In much of New Hampshire and Vermont, our beloved maples are showing signs of distress. Sure, the reds and oranges for which our forests are famous are now popping out of autumn hillsides. Everything seems to be in order. But behind the scenes, scientists are becoming alarmed at the signs: increasingly sluggish growth, dieback of branches and crowns, and death of entire trees and stands – particularly of sugar maple.
Hypotheses for the cause of this decline are not in short supply. Short-term weather cycles, insect and fungus outbreaks, and global warming are all under suspicion. Fortunately – and especially since sugar maples are an economic engine for the region (for both their leaves and their wood and syrup) – many experts, including some of our best scientists, have been brought into the investigation.
At the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, scientists have devoted six years to a long-term study measuring the response of sugar maples to artificial calcium additions, in an attempt to determine if nutrient deficiency might be a part of the sugar maple decline puzzle. The recently published results confirm scientists’ suspicions that depletion of soil calcium, due in large part to acid deposition, is a leading cause of sugar maple decline.
The story begins not in the misty autumn hillsides of Vermont and New Hampshire but instead above the industrial centers of the West and Midwest. Here, high above the treetops, weather systems concentrate pollution from power plants and vehicles. These systems ride the jet stream eastward, picking up more atmospheric nitrogen and sulfur pollutants on their way. When the weather reaches the Northeast, if it is wet, the moisture falls to earth as snow, fog, or rain, each droplet a liquid crucible carrying at its heart a particle of acid. The result is what we call “acid deposition” – the same culprit that is corroding man-made monuments of marble and copper. Acid deposition causes a host of problems to natural systems, too, including acidification of waterways. Even worse, acids in rainwater leach base cations crucial to plant health, such as calcium, from the soil.
The scientists at Hubbard Brook, where sugar maple growth has declined over the past 25 years, set out to discover what would happen if enough calcium to replicate pre-industrial levels was added back into the equation. In 1999, pellets of calcium, in a mineral form known as wollastonite, were dropped over the experimental watershed from a huge hopper attached to a helicopter.
Just two years later, leaves of canopy sugar maples in the study area showed increased concentrations of beneficial calcium; in the fourth and fifth years after the wollastonite application, concentrations of potentially toxic leaf manganese had declined. By the sixth year, the crowns of study-area sugar maples were noticeably healthier than those in nearby, untreated areas.
The researchers also noted significantly more sugar maple seedlings in the treated study area, which they attribute to either higher germination rates, better survival, or both; the seedlings from the treated area were consistently about a third more substantial than those in untreated areas, and they a higher chlorophyll concentration in their leaves.
Finally, the team discovered that colonization of sugar maple seedling roots by beneficial mycorrhizae (fungi that make soil nutrients readily available, thereby helping maples grow and thrive) was significantly greater in the treated area. Even the relationship of mature maples with their mychorrhizal partners was enhanced, though to a somewhat lesser degree.
This study strongly supports previous work demonstrating the link between acid deposition and sugar maple decline throughout the Northeast, though the mechanism of decline is not fully understood. The authors suggest that calcium may ameliorate manganese or aluminum toxicity, but they note that the complexity of the interactions between trees, soil minerals, and mycorrhizae precludes an easy answer.
The Statue of Liberty, herself a victim of acid precipitation, has recently undergone renovation that has restored her to her former coppery glory. But what will it take to restore the glow of our autumn hills to the splendor of the years before industrial combustion? Or to prevent further loss? Carpeting a 30-acre forest with pellets of calcium is one thing, but is it even possible to cover the nine million acres of forest in Vermont and New Hampshire?
Perhaps the most valuable outcome of the Hubbard Brook experiment has been its ability to clarify and emphasize the important relationship between calcium and maple health. The relationship between calcium loss and acid deposition has already been documented by previous studies, as has the interplay between acid deposition, weather, and industrial combustion. The answer to sugar maple decline may lie not at the “sink” but rather the “source” – with the electricity and goods created elsewhere whose unhealthy byproducts inexorably flow east with the wind and the rain.