The study of cyclic changes in the natural world based largely on climate’s effects on plants and animals is called “phenology.” In northern Michigan, many of us welcome the cyclic change we observe this time of year as the days get longer and warmer. And perhaps nothing is more a harbinger of spring than sap flow in our sugar and red maples, when cold nights are followed by warm days. But how does the process of sap flow work and what can a private landowner do to promote vibrant trees from which sap can be collected and syrup made? “Physiology” refers to the mechanical and chemical workings of plants and animals. Sap flow in maple trees is a physiological process driven by phenological change. To understand how and why sap flows in a tree one must understand how a tree produces and stores its energy and how it transports water and other materials from its roots to its leaves. Most green plants, including trees, produce energy through “photosynthesis.” Plants are called “phototrophs” because they use light energy (photons) to combine the gas carbon dioxide and water to produce sugars. In this process, oxygen is released. Without green plants, animals would not have oxygen they need to live. And without the sun, plants would not be able to produce energy. While all green plants need sunlight, some tree species need more sunlight than others. We refer to tree species that require less sunlight as “shade tolerant” species, and those requiring more sunlight as “shade intolerant” species. All native maples, as well as American beech, ironwood, and a few other of our deciduous tree species, are shade tolerants. These species are able to produce energy with relatively little sunlight. This energy (in the form of sugars) produced by photosynthesis is used to meet the demands of tree growth and maintenance. When excess sugars are produced, they are stored in the tree roots. Conversely, other tree species, such as aspen and oaks, are shade intolerant and need more sunlight. These tree species rarely produce extra sugars and do not typically store much energy. Tree sap is primarily a mix of water and sugars. Tree sap move in tubes in a manner slightly similar to the way blood moves in our arteries and veins. The tubes in which maple sap typically flows is called “xylem” tissue. Dead xylem is the “heartwood” of a tree and living xylem is the “sapwood”. Most of the material used in making lumber from a tree is xylem tissue. In other tree species, sap can also flow in the other tubes (phloem). Syrup made from other tree species has a distinctive taste quite different than maple syrup. During the spring of each year, sap flows from the roots, where sugars are stored in the winter, to the crown of the tree. Sap flows because of the pressure in the roots is greater than the pressure in the crown. What can a landowner do to promote vigorous maple trees form which syrup can be made? Tree vigor is the primary consideration for syrup production. Vigorous trees with large, healthy crowns tend to make and store more energy and produce more sap. Site (soil type) and location, as well as tree genetics, are factors that influence tree vigor. On sites with well drained, upland soils and plenty of sun (often south facing slopes), maple trees can thrive and be vigorous, especially if competition with other trees is reduced. When provided more sunlight, a maple tree will respond by growing more leaves and a bigger crown. This is the type of tree that if not impacted by disease or physical harm can produce sap for decades. Thinning a forest to produce trees of 10-25 inches in diameter separated by 20-25 feet is a good starting point. Removing diseased trees or trees of low vigor can promote more diameter growth on the retained trees. Do not, however, reduce overall stand tree diversity. Forests with more tree diversity tend to be healthier overall, and a vibrant maple stand (sugarbush) produces more maple sap. To capture sap, the general recommendation is that trees less than 20 inches in diameter should have one tap, while trees over 25 inches may have up to 3 taps in a given year. Just like all types of forest management, managing a sugarbush depends on many factors and involves a great deal of art guided by science. In the end, an understanding of how a tree, and a forest, forms and functions provides a solid foundation for planning and management. Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (firstname.lastname@example.org) or phone (989.356.3596 x102)
Long Term Considerations for Managing a Sugarbush
How to Manage a Woodlot for Sugar Maple Syrup Production...Fewer Trees, Bigger Crowns
Michigan forests provide innumerable benefits. To start, forests help maintain biodiversity and clean air and water. They are also used for hiking, hunting, camping, wildlife observation, and collecting berries and mushrooms. Our forests also feed local economies. How then do we sustain forests ecologically in our quickly changing world? All forests are dynamic; they change in composition and structure over time. Different forests naturally change in different ways by different means over different time frames. As defined before, a “disturbance” is anything that impacts the amount of living material (biomass) in a forest. Jack pine forests, for instance, are as fire-dependent as any forest type in North America. Fire is an essential disturbance for naturally regenerating these forests and providing complexity. Fires shape these forests in dramatic ways every hundred years or so. Fires kill most mature trees and provide for biodiversity. Fires also open up cones so seeds are released. These seeds germinate on the forest floor on which the leaf litter is removed by fire. Not surprisingly, a jack pine rarely lives much more than a hundred years. It is adapted to fairly frequent disturbance. Conversely, our forests of American beech, sugar maple, and eastern hemlock are more stable; natural change occurs in these forests, but less dramatically and over longer time frames. Not surprisingly, trees in this forest type live many hundreds of years. They also commonly grow quite large and become more vulnerable to the effects of wind. “Windthrow” is a natural disturbance and effects not only a single tree, but other trees knocked down as a large tree falls to the forest floor. The downed log provides habitat for many organisms, including male ruffed grouse that “drum” on logs. The “canopy gap” provides an irregular patch of sunlight in the forest. In gaps, saplings and seedlings take advantage of pulses of sunlight and grow more rapidly. Forests regulated by windthrow are structurally complex. While forests have always undergone change of different types and over different time frames, science suggests that forest disturbances are changing in type, return interval, and severity (degree of impact). For instance, many non-native organisms are now competing with native species for space and resources in our forests. Some of these exotic species are causing tree mortality. Oak wilt, Emerald ash borer, beech bark disease, and hemlock wooly adelgid threaten the sustainability of our forests. These new agents of disturbance, and potential changes to fire and wind patterns, require us to reevaluate forest management. “Resistance” is the ability of a forest to remain unchanged when challenged by disturbances. “Resilience” is the ability of a forest to reorganize itself after a disturbance so that it still functions. These two terms can be used as a basis for planning and can guide our forest management actions. In other words, a landowner could have these as specific goals in a forest management plan and can use these to help devise forest treatments. To promote resilience and resistance, early evidence suggests that forest diversity is important. Forest diversity is a product of site conditions of soil and climate, as well as past management activities. To maintain forest diversity, landowners should focus on what will be left behind after logging is done. In other words, think about “desired future condition.” This can include leaving less common tree species in the forest after a treatment. In many of our forests now dominated by oaks, there were once red pine and eastern white pine. If scattered pine trees are still found, these should be retained to provide seed and increase the pine component of the future forest. In cases where no pines or other conifers are found, underplanting seedlings in gaps created by logging would increase diversity. In a similar way, many pine forests historically managed by fire had scattered oaks and aspen. Future management should aim to keep these forests mixed and not solely one species (called a monoculture). When a future disturbance occurs that is not planned for (like a new exotic species), these forests would be provided a greater likelihood of resilience and resistance. A repeated principle in this column has been the importance of forest diversity. While previously discussed in the context of wildlife, tree species diversity is also an important factor in having resilient and resistant forests for the future. Diverse forests theoretically have more potential management options. In the end, however, uncertainty abounds. Understanding what our forests once were like in terms of composition and structure and how they once functioned provides us with ways to understand current conditions and plan for an uncertain future. Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (email@example.com) or phone (989.356.3596 x102).
The first wildlife management text in the United States was Aldo Leopold’s Game Management (1933). Leopold saw that game populations responded positively to habitat management. He also proposed that species-specific wildlife habitat consists of food, water, and cover. Nearly 100 years later, we know that wildlife populations are impacted by many other things. Nonetheless, food, water, cover, and space (as a newly added fourth variable) still structure how we think about wildlife habitat for individual species. How does space come into play in wildlife habitat? For some wildlife species, including many Neotropical migrant songbirds, space is used differently during the breeding season, during migration, and on the wintering grounds. Throughout migration routes, space provides birds the ability to restore energy reserves by resting and foraging for food. Birds that struggle to find suitable space during migration from their wintering grounds to their breeding grounds may arrive in poorer physical shape, potentially reducing their ability to reproduce successfully. Space is also critical for wildlife species with exceptionally large home ranges or “area-sensitive species”. Area-sensitive species not only require a certain condition of vegetation structure (vegetation arrangement) and composition (mix of plant species), they also require large blocks of habitat (space). In northern Michigan, the upland sandpiper and sharp-tailed grouse are bird species of fire-dependent, pine and oak barrens, as well as pasture and hayfield complexes. Both species require hundreds or thousands of acres of unfragmented open habitat. Similarly, black-throated blue warblers are only found in large tracts of mature, mixed forests of northern Michigan. A forest must be many hundreds of acres or more to be occupied by this migratory bird species. Of the four variables of wildlife habitat, space is perhaps the most challenging to provide in a world increasingly dominated by human land uses. Not surprisingly, many species requiring large spaces are of conservation concern. [Authors note: the importance of “landscape context” previously covered applies here.] How can a landowner provide wildlife habitat, especially when space is limited? Let’s think specifically about our winter wildlife. While wildlife diversity in northern Michigan is reduced during the winter months due to migratory birds wintering elsewhere, it is also during the winter when some species move into our area. Snowy owls, pine grosbeaks, red crossbills, common redpolls, pine siskins, and American tree sparrows are encountered more often during our winter as birds from farther north move south. For many of these and other species, cover and food are very important during the winter. Fortunately, cover and food can be provided in a number of ways, regardless of whether a property is in the city or in the country. For example, landowners can provide cover by having patches of their land devoted to native coniferous trees. When a property has coniferous trees of different ages, sizes, or species, a “layering” of vertical structure results. In the winter, these areas provide some refuge from the wind and precipitation, while still retaining heat. One of the reasons white-tailed deer seek swamps of northern white-cedar, black spruce, and tamarack is the “thermal cover” and reduced snow depth these areas provide. Private landowners can also provide cover by retaining large live or dead trees with flaking bark or cavities. Wildlife such as southern flying squirrels and white-breasted nuthatches find refuge from harsh winter conditions in these spaces. And dead material, like a pile of decomposing leaves and branches on the forest floor or in the corner of a backyard, can provide many small mammals, such as meadow voles and deer mice, with cover. Some reptiles and amphibians may use these warm, quiet places to hibernate. Mature coniferous trees, such as red pine and eastern white pine, also provide energy-rich seeds. Red crossbills evolved with beaks that are offset from one another. These beaks allow the birds to pry open pine cones. Birds then extract seeds with their strong tongues. Some deciduous trees also provide winter food. Young aspen is browsed readily by white-tailed deer, elk, and snowshoe hare; ruffed grouse eat the buds of more mature trees. While not a favorite for those interested in timber products, ironwood is favored by many birds in the winter because it retains seeds throughout the year. Like all professional fields grounded in science, our knowledge of forest and wildlife ecology is dynamic. As our forests change due to succession, management activities, or other reasons out of our immediate control, wildlife use can also change. By studying and understanding how the natural world operates, forest managers can devise wildlife habitat management activities best suited for a forest and the suite of native wildlife species adapted to its conditions. Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (firstname.lastname@example.org) or phone (989.356.3596 x102).
Dr. Greg Corace
Want to hear about what is new in the science world? Maybe get more information on the birds around us? Or maybe you want to keep up to date on what is happening in our current environment and with the natural resources we love. Check out some interesting articles shared by our Forester, Dr. Greg Corace.