By: Sanglin Lee and Alan Raflo.
Sanglin Lee was a Virginia Tech English Department intern at the Virginia Water Resources Research Center during the Fall 2011 semester.
The authors thank Carlos Evia of the Virginia Tech Department of English, Jennifer Gagnon of the Virginia Tech Department of Forest Resources and Environmental Conservation, and Kevin McGuire of the Virginia Water Resources Research Center for their assistance with this article.
Extra! Extra! Read All About It: Trees Affecting our Community and Beyond
"We All Benefit From Trees, But How Much?" Conservation Currents, Northern Virginia Soil and Water Conservation District, Summer 2011
"Nationwide Insurance Joins with American Forests to Support Poultry Farm Tree Plantings," Delmarva Poultry Industry News Release, 05/03/10
"Trees and Shrubs Help Filter the [Chesapeake] Bay," Newport News Daily Press, 04/02/09
"A Natural Solution: Trees Planted to Remove Pollution from Streams," Staunton News Leader, 11/6/08
"The Forgotten Forest Product: Water!," New York Times, 01/03/2003
Since at least the late 1800s, scientists and forest managers in the United States have recognized that forests have strong influences on water resources.1 Today the connections between forests and water are widely reported in the news media (as shown by the headlines above), promoted by natural resource agencies, and investigated by specialists such as hydrologists, dendrologists,2 foresters, forest biologists, and water managers. Tree structures and functions influence at least four major areas of water-resources concerns: water quality; aquatic habitat; water quantity; and the interactions among water, climate, and energy use. In this article, we seek to give readers a basic introduction to Virginia's trees and a foundation for assessing the interesting and complicated connections between water and trees.
Some Tree Basics
Trees, shrubs, and some vines are distinctive among plants in that they produce woody structures. According to the National Park Service and U.S. Department of the Interior, the difference between trees and shrubs is that trees usually grow over twenty feet tall and have trunks that are at least two inches in diameter and rise above four and a half feet above ground. Shrubs, on the other hand, are smaller than trees and start splitting into smaller stems near the ground. Under certain environmental conditions, however, some trees may grow more like shrubs (with multiple stems and lower height).
Along with other plants that reproduce by seeds (many plants reproduce by spores, not seeds), trees are classified into two large groups, based on whether or not they produce flowers. Angiosperms are flowering plants, with their seeds enclosed within fruits that develop from the flowers; oaks, hickories, and maples are all angiosperms. Gymnosperms, on the other hand, do not have flowers; their seeds are not enclosed in a fruit (the term means "naked seeds") but are typically borne on cones, so most gymnosperms can also be called conifers (cone-bearing trees); pines, spruces, and firs are gymnosperms and conifers.
Most deciduous trees (those that shed all their leaves in autumn) are flowering plants, and most evergreen trees (which do lose their leaves but not all of them at once in a given season) are gymnosperms; there are, of course, exceptions, and the photos below show two. According to Forest Biology Textbook by J. R. Seiler et al. "deciduous, flowering trees often are referred to as hardwoods and evergreen conifers as softwoods, because conifers' wood, with some exceptions, is typically lighter and softer than wood from deciduous trees." Finally, flowering trees are often called broadleaf trees, because their leaves are relatively wider than the needles found on many conifers. The following chart organizes the bold-faced terms for the two main tree types.
Angiosperms (flowers, with seeds within fruits)
Gymnosperms (no flowers, with seeds on cones)
Known as hardwoods
Known as softwoods
Needle-like or scale-like leaves
Familiar examples: hickories, maples, oaks
Familiar examples: firs, spruces, pines
Several hundred species of trees are found in North America. These trees are either native to this continent or naturalized, that is, originating in some other area but having become widespread and capable of reproducing outside of cultivation. Each species has a range, or the broad area in which temperature, rainfall, and other environmental conditions allow the species to grow and reproduce. Of course, planted individuals can be found far outside of a species' native range; for example, the Rocky Mountain native Blue Spruce is often planted in Virginia. Within their native ranges, certain tree species typically occur together in identifiable forest types. The National Wildlife Federation's (NWF) Field Guide to Trees, for example, lists 15 forest types that naturally occur in the United States (excluding Hawaii), ranging from the boreal forests of Canada and Alaska to the southeastern coastal plains forests of the Atlantic coast. The next section looks more closely at the kinds of forests and trees found in Virginia.
Trees and Forests in Virginia
The NWF Field Guide to Trees (p. 18) describes three main forest types found in Virginia. First is the eastern deciduous forest, characterized by high diversity of mostly deciduous species (perhaps 200 native species) with relatively few conifers. Second is the southern oak-hickory-pine forest, widely occurring in the Piedmont areas of southeastern states and differing from the eastern deciduous forest by having more types and more abundance of pines. Third is the southern coastal plains forest which, according to Kershner's National Federation Field Guide to Trees of North America, is characterized by pines in upland areas with deciduous trees along waterways and in other low, moist areas.
According to the 2010 State of the Forest report from the Virginia Department of Forestry (VDOF), in 2008 about 15.7 million acres of Virginia, or about 62 percent of the state, were classified as forest land, defined as "land at least 10 percent stocked by forest trees of any size, or formerly having such tree cover, and not currently developed for non-forest use" (p. 4). Of these forest-land acres, 78 percent (12 million acres) were hardwood or hardwood-pine forests (mostly oak-hickory forests), and 20 percent (three million acres) were pine forests (with over 50 percent of the pine acres in managed pine "plantation"). Bottomland hardwood forests occupied four percent of Virginia land in 2007, and deciduous forests dominated by maples, American Beech, and birches occupied two percent.
The annual VDOF report identifies the most common Virginia trees in two ways: by estimates of the number of individual trees and by estimates of the volume occupied (in cubic feet). The table below shows the top ten Virginia trees by number and by volume in 2010.
|Top Ten Virginia Trees by Number (2010)||Top Ten Virginia Trees by Volume (2010)|
|Loblolly Pine||Loblolly Pine|
|Virginia Pine||Northern Red Oak|
|American Holly||Virginia Pine|
|Chestnut Oak||Scarlet Oak|
|Flowering Dogwood||Black Oak|
Source: Virginia Department of Forestry,
2008, 2010 State of the Forest, p. 7.
VDOF's 2010 report noted that several native Virginia trees are suffering substantial decline due to insect infestations. Traditional pests such as Gypsy Moth and Southern Pine Beetle have been declining while new pests, such as the Emerald Ash Borers, invasive weeds, and Thousand Cankers Black Walnut Disease, have been on the rise. In general, invasive species remain the most significant threat to forest health, according to VDOF.
Water is an important factor in how trees respond to these and other insect pests. Effects on trees from insect pests and diseases can be more severe if trees are already stressed by inadequate water. In turn, the consequences of drought magnify the negative impacts on trees that are already suffering from insects or diseases. VDOF reported that the wave of Gypsy Moth outbreaks that occurred during the drought between 2005 and 2008 resulted in 114,000 acres of severe defoliation in 2008. The lack of water combined with high temperatures contributed to the death of many trees that were already stressed by insects. According to the VDOF, the Gypsy Moth numbers were much reduced after the excessively wet spring and summer of 2009, which allowed a naturally occurring virus to take over and subdue the moth population, including a steady decline in 2010 and 2011.
But, of course, water is important to trees in many ways, not just in response to insects and diseases. The next section looks at some basic aspects of water relations in trees.
Water's Influence on Trees
Like all living things (except viruses), trees are made up of cells that consist largely of water. According to Seiler et al.'s Forest Biology Textbook, water makes up as much as 90 percent of the leaves and other tree tissues during periods of growth. With water making up a large percentage of a tree, the availability of water is crucial to a tree's ability to grow as it affects cell division and stem elongation. Like other living things, water in tree cells provides a solvent (a substance in which other materials can dissolve) to contain and transport the many substances needed for biochemical reactions.3 Water is itself involved in many of these reactions, particularly in photosynthesis, the process whereby green plants convert light energy into carbon-based chemical energy (or "food"). When plants absorb carbon through leaves for photosynthesis, they also lose water at the same time. This tradeoff between carbon and water drives the plant's circulatory system and cools plant leaves through vaporization where heat energy is released to the environment. Water in trees and other plants (but not in animals) is also needed to maintain pressure in cells that supports leaves and other non-woody plant parts; without sufficient pressure from water, plant cells lose the structure needed to function properly. Wilting is a common response to inadequate water pressure in plant leaves or stems; the accompanying photo shows an example of wilting.
A tree's roots absorb water and the structures functioning in the tree combined with certain physical properties of water allow trees to get the water and dissolved substances that trees need. Water molecules, which tend to stick to other water molecules, are absorbed by the roots and pulled through thin tubes inside the tree into stems and leaves. This is known as capillary action. Water properties of tension and cohesion determine movement of water up trees. Water also influences cell division and stem elongation in trees.
As noted above, water, along with temperature and other environmental conditions, is a key factor in determining the range of tree species; that is, the broad geographic area where a tree species exists. A Sugar Maple, for example, could never survive in the southwestern U.S. deserts, where species of mesquite trees live. But water availability is also a key factor in determining tree distribution—that is, where within the range one actually finds populations of a given species. Areas in Virginia range from being consistently wet, such as wetlands found throughout the state but particularly along in Tidewater, to relatively dry south-facing mountain slopes in western Virginia. While trees are found in all of these conditions, some require really wet or really dry conditions in order to survive. Others have preferred conditions but can survive in a wide variety of circumstances. Following are some examples of how tree distribution is related to water condition, using various species from the VDOF Top Ten Virginia Trees and other trees from their related groups. The information and quotes in these examples are from Virginia Department of Forestry's Common Native Trees of Virginia Tree Identification Guide, and the Virginia Tech Department of Forest Resources and Environmental Conservation's "Tree Identification Factsheet," available at http://dendro.cnre.vt.edu/dendrology/data_results_with_common.cfm?state=unknown# as of 11/30/11. The scientific name of each tree is provided at the end of this article.
Virginia Pine can be found throughout the state in dry conditions, and definitely would not be found in wetlands. On the other hand, Pond Pine is normally found within a limited area in southeastern Virginia and can only grow in moist sites.
Silver Maple is commonly found in the Appalachian region and along the southern border of the state. Silver Maple grows better in wetter conditions, so native trees are most commonly found along stream banks, flood plains, and lake edges. Like the Silver Maple, Sugar Maple is also commonly found in the Ridge and Valley and Appalachian Plateau regions. Unlike the Silver Maple, it thrives in "moist but well-drained soils," which is why it would not typically be found on river bottoms. A third example, Red Maple is different from the other two in that it is naturally found across Virginia and can live in a wide variety of sites, from very dry to wet conditions. These three and other species of maple are found planted (not occurring naturally) in all parts of Virginia, including regions outside of their natural habitat.
Blackgum and Water Tupelo
Blackgum also known as Black Tupelo, is found throughout Virginia and can live in a wide range of conditions from very wet to relatively dry conditions. In contrast, Water Tupelo is found in a very limited area of the state, mostly on the coast of the southern border of the state. According to the Common Native Trees of Virginia Tree Identification Guide, its natural habitat is near water, such as in "deep river and coastal swamps" (p. 93).
Twenty-six species of oaks are native to Virginia. Several of these oaks are on the 2010 list of "Top Ten Virginia Trees by Volume." This diversity of species of oaks shows a variety of preferences and tolerances for water conditions among trees in the same genus. Post Oak, for example, can be found throughout most of the state, but needs "rocky and sandy ridges or dry woodlands" to thrive. In contrast, Water Oak is commonly found only in the southeast coastal areas of Virginia along waterways such as streams and swamps. Other oak species respond to water in more subtle ways, such as growing on various parts of a slope of a mountain. Northern Red Oak is found throughout the state but it thrives in "well-drained soils and fertile coves, and reaches best growth on north and east slopes—in cooler, moister areas." In contrast, Scarlet Oak would typically be found on drier, rockier parts of mountain slopes.
Trees Providing Direction
In recent years, Virginians who follow news about water, the environment, or their local community may have seen headlines highlighting trees and water in the news. Streamside hikers may have seen dozens or hundreds of plastic tubes staked in the ground, indicating newly planted trees intended to help improve stream water quality and habitat. Cities are encouraging planting of trees to help reduce stormwater runoff and the pollutants it can carry to waterways. Travelers booking flights online may have been asked if they wish to offset their carbon footprint by making a contribution to help plant trees that will absorb the greenhouse gas carbon dioxide and store it in tree structures.
In these and other ways, trees are being recognized as part of the solution for an array of issues connected to water. Virginia has a rich diversity of tree species that both are influenced by water conditions and in turn affect the quality and quantity of water around them. Increasing one's awareness of this resource and its water relationships will help provide direction for public and private decisions that will affect our landscapes, our communities, and ourselves.
Scientific Names of Trees Mentioned
Virginia Pine- Pinus virginiana
Pond Pine- Pinus serotina
Silver Maple- Acer saccharinum
Sugar Maple- Acer saccharum
Red Maple- Acer rubrum
Blackgum and Water Tupelo
Blackgum- Nyssa sylvatica
Water Tupelo- Nyssa aquatic
Post Oak- Quercus stellata
Water Oak- Quercus nigra
Northern Red Oak- Quercus ruba
Scarlet Oak- Quercus coccinea
1 J.D. Hewlett, Principles of Forest Hydrology (Athens: University of Georgia Press, 1969), pp. 4-5.
2 The science of hydrology seeks to understand, describe, and predict water movement over and below the land surface, in water bodies, and in the atmosphere-movement collectively referred to as the hydrologic cycle (or water cycle) (for more on the science of hydrology, please see the Dec. 1998 Water Central, p. 7. Dendrology is the scientific study of the identification, classification, distribution, and general growth habits of trees and other woody plants.
3 Water-based solutions are the transport medium both within cells and throughout a tree; rising sap in the spring, for example, brings nutrients dissolved in water from roots to newly growing cells in leaves and stems.
Catanzaro, P., D'Amato, A. November 2006. High-Grade Harvesting: understand the impacts, know your options. Cornell University. 09/12/2011. Available at http://na.fs.fed.us/pubs/silvics_manual/volume_2/liriodendron/tulipifera.htm as of 11/30/11.
Harlow, W.M., Harrar, E.S., and White, F.M. 1979. Textbook of Dendrology, Sixth Edition. New York: McGraw-Hill.
Hewlett, J.D. Principles of Forest Hydrology. Athens: University of Georgia Press. 1969.
Kershner, B., et al. 2008. National Wildlife Federation Field Guide to Trees of North America. New York: Sterling Publishing.
Seiler, J.R., Groninger, J.W., and Peterson, J.A. 2008. Forest Biology Textbook (compact disk). Blacksburg, Va.: Virginia Tech. 11/29/08. Available at www.fw.vt.edu/dendro/forestbiology/syllabus3.htm as of 11/30/11.
U.S. Fish and Wildlife Service. "Fire and Wildlife." 09/12/2011. Available at http://www.fws.gov/northeast/refuges/fire/firewildlife.html as of 11/30/11.
U.S National Park Service and U.S. Department of the Interior. "Trees and Shrubs." 11/16/2011. Available at http://www.nps.gov/noca/naturescience/treesandshrubs.htm as of 11/30/11.
Virginia Department of Forestry. 2010 State of the Forest. November 2011. 09/12/11. Available at www.dof.virginia.gov as of 11/30/11.
Virginia Department of Forestry. Common Native Trees of Virginia. 2007.
Virginia Tech Department of Forest Resources and Environmental Conservation. Dendrology at Virginia Tech. April 2010. 10/26/11. Available at http://dendro.cnre.vt.edu/dendrology/data_results_with_common.cfm?state=unknown# as of 11/30/11.
United States Department of Agriculture. "Interpreting Wetland Indicator Status." 10/19/11. Available at http://plants.usda.gov/wetinfo.html as of 11/30/11.
The National Tree Benefit Calculator is an interesting resource that can be used for figuring out the impact and contribution the trees in your neighborhood are making regarding property value, energy, stormwater, and carbon and air quality. This Benefit Calculator can be found athttp://www.treebenefits.com/calculator/index.cfmas of 11/30/11.
It may also be useful to refer to the Virginia Department of Forestry's guide to tree identification, http://www.dof.virginia.gov/trees/index.shtmlas of 11/30/11, in order to help identify the trees in your area.
United States Department of Agriculture's Interpreting Wetland Indicator Status provides a list of trees in the US and indicates their levels of need and tolerance for water. This list is available at http://plants.usda.gov/wetinfo.htmlas of 11/30/11.Top of Page