Photo by Bob M. Montgomery Images, www.bmmimages.com.
2012 was an unseasonably warm year for sugaring. According to the National Climatic Data Center, March - typically the principal month for sap flow - was the warmest on record for almost the entire U.S. maple region. While early April featured a return to more seasonable weather, the March warm spell had already ended the season for most producers. Old-timers may recall other open winters similar to 2012, as well as previous warm spells that interrupted the sugaring season, but there is increasing evidence that we are experiencing not just random variation in the weather, but real change in the climate of the Northeast. It may be many years before another sugaring season like 2012 occurs, but sugar makers need to be prepared for change, both in the way we manage our forests, and how we manage our sap and syrup production. This article will describe changes that may occur in our resource, the maple forest; a future article will describe possible ways to adapt our sugaring technology to warmer seasons.
According to many climate models, warming of the Northeast climate could affect forests in multiple ways, and it may already be doing so. While dire predictions that in 100 years our maple forests will be entirely replaced by a southern oak-hickory forest or an open savanna are easy to dismiss, more subtle changes are likely to occur much sooner. Among these are an increase in the length of the growing season, less severe winter temperatures, heat stress on some plant species in the summer, and periods of drought interrupted by extreme wind and rain events.
Related to these changes are possible current and future expansions of many "nuisance" species that can affect our forests. Nuisance species include both native insects and diseases, such as Armillaria root rot, Nectria canker and forest tent caterpillar, as well as many introduced species, such as sudden oak death, hemlock wooly adelgid and Asian longhorned beetle. Invasive plants, such as Japanese barberry, Norway maple and Oriental bittersweet are by definition introduced species, although many are so widespread that it is hard to imagine them absent from our landscape. Some nuisance species can withstand severe cold temperatures, others cannot, but are predicted to expand their ranges with an increase in the minimum winter temperatures. Among the latter are hemlock wooly adelgid, which has devastated Appalachian hemlock stands and has recently been found as far north as southern Vermont, as well as balsam wooly adelgid, which recently reappeared, killing balsam fir in the Northeast, following decades of low populations. Winter moth, a defoliator of hardwoods, has recently expanded its range and is a serious pest in Massachusetts - winter moth caterpillars feed voraciously on young leaves of many trees species, including maple, and may be able to move northward with milder temperatures. Scale insects that are precursors to beech bark disease may expand with less extreme winter temperatures; this can affect sugar bush management because the death of mature beech trees often leads to dense thickets of saplings from root sprouts.
Predicting exactly how climate change will affect populations of insects is difficult because there are many factors involved. For example, less severe winter temperatures may be accompanied by reduction in the protective snow cover that allows many dormant insects to withstand the cold, and this could affect nuisance insects and helpful predator insects differently.
Some nuisance species gain a foothold only in maple trees that are stressed. Among these is the sugar maple borer, which preferentially attacks less vigorous trees, creating galleries in maple trunks that make them susceptible to wind damage, and Armillaria root rot, which is present throughout northeast forest but kills only weakened trees. Maples that are stressed by high annual temperatures, severe storms and drought, as well as by insect defoliation, are much more likely to become infected, and higher winter temperatures will allow decay pathogens like Armillaria to function for more months in the year.
Introduced invasive plants affect maple forests in many ways, including the production of a dense understory that shades out native tree seedlings, alterations of soil chemistry to make it less hospitable for the growth of sugar maple roots, and increased deer browsing on native hardwoods due to the animals' preference for native over the nonnative plants. Many of these invasive plants, including barberry, honeysuckle and common buckthorn, have arrived here from latitudes similar to ours and can withstand severe winter temperatures.
One particularly noisome pest that is likely to expand its range with milder winters is Oriental bittersweet. This vine, already a serious problem in the southern portion of the maple region, not only shades out young maples with its thick foliage, but also girdles trunks and climbs to the tops of trees, where it promotes breakage in wind and ice storms. Oriental bittersweet, now present in warmer parts of Vermont, is predicted to aggressively move north. Like several other forest invasives, it can survive in the shade of a mature forest, but is most worrisome when a disturbance creates an opening in the woods. Under these conditions the rapid growth rate of alien plants often far exceeds that of native species. Disturbance caused by the death of forest species due to insects, diseases and weather events will create openings that are ripe for populations of invasive plants to explode. To prevent this occurrence, managers of sugar bushes should always strive to identify and eliminate small populations of invasives before they become too large to control.
I recently spoke with Nancy Patch, county forester for Franklin County, the number one syrup-producing county in Vermont. Patch, who works for the Vermont Department of Forest, Parks and Recreation, is concerned with the way many sugar bushes are currently being managed in light of predictions for future climate-related stress. She argues that sugar maple monocultures, which are being created in many areas in an attempt to maximize production, are ripe for collapse. Our native forests were complex, with multiple species and multiple age and height classes, and these forests were much more resilient than many of today's sugar bushes. She is a strong proponent of diversity in the sugar bush, in part because the presence of a mix of tree species has been shown to reduce the populations of destructive insects and increase the presence of helpful insect predators. She says that trees like oak and cherry, which are well-adapted for warmer temperatures, should be left in the maple forest; both are valuable timber species, and trees like oak can withstand future drought while shading drought-sensitive species like sugar maple. Some tree species such as yellow birch will be stressed by a changing climate, and ash will be eventually killed off by the emerald ash borer; managers should include them in the current species mix but not build future plans for diversity around these trees. Red maple, which has great genetic diversity and is a natural survivor, should be part of the diverse forest, but sugar makers should promote strong single-stemmed trees rather than multistemmed clusters. Some stands with wet soils should not be converted to sugar bushes, as storms will blow over these shallow-rooted trees. Thinning of maple stands should be "gentle," as too much opening of the canopy coupled with drought will promote soil drying that will stress and kill sugar maples and make the forest more susceptible to flash floods. Many birds that eat insect pests prefer to nest in the zero to 6-foot-high level of the forest, and this layer should remain healthy and complex in the sugar bush rather than chopped out for more convenient tapping.
One more species that is likely to expand its range in our maple forests with warming temperatures is the black-legged, or deer, tick, the carrier of Lyme disease. While I have never found ticks where I work at the University of Vermont Proctor Maple Research Center, in the past few months while working in a sugar bush closer to Lake Champlain, I encountered deer ticks in November and during the sugaring season in March. In a 2012 survey of 200 sugar makers in Vermont and New Hampshire, 55 percent of the producers said they had ticks in the sugar bush, another 14 percent said they might be present, and several producers claimed that their tick population had arrived in recent years. Only two maple counties in northernmost Vermont seemed to be free from deer ticks. These data coincide with research that shows that less severe winter temperatures promote the expansion of the tick population. Additionally, an abundance of invasive plants with their dense cover promotes the survival of ticks and the small mammals, such as mice, that are necessary for ticks to complete their life cycle. As tick bites are not harmless, due to the threat of Lyme disease, and many people cannot avoid working in infested areas, adequate protection and self-inspection for ticks should become a standard policy for sugar makers in much of the maple region.
Whatever sugar makers feel about the root cause of global warming and new methods for the reduction of atmospheric carbon dioxide, a warming climate is likely to have an increasing effect on maple production and our resource, the maple forest. There will certainly be future years where climatic variability leads to cold, old-fashioned winters and springs, but accumulating data indicate that this kind of season may become rare. Changes and adaptations for the timing of tapping, the management of warm sap, and the increased production of darker syrup may accompany climate change for sugar makers, but most important for the sustainability of this industry is the continued vitality of our treasured maple forests.
The author is maple specialist with University of Vermont Extension and Proctor Maple Research Center in Underhill Center, Vt.