Those reddish spruce needles you see adorning some mountaintop spruce trees are dead, killed by freezing. They are a flagrant indication that the host tree could not stand the weather. Such intolerance would be problematic for any tree, since none can hibernate or move south to avoid the winter. But it is especially odd for red spruce, a tree species that for thousands of years had been doing quite well atop the Northeast’s mountains in some of the coldest, harshest winter environments anywhere.
Clearly something has changed, and it’s not good for spruce.
A primary culprit is acid rain. Yes, it’s still happening and it’s still causing a range of environmental problems. As our rain, fog, snow, sleet, and ice become acidified from sulfur and nitrogen pollutants produced upwind, a cascade of negative effects is triggered in our forests downwind. For decades now, ecologists have implicated such acidic precipitation in forest health decline and tree mortality – especially in our high-elevation sprucefir forests, which tend to receive more precipitation. But only relatively recently have forest scientists begun to piece together the specific causal mechanism of spruce decline. And those reddish-orange needles have helped guide the way.
Dr. Paul Schaberg is a research plant physiologist with the U.S. Department of Agriculture Forest Service in Burlington, Vermont. Schaberg and his long-time collaborators, including University of Vermont Research Associate Gary Hawley, have published dozens of scientific studies on the ecology of northeastern forests. They report that winter injury occurs to some extent every year somewhere on the landscape, but that severe, region-wide injury was documented with a four- to five year return cycle for much of the 1980s and 1990s. Then the phenomenon appeared more muted for a time, as Schaberg explains, possibly because many of the most susceptible trees had already died. And then came 2003.
Schaberg tells of riding a chairlift at Vermont’s Mt. Ellen that winter and noticing – where he hadn’t just weeks before – great numbers of red spruce showing the classic reddening of needles. It turns out he was witnessing one part of the most intense episode of red spruce winter injury ever quantified. Curiously, the balsam fir growing right among those spruce in those same conditions showed no evidence of freezing damage, while the spruce were announcing it loudly in color. This is a fundamentally important part of Schaberg’s and Hawley’s story: red spruce is uniquely sensitive to the effects of acid precipitation on those mountains.
All northern tree species undergo physiological changes in autumn as they prepare for winter. Chief among these changes is acclimation to shorter days and lower temperatures. Each species develops cold tolerance to different degrees – literally. Red spruce is inherently and naturally less cold tolerant than balsam fir. That is, in controlled laboratory tests, red spruce often show significant freezing injury at about -25°F, whereas balsam fir can survive artificially induced temperatures below -75°F. But acid precipitation decreases the red spruce’s ability to withstand cold.
Here’s how Schaberg and Hawley explain it. Acid deposition leaches calcium from red spruce needles and depletes soil calcium as well. This is critical because trees absorb calcium from soil through their roots and use it in a great variety of protective functions, such as in the creation of antioxidants and in the development of cold tolerance in needles. In other words, increases in acids lead to decreases in tree calcium, and decreases in tree calcium lead to decreases in cold tolerance. Because of acid deposition, red spruce needles become vulnerable to freezing at temperatures they had typically tolerated back when more calcium was available.
According to Schaberg and Hawley, winter injury in red spruce due to decreased cold tolerance is but one example of how acid precipitation-induced calcium depletion in forests disrupts the stress-response system of trees. In the case of red spruce, it decreases a tree’s ability to deal with cold. But a shortage of calcium may cause other problems for spruce, and for other species, too, when they are confronted with different stresses, such as drought, insects, and disease.