative ways. While the increase in red maple is bad for the sustainability of oak, hickory, and pine forests, it is one of our brilliant fall leaf color species. In addition, the longer growing season and increased CO₂ are increasing photosynthesis and, therefore, the leaf pigments that produce the brilliant fall colors. However, the timing of peak colors is now about 5–14 days later than it was about 40 years ago, when I first started carefully observing year-to-year variation in fall colors. In New England, peak colors are coming about two weeks later in most years.

One potentially negative aspect of climate change on fall colors is when warm and wet weather extends to mid-October. This type of weather is counter to the normal cooldown and will keep tree leaves green by extending the growing season. This is often bad for colors, with leaves going from green to brown, particularly if there is a sudden frost. Fall colors are quite resilient, even in the face of climate change, and in most years, we have had good to excellent colors in Pennsylvania.

We should not be dismissing the future potential impacts of climate change. The eastern US has been lucky so far. This is very different from the western US, where more dramatic warming and extreme drought, along with Smokey the Bear fire suppression, has resulted in high fuels, catastrophic fires, and extensive tree mortality over the last several decades. Indeed, the eastern US has seen droughts and large wildfires in the southeast in recent years. This may be a localized phenomenon or a sign of the future; time will tell. Moreover, global warming may be exacerbating the devastating impacts of exotic insects, diseases, and invasive plants that are now ubiquitous.

Forests and other plant communities are vitally important in combating climate change. They shade the earth and are major sinks for taking up CO₂, which slows down its increase in the atmosphere. Foresters can play a vital role in combating climate change through the reforestation of open land and cutover areas, fostering tree populations with high genetic diversity, using highly adaptable, generalist tree species, promoting trees with the ability to migrate, trees with high seed production, trees with large ranges and niches and trees with greater longevity. I am proud to see an increasing number of foresters getting involved in mitigating the impacts of climate change through ecological forestry, which will clearly be a boom for the profession.

If you have any questions or are interested in collaborating with FOCCE, please reach out to Melissa Kreye at mxk1244@psu.edu.

Literature Cited

Abrams, M. D. and G. J. Nowacki. 2015. Exploring the Early Anthropocene burning hypothesis and climate-fire anomalies for the eastern U.S. Journal of Sustainable Forestry 34:30–48.

Nowacki, G. J. and M. D. Abrams. 2015. Is climate an important driver of post-European vegetation change in the eastern U.S.? Global Change Biology 21: 314–334. doi 10. 1111/gcb.12663.