Ecology for Gardeners
Our gardens, as Rachel Carson showed us in her book, Silent Spring, are inextricably linked with the larger landscape. It's our task as stewards of the Earth to make sure that the ecological role our gardens play is a positive one.
Introduction
Ecology: The science of the relationships among living organisms and between organisms and their environments.
Ecosystem: A functioning unit of nature that combines biological communities and the environments with which they interact. Ecosystems vary greatly in size and characteristics.
The science of ecology had been developing for over a century, yet most people never heard the word until the 1960s and the publication of Rachel Carson's book, Silent Spring, which documented how chemical pesticides were spreading insidiously through the food chain and threatening the survival of many bird species. The book brought into sharp focus the ease with which the natural balance of plants, animals, and the environment that comprise life as we know it on this planet can be tipped. From the oil crises of the 1970's, to the nuclear fallout from the Chernobyl reactor in the 1980's to the more recent evidence of ozone depletion and climate change on a global scale, we continue to be reminded of the need to protect the biosphere.
Gardeners are mainly concerned with plants, specifically the ones we grow in our gardens. But our gardens, as Rachel Carson showed us, are inextricably linked with the larger landscape. As cities and suburbs spread, gardens and other human-influenced environments predominate. It is our task as stewards of the Earth to assure that the role our gardens play is a positive one. This requires an understanding of ecological relationships.
The first step in understanding ecological relationships in the garden is understanding those in plant communities in the wild. Have you ever wondered why oaks grow in some woods and not others? Why the East is covered largely with deciduous trees, while in the West conifers dominate? For decades, scientists have been studying vegetation associations, the distinctive mixes of species that predominate in different areas of different regions. Over the past century or so, ecologists have learned that such natural systems are defined by two major kinds of order: structural and functional. A terrestrial ecosystem's structure, or form, depends primarily on its vegetation: the structure of a grassland is very different from that of a forest, for example, because of the form of the predominant plant life. By the same token, the functions of these two plant communities - how they respond to fire, wind damage, and other types of disturbance, how they use and recycle water, nutrients, organic matter, and so on - also differ because the physical environment, the soils, climate, and other conditions that give rise to them, are different.
What follows is a basic introduction to the structure and function of the major vegetation associations of the Southeast, particularly North Carolina, and how gardeners in these various regions can use them as models for transforming the garden landscapes that sustain us, without destroying the creatures and natural communities with whom we share the Earth.
Eastern Deciduous Forest
This vast province, which stretches from the Atlantic Ocean to the tallgrass prairies of the Midwest, encompasses many associations. Eastern forest associations are determined by climate, soils, and moisture. The common denominator is the dominance of deciduous canopy species. In rare instances, there are codominants that are coniferous, usually pines or hemlock. Meadows occur as scattered breaks in the forest where the soil is too wet or dry to support trees. Old-field meadows occur on abandoned agricultural land.
Maple-Basswood/Beech-Maple Forest
American beech, Fagus grandifolia
Basswood, Tilia americana
Canada hemlock, Tsuga canadensis
Northern red oak, Quercus rubra
Sugar maple, Acer saccharum
White oak, Quercus alba
Mixed Mesophytic/Western Mesophytic Forest
American Beech, Fag's grandifolia
Basswood, Tilia americana
Black oak, Quercus velutina
Hickories, Carya species
Sugar maple, Acer saccharum
Tulip tree, Liriodendron tulipifera
White oak, Quercus alba
Oak/Hickory Forest including the former Oak/Chestnut Forest)
American chestnut, Castanea dentata (devastated by chestnut blight)
Black oak, Quercus velutina
Bur oak, Quercus macrocarpa
Yellow chestnut oak, Quercus muehlenbergii Hickories, Carya species
Northern red oak, Quercus borealis
Northern pin oak, Quercus ellipsoidalis
White oak, Quercus alba
Floodplain Forest
Cottonwood, Populus deltoides
Green ash, Fraxinus pennsylvanica
River birch, Betula nigra
Silver maple, Acer saccharinum
Sycamore, Platanus occidentalis
Coastal Plain
The Coastal Plain of the East and Gulf coasts is a vast area with a variety of climates, soils, and moisture regimes. On rich soils, mesic forests of oaks and other hardwoods dominate, similar to forests in the Eastern Deciduous Forest province. In sandy and waterlogged soils, savannahs are found, with open pine woodlands with well-defined shrub or grassy ground layers. Bottomlands are dominated by deciduous trees interspersed with shrubs, wetland sedges, and forbs.
Northern Pine Barrens
Pitch pine, Pinus rigida
Scrub oak, Quercus ilicifolia
Shortleaf pine, Pinus echinata
Upland Hardwood Forest
American beech, Fagus grandifolia
Bullbay magnolia, Magnolia grandiflora
Live oak, Quercus virginiana
Loblolly pine, Pinus taeda
Red maple, Acer rubrum
Southern red oak, Quercus falcata
Sweet gum, Liquidambar styraciflua
Water oak, Quercus nigra
White oak, Quercus alba
Xeric Pine Forest
Blackjack oak, Quercus marilandica
Bluejack oak, Quercus incana
Slash pine, Pinus elliottii
Longleaf pine, Pinus palustris
Turkey oak, Quercus laevis
Wiregrass, Aristida stricta
Mesic Pine Forest
Loblolly pine, Pinus taeda
Longleaf pine, Pinus palustris
Saw palmetto, Serenoa repens
Scrub oak, Quercus inopina
Slash pine, Pinus elliottii
Savannah and Pocosin
Loblolly bay, Gordonia lasianthus
Longleaf pine, Pinus palustris
Lyonia, Lyonia lucida
Pond pine, Pinus serotina
Titi, Cyrilla racemiflora
Wax myrtle, Myrica cerifera
Zenobia, Zenobia pulverulenta
Bottomland Forest
Atlantic white cedar, Chamaecyparis thyoides
Bald cypress, Taxodium distichum
Laurel oak, Quercus laurifolia
Overcup oak, Quercus lyrata
Pond cypress, Taxodium ascendens
Red bay, Persea borbonia
Red maple, Acer rubrum
Swamp tupelo, Nyssa sylvatica var. biflora
Tupelo, Nyssa aquatica
Water hickory, Carya aquatica
Maritime Communities
Beach grass, Ammophila breviligulata
Groundsel bush, Baccharis halimifolia
Junipers, Juniperus horizontalis and Juniperus virginiana
Live oak, Quercus virginiana
Poison ivy, Toxicodendron radicans
Red bay, Persea borbonia
Sea oats, Uniola paniculata
Wax myrtle, Myrica cerifera
Plant Community Ecology Basics
Plant community ecology is the science that explores the processes that form and influence plant communities and the patterns of distribution within them. For example, plant communities are constantly changing in a process often called succession. As young grasslands and forests comprising a certain mix of species mature, new assemblages of species take over, until storms, fires, diseases, or insects disturb them and the process begins anew. Healthy ecosystems are also characterized by biodiversity, a mixture of many different individuals of many different species. In any given plant community, there are dominant, subdominant, and subordinate species. Dominance refers to the one or more species that are most essential to the life and character of the plant community.
Gardens are plant communities, too. Successful gardening requires an understanding of the ecological processes at work on cultivated land. Our models for understanding these processes are local plant communities. Of course, structure and change in the garden are orchestrated by the gardener, not the forces of nature alone. The closer a gardener works with nature, however, the less work and the more successful the garden will be.
The Physical Structure of Plant Communities
Every native plant community has a recognizable, and somewhat predictable, structure based on the dominant and subordinant vegetation. A forest has a towering canopy of trees that influence what can and cannot grow beneath them. A grassland has a similar structure, but the plants are herbaceous instead of woody, and this vertical structure is re-created above ground each growing season, not over decades and centuries as in a forest.
Plants within a given community also create patterns of distribution on the landscape as a result of environmental factors such as soil, moisture, and light. Different species thrive with different amounts of moisture, for example. In a prairie, shrubs will grow in the wettest areas, grasses and forbs on the slightly higher and drier land. As a result of these factors, plant communities have recognizable vertical and horizontal structure.
Ecological gardeners use the structure of the native plant community as the basis for structuring their gardens. In the eastern forest region, for example, canopy trees lend a grand vertical scale to the garden, creating a cathedral-like enclosure, while understory trees add a more intimate, human-scale "ceiling." Shrubs can become "walls" that divide spaces horizontally and create privacy. The ground layer is where gardeners can create a tapestry of wildflowers, ferns, and grasses in beds and borders. Ecological gardeners know that structuring a garden like the native plant community also helps support birds and butterflies and other wildlife by offering them an array of spaces for feeding, breeding, resting, and nesting.
Reestablishing Broken Connections
Vertical Structure
All plant communities have a vertical structure based on the size and growth pattern of the dominant species. This pattern is called vertical stratification, or vertical layering. Vertical structure is most obvious in a forest. The tallest layer is called the canopy. It is composed of mature trees that cover the entire forest. The tallest canopy trees may be one hundred feet in height or more. The lowest canopy trees grow to about 30 feet. The canopy is interwoven, forming a fairly continuous ceiling over the entire forest. The canopy sets the stage for everything that happens in the layers below it.
The next layer down is called the understory. This layer is composed of saplings of canopy tree species as well as smaller flowering trees such as dogwoods, redbud, shadblow, ironwood, and hop hornbeam in the eastern United States. The understory extends from 30 to 12 feet.
The shrub layer is the lowest layer of woody vegetation. It occupies the area between 12 and 3 feet above the ground. Shrubs grow in patches where light and space are sufficient. A variety of forest birds, including vireos and some warblers, use the shrub layer for foraging and nesting. Many shrubs produce edible fruits that enable birds to lay on fat reserves in preparation for migration.
The lowest above-ground layer of the forest, below 3 feet, is called the ground layer. Here, wildflowers, ferns, grasses, and sedges grow in often-spectacular assemblages. Plants in the ground layer also partition their environment vertically. The spring ephemerals bloom first, typically raising their foliage only a few inches above the leaf litter. As they are going dormant the taller ferns, trilliums, and other herbs overtop them.
The age of a plant community affects its structure. Young forests have a well-defined shrub layer and understory. The canopy becomes more distinct as the forest ages. Different forests have different structures. Deciduous forests have the most elaborate structure, as described above. By contrast, coniferous forests, with their dense stands of tall and narrow trees, typically have very little understory, but can have a dense shrub layer and ground layer of herbs and mosses. Pine forests have the most open canopies of the coniferous forests, with scattered understory trees and a well-defined shrub layer. Ground-layer species are scattered in the sunny openings. Oak woodlands have a structure similar to that of the pine woods, except the trees are smaller in stature and the canopies are wider. The ground layer of grasses and annual wildflowers is exceptional during the rainy season.
Shrub communities have mixed layers of different-sized shrubs, with a ground layer of herbs, grasses, and sedges. In communities dominated by herbaceous plants, the vertical structure is no less distinct. The plants resprout from their roots each year. The earliest plants to emerge in the spring are low to the ground. Each successive emerging plant overtops the next, culminating with the tallest grasses and late-blooming composites that end the growing season.
Reestablishing Broken Connections
Horizontal structure is important to the long-term health of ecosystems. As we have parceled, subdivided, and cleared the land, we have interrupted or destroyed the horizontal connectivity of plant communities. When we do this, we cut off vital links used by wildlife and plants to move freely across the landscape. In effect, we create islands or pockets, isolated fragments of vegetation surrounded by cities, suburbs, and farm fields. The health of the plant community, both locally and globally, depends on the connections we have severed. In isolated pockets, plants and animals interbreed, reducing genetic diversity and endangering their survival over the long haul. Isolated patches are also more vulnerable to destruction by pests and diseases. Once plants are eliminated, the island cannot be re-colonized by many species, and the diversity of plant life is reduced forever. As gardeners, we have the ability to repair these broken connections. Our gardens, one by one, can re-establish links to parks, nature preserves and other green spaces, and restore the health of the environment.
Horizontal Structure
Light, moisture, slope, and soil have a direct effect on where a plant or group of plants grow in a forest, grassland, wetland, or other plant community. Plants form horizontal patterns of distribution in response to these environmental and edaphic, or soil-related, factors, better known to gardeners as microclimates. The resulting diversity of associations and species across the landscape is called horizontal heterogeneity.
In deciduous forests, oaks are found on the drier sites such as sunny slopes, while maple and basswood grow in the moister soil on east and north slopes. Individual trees are spaced according to their canopy size and shape. In marshes, sedges dominate the wettest areas, shrubs the intermediate regions, and wet meadow or woodland plants the upland zones.
Ecotones
Ecotones are the transitions between two plant communities. Because they include species from both communities and often their own unique species as well, ecotones are usually the most diverse ecosystems. In the Midwest, the eastern deciduous forest grades into the prairie in an ecotone called oak savannah. In the West, conifer forests grade into pinyon, juniper, or oak woodlands or into shrubby grassland ecotones. Ecotones also occur at a smaller scale, where wind throw has created an opening in the forest, for example, or at the edges of small tracts of woods. Ecotones support not only a variety of plants but also generalist animals such as skunks, deer, robins, and jays.
Today, the most common ecotones occur on abandoned agricultural land. These fields are first overtaken by pioneer species, generally a mixture of native and naturalized annuals that colonize open or disturbed ground. As the soil stabilizes and grows richer, an increasing variety of plants parades across the landscape, depending upon soil, moisture, and exposure. In forested regions, canopy trees ultimately become dominant. This new edge of grasses, shrubs, and young trees becomes an ecotone between open field (meadow) and forest.
The Biological Structure of Plant Communities
The biological structure of plant communities gives them the character we observe in nature. A deciduous forest dominated by tall, slender oaks looks very different from one composed of wide-spreading beech trees and rich evergreen hemlocks. A prairie dominated by grasses is altogether different from a forest of stately trees. Within different plant communities a vast diversity of species is found. In a tallgrass prairie dominated by three grass species, you may find more than nine different grasses and up to three hundred different forbs. A mixed mesophytic forest may have over thirty different tree species, even though only two or three are codominant. Ecological health depends on diversity. Plant communities are naturally diverse, but our gardens are comparatively barren. A typical garden has one grass, bluegrass turf, a dozen perennial plants, three different shrubs, and one or two different trees. If we are to make our gardens as attractive to wildlife as they are to us, we must understand biological structure and put that knowledge to work. We must learn to appreciate both biological and visual complexity.
Species Dominance
Dominance refers to the influence of species that contribute the most cover and/or biomass (total mass or weight) to a plant community. In a forest, the dominant species is the tree that contributes the most cover to the canopy. In a prairie it is a grass or a forb. If more than one species make up the dominant cover, then individuals in the suite of species are called codominants. Dominant species are often called visual essence species, for they are the ones that give a plant community its unique look.
Dominance is determined by density in a plant community - that is, the total number of stems or individual plants within a given area. Ecologists use precise sampling techniques to determine dominance. One common technique is the use of quadrats, or plots of land with fixed dimensions. Within each quadrat, the trees, saplings, shrubs, and herbs are counted and measured. In forests, relative density, relative cover, and relative frequency are weighted to determine dominance. In herbaceous communities, dominance is determined by relative cover and frequency.
The Benefits of Biodiversity
Do you ever wonder why you don't see as many birds and butterflies as you used to? You need look only as far as your garden for the answer to this question. The typical North American garden consists of bluegrass or some other turf, a dozen annuals and perennials, and a few shrubs and shade trees. If you are a bird looking for food, visiting the typical garden is like going to a restaurant where there is only one dish on the menu. Nature offers a smorgasbord to animals. The diversity of ecosystems and species within them offers a complete menu for a rich variety of creatures.
Our gardens can do a lot better. Plant thirty or even fifty perennials instead of a dozen. Use many different fruiting shrubs and trees. Choose half a dozen shade trees. Not only will this diversity help wildlife, it will make your property more interesting. A hidden benefit of diversity is resistance to outbreaks of pests and diseases. It is harder for an epidemic to spread if an infected plant is surrounded by other species that are not susceptible. It is also more likely that some of the plants will be resistant if you have many individuals with diverse genetic makeups, not just a group of the same cultivars or clones. Instead of choosing several plants of Aster novae angliae 'Purple Dome', a cultivar of New England aster with a compact habit, for example, opt for a genetically diverse collection of New England aster plants that have been propagated by seed .
Biodiversity
Biodiversity, a much used term these days, refers to a healthy mixture of ecosystems, communities within the ecosystems, species within the communities, populations within the species, and individuals within the population. An individual is a single plant of a given species -- a barrel cactus, for example. A population is all the barrel cacti in the area. The community is the barrel cacti, along with associated vegetation such as prickly pears, palo verde trees, ocotillo, and mesquite. The ecosystem is the Sonoran Desert. The Sonoran Desert is one of the four associations in the Western Deserts province.
Ecosystem diversity is the broadest aspect of biodiversity. Globally, as well as within North America, the variety of ecosystems contributes to the health and stability of the biosphere. Forests, grasslands, deserts, and other vegetation formations comprise the global ecosystem. On a smaller scale, biodiversity is important to the health and resilience of ecosystems as well. For example, food chains are built on the foundation of a healthy, diverse ecosystem of many different native plant species. This is referred to as species diversity. Native plants provide food and cover for insects. Insects in turn are a vital food source for many birds and mammals. The greater the diversity of plants, the greater the diversity of other species the ecosystem can support.
Species diversity is a measure of both richness and evenness. Richness refers to the total number of different species in an area or community, while evenness is a measure of the number of individuals of each species. In general, diversity increases with richness and evenness. In other words, the more different species and the more evenly distributed the individuals of each species, the more diverse the community.
On the smallest scale, the genetic diversity of individual species is also critical. Species with the largest number of populations with large numbers of individuals generally have the broadest genetic base. The more diverse its gene pool, the more resilient the species. Species with few populations and few individuals are more likely to lack the genetic diversity necessary to enable them to adapt to environmental change, disease, or other types of stress. In other words, genetic diversity enables a species to adapt and survive.
The Functions of Plant Communities
Living systems do not function in isolation. Interactions are the hallmark of healthy ecosystems. Some interactions have positive, others seemingly negative consequences. Not everything that we see as negative is detrimental to the long-term function of ecosystems, however. An animal eats a plant to survive. This is good for the animal, but bad for the plant. Or is it? Some plants respond well to being grazed, while others may die. Death is part of natural renewal. A dead tree feeds insects, provides a home for birds, and ultimately falls to the ground and creates humus. Catastrophic floods and hurricanes can alter the landscape in the blink of an eye. These disturbances set changes in motion that over time alter the nature and composition of plant communities. The landscape as we see it today is the product of all this change, seen and unseen, dramatic and subtle, fast and slow. Ultimately, the system prevails in spite of the individual and local dramas that are played out over time. Without dynamic interaction, there would be no healthy function.
Disturbance and Succession
Succession is the term that scientists use for the vegetation changes in plant communities over time. Succession works at many scales, from the thousands of acres burned in Yellowstone National Park several years ago to the comparatively tiny gap in the canopy left when a tree routinely topples in a forest.
Early ecologists believed that there was a fixed and predictable end point to succession, which they termed the climax stage, and that this climax stage was stable and self-perpetuating for long periods of time. They believed that vegetation changes in a plant community are inherent in the community itself, and outside factors such as storms or fire play little or no role in the process. Today, ecologists realize that no self-perpetuating end point is ever really reached, and that periodic disturbance - fire, flooding, damage by insects and diseases, and windstorms, to name a few - plays a critical role in maintaining the diversity of species and habits in a region. Rather than an anomaly that occasionally disrupts climax communities, disturbance is now viewed as the key recurring factor that keeps a mosaic of habitats in different stages of vegetation development in fairly close proximity to one another. This, in turn, assures the presence of a diverse mixture of plants and animals that characterize each phase of the change from, say, bare land to mature forest.
Although how vegetation succession or change occurs is more complicated than previously thought, what will ultimately happen in most places is still generally predictable. It certainly is true that specific types of vegetation will eventually predominate on most sites in particular floristic provinces.
In the Eastern Deciduous Forest province, vegetation change on abandoned farmland left undisturbed for many years will work something like this, with regional variations: Millions of seeds that lay dormant in the exposed soil germinate, causing an explosion of physiologically tough, aggressive annuals like horseweed and common ragweed. These plants, called pioneer species, dominate the first season. In a few years, biennials (today, many of them non-natives such as common mullein, chickweed and Queen Anne's lace) become common, along with a few perennial wildflowers like asters and goldenrods. After five years or so, grasses and wildflowers turn the area into a meadow. Within a few years young maples, ashes, dogwoods, cherries, pines, and cedars, many present as seedlings in the earliest stages, rapidly transform the meadow into "old field," an extremely rich, floriferous blend of pioneer trees, shrubs, and herbaceous species particularly favored by wildlife. Given enough time without major disturbance, perhaps several centuries, a mature or old-growth forest will once again be found on the site.
Succession and Disturbance in the Garden
The twin forces of succession and disturbance are constantly at work in the garden, just as they are in the native landscape. When you plant on bare earth you are creating a plant community. This community is a human invention, but it will change with time. You not only set in motion the forces of succession but ultimately become the agent of disturbance as well.
In traditional ornamental gardening, the hand of the gardener must always be at work to quell the influence of the dominant native plant community. Consider what happens when you weed your flower border. When you weed, you are thwarting succession. Annual weed seeds exposed by tilling and planting germinate and grow to cover the bare soil. Wildflowers such as goldenrods may blow in as seeds on the wind and find an empty spot in the garden. In forested regions, tree seedlings germinate and start the long process of reforestation. Without constant tending, weeds, unplanted wildflowers, and tree saplings would quickly overtake the garden, turning it into a thicket.
Even in the natural habitat garden, it is often beneficial for the gardener to guide change by thwarting succession, in this case by substituting for natural agents of disturbance such as fire. Meadows remain open grasslands because annual mowing keeps trees out. A meadow would otherwise soon start to become a forest. Prairies are renewed by wildfires that burn off the thatch, clearing the soil and releasing nutrients. Thankfully, most of us do not have wildfires in our gardens. To keep a prairie healthy, however, we must either mow it, or do a careful prescribed burn. We must lend mother nature a helping hand. The California chaparral is still another fire-dependent community. One of the reasons wildfires are so devastating in southern California is that regular fires are suppressed. Without regular burning, the large amounts of flammable litter accumulate. When a fire finally does come, it has so much fuel that it spreads out of control, consuming houses as well as native vegetation.
When we build a garden in a woodland, we are acting as agents of disturbance, too. If we thin the canopy or remove a tree, we are initiating what ecologists call gap phase succession. We let in more light and alter the dynamics of that piece of forest. When we plant, we disturb the soil, allowing new seeds to germinate. All our actions as gardeners have consequences. In the interests of both low maintenance and environmental harmony in the garden, it is important to understand and work with natural forces like succession.
Water and Nutrient Cycles
Energy and nutrients are constantly flowing through ecosystems unseen. Water is taken up by plants, transpired through their leaves, and given off to the atmosphere. This invisible process called evapotranspiration can influence global weather patterns. Eventually, the water falls back to the earth as precipitation. Similarly, the nutrients that sustain every living creature on Earth are transformed and distributed by a perpetual cycle of decay and renewal. The movement of chemical elements among the planet's living organisms and the physical environment is called the biogeochemical cycle. Nutrient and water cycles are two of the most important biogeochemical processes.
Nutrient Cycling
The nutrient cycle is the basis of life. Plants are the foundation of the nutrient cycle. Plants convert the energy of the sun and atmospheric carbon into biomass through a process called photosynthesis. Plants are called primary producers, because they are the starting point, where energy and nutrients enter the system. Animals that eat plants are called herbivores. They vary from the krill in the sea to the bison on the prairie. Herbivores are the lynchpin of the food chain, which is built upon production and consumption. Herbivorous animals are fed upon by meat-eating predators called carnivores. Wolves eat the deer that eat woody twigs and leaves, grasses and forbs. People are also part of the predator/prey system. People also eat the deer or, more likely, the cattle that have replaced them on the farms. Carnivores are at the top of the food chain.
Not all biomass is consumed in a living state. When a plant or animal dies, or when a portion of a plant dies, the nutrients are still recycled. When leaves fall from trees in autumn, for example, the nutrients are released by soil insects, fungi, and microbes, which break down this dead living material, called organic matter, through a process called decay, or decomposition. Decomposition of organic matter forms humus, a stable colloidal material that provides a variety of nutrients to plants. Colloids bind nutrients tightly until they are utilized by plants. Some of the nutrients are lost through movement of water down through the soil in a process called leaching. As any good gardener knows, the more organic matter and humus in the soil, the less leaching of water and nutrients there is. In such an efficiently operating ecosystem, most nutrients can be taken up by plants and become new biomass -- new plant growth -- and thus the cycle begins anew.
Nitrogen, an important nutrient for plant growth, also enters the ecosystem directly from the atmosphere through a process called fixation. Nitrogen fixation is the conversion of atmospheric nitrogen into a form usable by plants. Fixation occurs during thunderstorms, when electricity converts nitrogen into a soluble form that dissolves in rainwater, which then falls to the ground. Fixation also occurs in the roots of some higher plants, made possible by bacteria that live symbiotically in thick swellings on the roots called nodules. These bacteria absorb nitrogen gas and convert it directly to a form the plants can use. The forests and prairies are filled with native legumes (nitrogen-fixing plants) like wisterias, baptisias, indigobush, locust trees and redbuds.
Restoring the Nutrient Cycle
In the typical yard, the nutrient cycle is interrupted. A primary reason for this is that most gardeners like to keep things neat. Raking and disposing of leaves and lawn clippings eliminates the natural source of nutrients. As a result, fertilizer must be added to the soil to keep the lawn and garden healthy. Fertilizers typically release massive amounts of nutrients to the garden ecosystem at once. The soil cannot store them, nor can plants use them fast enough, and so they leach away. Leached nutrients, especially nitrogen and phosphorous, are a major source of water pollution.
Gardeners can repair the natural nutrient cycling process in their backyard ecosystems. Fallen leaves, lawn clippings, and other potential organic matter can be left in place in many areas of the yard. For a tidier look, leaves can be chopped with a mower or shredder and applied in flowerbeds and around shrubs as a natural mulch. Leaves removed from areas that must be raked can be added to a compost pile. There, along with other garden trimmings and kitchen scraps, they will be transformed into compost, that is rich in humus - nature's soil conditioner and natural, slow-release plant food. This not only saves money that otherwise would be spent on fertilizer, but also helps prevent pollution of wetlands and streams. Another way to restore the natural nutrient cycle is to integrate some beautiful native legumes (nitrogen-fixing plants) to beds, borders, and lawn areas. They will increase the soil's fertility while adding aesthetically to plantings.
Water Cycling
The movement of water through the environment is called the water cycle. Precipitation falls to earth, and moves through the landscape in a variety of different ways. Some of the water is intercepted by the canopy of trees and other vegetation, where it evaporates before it hits the ground. Evaporation also occurs from the ground itself. Most of the water infiltrates into the ground, where it is either taken up by the roots of plants or seeps into the ground and enters the water table. Some of the water runs off the surface into wetlands, streams, ponds, and, today, storm sewers.
A portion of the water that is absorbed by the roots of plants is returned to the atmosphere by the process called transpiration. This loss of water through the leaves of plants draws water into the roots and pulls it up the stem or trunk of the plant. Even water that flows to the water table eventually reaches the atmosphere, but it is a longer journey. Groundwater moves with the topography of the land until it reaches the lowest point in the water table - a pond, stream, or wetland. Water from these wetlands and waterbodies is constantly evaporating as it moves downstream through the watershed and ultimately to the ocean. Evaporation from the world's oceans returns massive amounts of water to the atmosphere, where it joins the water lost via transpiration, and returns to the earth as rain. Thus the cycle is renewed.
Modern cities and suburbs, with their impervious pavement, wreak havoc with the natural water cycle. In natural systems, surface runoff is minimal. By contrast, parking lots, sidewalks, and rooftops often produce torrents of runoff that never infiltrate into the ground. There are two serious consequences of reduced infiltration. Flooding is the most dramatic, and destructive. As water runs off roofs and streets, it collects in storm sewers and is rapidly deposited in streams and rivers. This fast-moving, enormous volume of water causes massive erosion of stream and river banks. Widespread and damaging flooding also occurs, in the worst cases washing away houses and crops. An ecologically sensible regional landscape would be designed to reduce runoff and maximize infiltration from the top of the watershed to the bottom. Since water accumulates in volume and speed as it runs through the system, the key to reducing flooding is to catch the water at the top of the watershed. This can be accomplished in many ways. The most obvious is to reduce or eliminate runoff. Reducing paved areas, using porous pavement that allows infiltration, and running downspouts into the ground are a few solutions. Where this is not possible, storm-water retention basins can be used to trap water. Keeping natural wetlands intact also reduces runoff.
Reducing Home Runoff
When rain or melting snow wash materials off the and into streams and lakes, the result is called non-point discharge. Runoff water can carry pesticides, fertilizers, soil, leaves, oil, and other pollutants that cause serious problems for our waters. Non-point source pollution can come from both urban and rural areas; however this information focuses on home landscape practices that can improve water quality. You can make a difference right away by preventing pollutants from being washed off your garden and into streams and rivers. The roof of a house, as well as sidewalks and driveways, and other impervious surfaces keeps water from infiltrating into the ground and contributes to erosion and flooding problems. Even lawns absorbs less water than more diverse types of cover. Gardeners, homeowners and landscape designers can minimize runoff by designing holding areas (planting beds) where water is encouraged to infiltrate into the ground over a 48 hour period. Planting beds provide an ideal location to direct downspout runoff where it can soak into the soil to be used by your plants. Planting beds can also replace selected mowed areas with an attractive alternative. Reducing the amount of yard area that is heavily maintained normally reduces the need for fertilizers and pesticides, which cause water pollution if misapplied. When it comes to chemicals, more is not better! Follow the directions exactly. Areas left to go "natural" or planted to create a prairie or woodland appearance require very low maintenance after the initial planting. Natural areas can supply their own nutrient requirements and absorb rainfall to reduce runoff problems. Properties that have a stormdrain located near them can release pollutants just like properties that have waterfront drainage. You can help prevent pollution by preventing anything except stormwater from washing down stormdrains. Buffer strips are vegetated areas along ditches, water courses, streams, or lakes, allowed to revert, or planted, to a natural state. They may be 5-to-15-to-25 feet or wider, depending on the site. Grass left unmowed in these areas or prairie or woodland plantings slow runoff of rainfall, settling out soil particles (sediment) and other pollutants before they enter surface waters. Well-planned buffer strips can be used to frame good views or screen unsightly ones. There are native plants that can help absorb pollutants from the runoff and even help neutralize the pollutants. Some of the natural functions of stream buffers are to decrease velocity of storm water, thereby reducing the risk of erosion; store flood waters; allows streams to meander, creating a diversity of habitats; filter out pollutants, sediment and excess nutrients; shade stream channels, thereby decreasing water temperature; provide wildlife habitat and corridors and provide greenspace for parks.
Plant/Animal Relationships
Plants and animals adapted together, so it is not surprising that there are many complex plant/animal relationships. Some relationships are beneficial to both parties, while others have a clear benefit for one at the expense, or even death, of the other. Four important plant/animal interactions are explored here: plant/herbivore, plant/pollinator, plant/disperser, and other examples of mutualism.
Plant/Herbivore Relationships
Herbivory is an interaction in which a plant or portions of the plant are consumed by an animal. At the microscopic scale, herbivory includes the bacteria and fungi that cause disease as they feed on plant tissue. Microbes that break down dead plant tissue are also specialized herbivores. Browsers and grazers, from aphids and caterpillars to deer and bison, are more familiar herbivores. Even insects and animals that eat seeds are considered herbivores.
Some herbivores consume entire plants, or enough to kill them. Others only eat a portion of the plant, and so the plant can recover. The plant/herbivore relationship traditionally has been seen as lopsided, with the animal as the beneficiary and the plant as the loser. Current research, however, is revealing that herbivory has some potential benefits to plants. One example is canopy grazing by insects, which allows more light to penetrate into the lower layers of the forest. Gypsy moth grazing on canopy trees in some areas of North Carolina's Blue Ridge Mountains, for instance, has resulted in more light penetration and therefore a more diverse and productive ground layer.
Herbivores and Their Food Plants
Bison, sheep, and other grazers - Succulent forbs, grasses, grass-like plants
Deer and other ungulate browsers - Leaves and twigs of woody plants such as willows, arborvitaes, forbs
Beaver - Tree bark, young shoots, leaves
Rodents - Succulent forbs, grasses, grass-like plants
Rabbits - Succulent forbs, grasses, bark
Voles - Roots, bark
Caterpillars - Leaves; in some cases, of specific species
Monarch butterfly - Milkweeds
Gypsy moth - Oaks and other hardwoods
Aphids - Plant juices; in some cases, of specific species
Many birds - Seeds, fruits, nectar and insects
Locusts - All plants; seeds, leaves, and stems
Some Animals and the Plants They Disperse
Ants - Many wildflowers, such as trilliums, bloodroot, violets
Birds - Fleshy fruits and grains, such as baneberry, viburnums, blueberries, ash
Woodpeckers - Poison Ivy
Mammals - Fruits, grains, nuts, berries
Squirrel - Nuts, such as those of oaks, hickories, pines
Fox - Berries, such as blackberry, grapes
Humans - Weeds such as plantain, dandelion, lamb's-quarters
Reptiles - Fleshy fruits, especially berries such as strawberry, groundcherry, jack-in-the-pulpit
Plants and Their Pollinators
Pollination is the transfer of the pollen from one flower to the stigma, or female reproductive organ, of another, which results in fertilization and, ultimately, the formation of seeds. The earliest plants were pollinated by wind, and for some modern plants this is still the most expedient method. Many trees, all grasses, and plants with inconspicuous flowers are designed for wind pollination. Bright, showy flowers adapted for another purpose - to attract a pollinator.
Many plants depend on animals for pollination. Insects, birds, even bats are important for perpetuating plants. The flowers of these plants evolved in concert with their pollinators, and their form reflects the form and habits of their pollinators. Bee-pollinated plants are often irregular in shape, with a lip that acts as a landing pad to facilitate the bee's entry into the flower. Butterfly-pollinated flowers are often broad and flat, like helicopter pads. The flowers of many plants are brightly colored to attract their insect pollinators, and many offer nectar as an enticement. Hummingbirds, with their long beaks, pollinate tubular flowers. Bats require open flowers with room for their wings, such as those of the saguaro cactus. In the tropics, birds and bats take the place of insects as pollinators. Hummingbirds and honeycreepers, for example, have distinctive beaks that have evolved to exploit flowers. Often, a beak may be so specialized that it is only effective on a small group of flowers. The pollinators, in turn, have evolved to take advantage of the flowers. A successful pollinator typically has good color vision, a good memory for finding flowers, and a proboscis, or tongue, for attaining nectar.
Animal pollination has obvious advantages for plants. Many pollinators cover great distances, which insures genetic diversity through outcrossing, or the transfer of pollen to unrelated individuals. The pollinator benefits as well by gaining access to a source of food. The relationship of pollinator and plant is an example of mutualism.
Plants and Their Dispersers
No two plants can occupy the same spot. In order to have room to grow, seeds must be dispersed away from the parent plant. Seed dispersal is accomplished by a variety of means, including wind, water, and animals. Animal dispersal is accomplished by two different methods: ingestion and hitch-hiking. Animals consume a wide variety of fruits, and in so doing disperse the seeds in their droppings. Many seeds benefit not only from the dispersal, but the trip through the intestine as well. Digestive acids scarify seeds, helping them to break out of thick seed coats.
Some seeds are armed with hooks and barbs that enable them to lodge in the fur of animals that brush past them. Beggar's ticks and bur marigold are two examples. Eventually, the seeds are rubbed or scratched off, and may find a suitable spot on which to germinate and grow. People are important for dispersing plants, too. The common weed plantain was called "white man's footsteps" by Native Americans because wherever settlers walked, the plantain came in the mud on their shoes.
Mutualism
Mutualism is an obligate interaction between organisms that requires contributions from both organisms and in which both benefit. There are many examples in nature. Pollination and dispersal, discussed above, are mutualistic because both plant and pollinator or disperser benefit from the relationship. The relationship between mycorrhizal fungi and many higher plants is another common example of mutualism. The bodies of the fungi, called hyphae, live on or in the tissues of plants, and make nutrients available for the plants to absorb. The plants provide the fungi with amino acids and other complex compounds. One of the most celebrated examples is the orchids. Whereas some plants may support as many as 100 different fungi, orchids have quite specific mycorrhizal associations (see "The Lovely Lady Slipper"). Different plant communities have different mycorrhizal associations. The microflora of a grassland is different from that of a forest. These differences, at least in part, may influence the distribution of plant communities.
The Lovely Lady Slipper
The reason lady-slipper orchids are so hard to grow in a garden is that the needs of both the orchid and its fungus must be attended to. The growing conditions in the garden must duplicate exactly those in the orchid's native habitat.
Anyone who tries to cultivate these beautiful plants learns before long that the pink lady-slipper (Cypripedium acaule) is much harder to grow than the yellow lady-slipper (Cypripedium calceolus). This is because of the fungus. Yellow lady-slippers grow in slightly acidic, rich soils. Their associated mycorrhizal fungus thrives under the same conditions as those in woodland and shade gardens. The pink lady-slipper, on the other hand, grows in sterile, acid soil, not the typical garden variety. Plant the pink lady-slipper in rich garden soil, and its associated fungus cannot survive. As a result, the pink lady-slipper slowly languishes and eventually dies. Most lady-slipper orchids are still collected from the wild, harming native populations. Buy them only from nurseries that propagate their plants.
Imperiled Pollinators
All is not well in the realm of pollinators. The age-old relationships between plants and pollinators is threatened, especially in urbanized and agricultural regions. Habitat destruction and fragmentation, pesticide abuse, and disease all have taken their toll on pollinators. As more land is cleared for human habitation, bees, butterflies, bats, and birds are left homeless. Our gardens offer little to sustain them. They need a constant source of nectar and pollen throughout the entire season. The few flowering plants most people grow will not suffice.
A related problem is fragmentation of plant communities. Plants must be pollinated in order to set seed for the next generation. Without pollinators, no seed is set and the plants eventually die out, leading to local extinction. Isolated patches of forest, grassland, or desert are particularly vulnerable. A small patch may not sustain enough pollinators, or may be too far from other patches for pollinators to travel. As a result, plants do not reproduce.
Pesticides have also reduced pollinator populations. Bees are often killed by chemicals applied to eliminate other pests. Honeybees are being destroyed by diseases and parasitic mites. The crisis is not just affecting native ecosystems. Fruit trees and many other food crops depend on pollination for production. We stand to lose over three quarters of our edible crops if we lose pollinators.
What can be done? Encourage pollinators by planting a diverse mixture of adult and larval food plants in your garden. Erect bat and bird houses, and where practical, place bee hives. Reduce or eliminate pesticide use. Help restore native plant communities not only in your yard, but also in parks and along roadways, and connect them through corridors and greenways to preserves and other natural areas.
References:
* Brookland Botanical Gardens, http://www.bbg.org/index.html
* North Carolina Department of Agriculture and Consumer Services, Cooperative Extension Service http://www.ces.ncsu.edu/
* Mecklenburg County, NC, Solid Waste Management, http://www.wipeoutwaste.com
