Study documents widespread growth slowdown
An extensive tree-ring study in the Northeast suggests a widespread and steady decline in the health of sugar maples, one of most economically and ecologically important trees in the eastern United States and Canada.
The decline started showing up in the 1970s a decline in the growth rate of of sugar maple trees, but the reasons are still unclear, according to the State University of New York researchers who recently published their findings in the open-access journal “Ecosphere.”
“Given their relatively young age and favorable competitive status in these forests, these sugar maples should be experiencing the best growth rates of their lives,” said Daniel Bishop, who conducted the study as part of his master’s thesis at ESF.
Bishop said he was surprised at the findings, but emphasized that the data are clear, showing that the downward trend in growth rates is continuing to this day.
Sugar maples are prized for high-quality wood and spectacular fall colors, and for fueling a multibillion dollar syrup industry. Sugar maple is also ecologically critical in Northeastern hardwood forests, fostering fertile soils and supporting a high degree of biodiversity of plants and soil organisms.
The new study started by looking at earlier research on acid rain impacts. Bishop said he looked at hundreds of trees across the Adirondacks in areas that had previously been assessed for the effects of acid rain. The researchers expected that poor growing conditions related to acid rain would influence how the trees responded to a warming climate.
“The last few decades have brought warmer and wetter conditions to the Adirondacks, which are typically good for plant growth,” said Dr. Colin Beier, an associate professor of ecology at ESF who supervised Bishop’s thesis research. “Meanwhile, there have been big strides in reducing acid rain, which is especially damaging to sugar maple. Given these changes, we would expect these trees to be thriving, but they are not.”
The detailed analysis conducted by Bishop and his co-authors did not find conclusive evidence that climate change or acid rain were the culprits. But Beier said this does not eliminate the possibility that these factors play a role. Such stressors can predispose trees to become less resistant to or tolerant of pests, diseases or other forms of stress. A number of factors, including insect outbreaks and late frost damage, are known to affect sugar maple.
“Outside of studies of red spruce in the 1970s, I have never seen anything quite like this,” said a study co-author, Dr. Neil Pederson, an ecologist at Harvard Forest in Massachusetts and an expert on tree rings and climate change. “Most tree-ring studies of canopy trees in the region do not show a decline like what we see in these sugar maple. Combined with evidence of reduced natural regeneration of sugar maple in the region, it is a concern.”
Bishop now works with Pederson as a researcher at Harvard Forest.
A question remains regarding whether similar declines are occurring in New England and eastern Canada. Prior research by U.S. Forest Service Northern Research Station scientists suggests similar growth declines might be occurring widely across the region, but further study is needed to verify their observations.
Beier said that, although a suddenly slower pace of growth is not a sure sign that a tree’s death is imminent, or that recovery is impossible, such changes when observed across a wide area can indicate a significant problem for a species.
Lower growth rates have direct implications for the management of sugar maple forests, whether the focus is on wood products or sap and syrup production. Guidelines for sustainable tapping require an understanding of growth rates to minimize long-term damage to tree tissues, while foresters creating management plans need to know how rapidly trees are reaching commercial size.
“Time will tell if slower growth is a harbinger of something more serious for sugar maple,” said Beier. “But given the ecological, economic and cultural importance of this tree, the stakes could be high. We need to sort out whether these declines are more widespread, the reasons why they are occurring, and what their implications might be for our ecosystems and local economies.”