Illustration by Adelaide Tyrol
An unusual number of sugar maple trees throughout their range are in decline, meaning that they aren’t growing well or generally showing much vigor. Explaining exactly why this is happening, however, is proving to be a maddeningly difficult task for forest ecologists.
This condition, called Sugar Maple Decline, is becoming widespread in Pennsylvania but has been observed across our area, too. Characteristics of the decline include: branch dieback, reduced growth, and, in some cases, unusually high levels of mortality. Since sugar maples are highly valued for their colorful foliage, sweet sap, strong lumber, and are a major component of the northern hardwood forest, scientists have been putting a concerted effort into understanding just why this decline is happening.
The story is not simple. Decline diseases, by definition, have no single cause; they occur as a result of a combination of factors, each of which plays a part in weakening the tree. Sugar Maple Decline has been observed in different places on and off over the past century, so some idea of which factors are important has emerged. Late frosts, insect defoliations, droughts, and winter freeze/thaw cycles have all been seen to play a role. A factor more recently identified has been soil nutrient imbalance. This factor, because it can be identified beforehand and is affected by human activity, is most relevant to landowners since it can be predicted and possibly mitigated.
A useful model for understanding decline diseases has been developed by Paul Manion at Syracuse University in New York. He describes three layers of factors that participate in “causing” a tree to decline. The first layer, called the “predisposing” factors, covers site conditions, climate, and even tree age. These alone won’t trigger a tree or population to decline, but they will make it more susceptible to subsequent stresses.
The second layer is called the “inciting” factors – the stresses that tip the balance. They could be either physical, such as a late frost, or biotic, like an insect defoliation. Trees without predisposing factors would still experience these events, but would be healthier to begin with and thus more likely to recover. Trees with one or more predisposing factors, on the other hand, are further weakened by the inciting factors and thus more susceptible to the final layer, the “contributing” factors. These are typically biotic stresses, such as a fungal or insect infestation. The contributing factors are often what finally kill a tree, but they would not have the chance to do so had the right combination of predisposing and inciting factors not weakened the tree enough to allow the infestation.
Understanding an outbreak of sugar maple decline means identifying the combination of predisposing, inciting, and contributing factors that are first reducing resistance and then reducing the health of the trees. Droughts, insect defoliations, and climate events are always likely candidates, but it is the recognition of the role of soil nutrients as a predisposing factor that is most relevant to us in New England, and it ties the story to our problem with acid rain as well.
In Pennsylvania, decline symptoms are most common on soils that are low in the nutrients called base-cations, particularly calcium and magnesium. Nearby trees growing on soils with high base-cation levels do not show decline symptoms as often. We have many soils low in base-cations in New England, as well as acid rain, so the concern is that acid rain could be turning some soils from adequate to somewhat depleted, leaving trees on those sites more vulnerable to stresses. A base-poor soil, therefore, appears to be a predisposing factor towards sugar maple decline, and acid rain appears to increase the number of base-poor soils.
So when we see sugar maple trees showing branch dieback or reduced vigor, can we conclude that acid rain is at work? Not necessarily. Other factors can act as predisposing stresses, and without knowing more about the soils, we would just be guessing. An individual tree’s ill health may come from combinations of insects, climate, or even just old age. But because base-poor soils seem to be a predisposing factor, and because acid rain is known to be turning neutral soils into base-poor soils, the cause for concern is obvious.
Decline diseases, by their complicated and interdependent nature, illustrate the limitations that scientists face in figuring out cause and effect in complex systems such as forests. But sugar maples are such beloved trees that scientists are striving to figure out just what, exactly, is harming them. As in Pennsylvania, the forests of Vermont and New Hampshire have to contend with both acid rain and some naturally poor soils, so only time will tell whether the right combination of other stresses comes along to trigger more widespread sugar maple decline in our area.
Geoff Wilson is an educator for the Hubbard Brook Research Foundation in Hanover, New Hampshire.