Sugar Maples and Acid Deposition in the Adirondacks

Like the polar bear of the Arctic or the coconut palm of the tropics, the sugar maple (Acer saccharum) defines northeastern forests. The neon flash of its leaves in fall draws tourists from around the world, and its sap drives an economy of millions. Sugar maples are also highly valued as sawtimber, and landowners often apply silvicultural prescriptions intended to maximize sugar maple yield. Such harvests may have unintended consequences, however, in regions where soils have been critically depleted by decades of acid deposition.

Because sugar maples have a high calcium requirement, they grow less vigorously and regenerate less successfully in nutrient-poor soils, as research in New Hampshire showed in 2013. In the western Adirondacks, a hotspot of acid deposition, naturally poor soils with little ability to buffer acidity have become so leached of calcium and other macronutrients that they may no longer support sugar maple regeneration after harvest. Researchers in the western Adirondacks modeled future forest composition by comparing the effects of four different silvicultural treatments on forest stands with high or low levels of soil nutrients. The treatments under scrutiny were an unmanaged reference treatment, a diameter-limit harvest in which all trees larger than 12.7 cm diameter-at-breast-height were cut, a shelterwood harvest in which stands were thinned from below the canopy in 2010 and larger trees were removed 10 years later, and a sugarbush treatment in which all trees except sugar maples were removed and the stand was maintained thereafter at a relative density of 50%.

If sugar maple fails to regenerate after harvest because of severe acidification, then what would future forests look like, both ecologically and economically, when we apply common management practices?

-Colin Beier, SUNY ESF

Using the Northeast Forest Vegetation Simulator, the researchers input field observations of current sugar maple regeneration at 50 plots across the region to model the effects of these management regimes over 100 years. The results of the simulation contributed to estimates of gain or loss in five primary ecosystem services provided by sugar maples: biomass production, sawtimber, maple syrup, greenhouse gas mitigation through carbon storage, and the aesthetic and economic benefits of fall foliage.

Regardless of management regime, sugar maple basal area after 100 years was highest in sites with well-buffered soils. These sites also produced higher values in terms of ecosystem services. Where the soil was poorer, harvest strategy played a more defining role. Stands that were left unmanaged (reference treatment) and stands that were managed exclusively for sugar maple dominance (sugarbush treatment) retained high sugar maple basal areas, while treatments intended to maximize yields of biomass and wood products effectively erased sugar maples from the forest. These harvest prescriptions removed mature sugar maples and left seedlings to be outcompeted by species that have greater success regenerating in acidic soils. A century later, plots on poor soils had shifted to dominance by beech and red maple.

Translating ecosystem services into economic terms revealed that profits from maple syrup significantly exceeded the value derived from biomass production, wood products, carbon storage, or fall foliage. As a result, stands left unmanaged or managed as sugarbushes were of significantly higher median value ($183,605 per hectare) than stands harvested for wood or biomass ($26,136 per hectare). Furthermore, unmanaged stands were not significantly less valuable than stands treated as sugarbushes, indicating that silviculture may not be necessary to maximize maple syrup yield.

Here in Vermont, although the lingering effects of acid deposition on forest soils may be less severe than in the Adirondacks, the impact of the maple syrup industry is even more substantial. Syrup sales contributed roughly $330 million to Vermont’s economy in 2013, and sugaring created over 4,000 jobs (Becot et al. 2013). In order to protect this valuable industry, forest landowners may need to take into account the levels of macronutrients present in their soils. Where soils are too poor to favor regenerating maples, a single harvest decision could mean the difference between a forest where mature sugar maples vie for canopy dominance, and a forest where they are scarcely to be seen at all.

Becot, F., J. Kolodinsky, and D. Conner. 2015. The economic contribution of the Vermont maple industry. University of Vermont Center for Rural Studies. 53 pp.