The Biology of Trees Tree Leaf Patterns. Leaf display on tree limbs are established by anatomical and morphological aspects of leaf development and growth. The blue-print for leaf orientation is genetic and the instructions are interpreted in the young growth tissue or meristem at the stem apex. This blue-print establishes whether leaves are alternate or opposite, the angle of rotation between adjacent nodes on the stem, and the rotation of leaves around the stem from node to node (e.g., phyllotaxy). Leaves also respond to light during early development as to maximize maximum light absorbance for shade leaves and minimize direct sunlight for those on the canopy edge. For example, trees may improve light penetration into the tree by decreasing the angle of incident light. Shade leaves maximize light absorption by maintaining a 90 degree angle to incident light and a growth response during development. Leaves in the understory appear to minimize overlap along branches because of the leaf growth in response to environmental variation in light intensity. The next time you visit a forest lay down and look upward at the leaf and branch orientation of sugar maple, beech and yellow birch. You will be amazed how little overlap there is of leaves on the same branch. You can also observe how branches are positioned around the trunk rather than stack one above another. These patterns are not haphazard by design, but rather an adaptive evolutionary response by the plants to maximize their survival and reproduction (Figure Leaf Orientation). In eastern deciduous forests, opposite leaves are common on maples (Acer), dogwoods (Cornus), ash (Fraxinus), horsechestnut (Aesculus), viburnum (Viburnum), and elderberry (Sambucus), although the alternate-leaved dogwood (Cornus alternifolia) violates the rule. Alternate leaved trees and shrubs are far more common and include oaks (Quercus), beech (Fagus), willows (Salix), poplar (Populus), cherry (Prunus), birch (Betula), witch hazel (Hamamelis), basswood (Tilia), and elm (Ulmus). In some groups (e.g., oaks), leaves may appear to have opposite orientation near the twig apex. This anomalous condition arises from shortened internode growth late in the spring. Similarly, birches frequently produce very short lateral branches and twigs that elongate just millimeters each year. These spur shoots will have leaves originating very closely along the stem and give the false impression of opposite orientation. Conifers also develop distinct needle orientations that help in identifying trees. Pines produce needles in bundles or fascicles of 2, 3, or 5. The number of needles in a fascicle is constant for a particular species. For example, white pines (Pinus strobus) have five to a fascicle whereas red (Pinus resinosa) and Virginia (Pinus echinata) pines have 3 and 2, respectively. The American larch or tamarack (Larix laricina) produces many needles in tightened whorls on short branches (i.e., spurs) except at the stem tip along the primary axis of the branch where needles alternate. Hemlock (Tsuga), firs (Abies), and spruces (Picea) all produce single needles along the stem. In hemlock, the needles are two-ranked in a single plane and give the branchlets a flattened appearance. Needles orient along all surfaces of the twigs in fir and spruce to give the branches a brushy or three-dimensional appearance. Spruces are easily distinguished from firs by their sharp needles that are four-angled in cross-section. Fir needles are flattened with rounded tips. Conifers and hardwood leaves produce leaves at different densities and leaf shape depending on light intensity. Balsam firs will produce shortened needles oriented upward in full sunlight. These leaves also possess a thick waxy cuticle that gives the leaves a bluish cast. Shade needles on balsam fir are longer, broader, greener, and flattened in comparison. Hardwood sun leaves are thicker and smaller than the thin, broad shade leaves. These adaptive features permit leaves within the canopy to maximize light absorption and therefore photosynthesis. The thicker cuticle of sun leaves retards water loss and may reflect excess light to prevent sun scorch (Figure Sun vs. Shade Leaves—Balsam Fir and Swamp White Oak). Novice field guide users are often challenged with understanding the fact that organisms vary. It is not uncommon to hear a first time user of the field guide claim that his/her tree leaves look nothing like the line drawing in the text. It is unfortunate that some “key” characters are ephemeral like the brownish pubescence along the lower leaf midrib of black cherry. On the other hand, leaf size and shape may vary not just between individuals but also within individual trees. Field guide authors and illustrators are challenged with allotting small amount of space to describing and identifying the important features for every individual of a given species. For a northern hardwood tree species in eastern deciduous forest, there may be hundreds of millions of individuals growing across the landscape. Imagine an alien from elsewhere in the universe arriving on planet year with its field guide to intergalactic carbon bodies. On page 2629 of the intergalactic field guide is the species account for Homo sapiens. There are two line drawings, male and female, to represent the 6 billion plus individuals. We recognize that humans vary greatly in size shape, skin color, hair color, facial features, musculature, posture, and behavior. Just how confused will the alien field guide user be if one accepts the condition that no species varies more than the line drawing shows! There are at least three reasons why we should expect tree leaves to vary in detail. First, like humans, other organisms possess considerable genetic variation that can translate into differences in physical traits. The final two reasons are attributed to the plastic nature of leaf growth on trees. Because trees are firmly rooted in terra firma, they retain some ability to modify the size and shape of leaves depending on the current environmental conditions. Thus (second), leaves produced in the shade tend to be broader and thinner than shade leaves (see above). Finally, some trees produce multiple forms of leaves evening within the same tree and often on the same branch. Sassafras and mulberry are two species that produce leaves with various forms of leaf lobes that results in unlobed, uni-lobed, and bi-lobed leaves (Figure Mulberry Leaf Variation). We often refer to these mitten-like leaves as possessing none, one, or two thumbs. Honeylocust is an example of a tree with compound leaves. Each honeylocust leaf may vary in the degree of dissection from once, twice, or thrice compound. Solar Forest. Nature has been solar powered by the sun for more than a billion years, nearly ¼ of Earth’s existence. Solar power provides the energy to life through photosynthesis. The ultimate source of all energized organic compounds for all organisms from the three domains and dozens of biological kingdoms is derived from solar energy whether they eat plants or not. Even ancient sunlight from geological periods from the dinosaurs and before was captured by chlorophyll and used to energize organic molecules. The modern combustion engines burn the fossil fuels that are the direct products of ancient sunlight and photosynthesis. Modern biological texts aren’t written without some reference to the historically important experiment of a 17th century Dutch scientist, Van Helmont. In not so simple of words, he asked whether plants really ate soil. He determined that in a five year periods, the 169 pound dry weight from a potted willow could not have been derived from the minerals missing in pot’s soil. Naturalists furthered Van Helmont’s line of inquiry by determining the plants rejuvenated the atmosphere with oxygen and removed the toxic gas, carbon dioxide, released by heterotrophs such as mammals into the atmosphere. Science now recognizes photosynthesis as a series light and biochemical reactions that occur in the chloroplasts of plants and algae. Plants draw on the natural resources of carbon dioxide, sunlight and water to produce high energy organic molecules and release water. Water provides the electrons to start the flow of energy through and from chlorophyll that is captured by energy carriers. Oxygen gas falls away from the split water and is liberated from the plant. The carrier molecules transport the energy away from the pigment centers to the manufacturing district of the chloroplasts. Here carbon dioxide is captured and locked onto an organic molecule. The labor forces of enzymes rearrange the molecule using energy from the energy carriers to make new molecules that possess considerably more energy than when the process began. Photosynthesis involves the three most abundant biological molecules on Earth. Chlorophyll is the most abundant pigment. Chlorophyll is built around four carbon ring molecules with nitrogen and a central atom of magnesium. Chlorophyll has a long carbon tail that positions it securely in the heart of a membrane pigment center. The central magnesium atom is critical in starting the flow of high energy electrons and splitting water to release oxygen. Nitrogen is often a limiting element in plant growth and plants deficient in nitrogen are chlorotic or yellowish. Hence fertilizers are rich in nitrogen molecules that plants can use such as either ammonium or nitrate. Ribulose bisphosphate carboxylase (RuBP carboxylase) is the most abundant and important plant enzyme used to capture carbon dioxide and fix the carbon atom onto a preexisting organic molecule. Almost as fast as carbon dioxide diffuses into plant cells and chloroplasts it is assimilated onto organic molecules by RuBP Caroxylase. Leaves capture carbon dioxide efficient from an atmosphere where less than 1% of the molecules are carbon dioxide. It air molecules where jelly beans in a jar and carbon dioxide is the one flavor you want, then you need to search through 3000 jelly beans to find just one of the desired flavor. Plants excel at accomplishing so much with so little. They capture miniscule amounts of carbon from thin air, but build carbon into the most massive natural living structures. Cellulose is one of many possible end products from photosynthesis. This molecule is a long chain of glucose sugar molecules. When cellulose is cemented together with other cementlike carbohydrates into rebar-like rods it forms a formidable shell around plant cells. However, unlike a building foundation, the cell wall of plants is flexible, capable of growth, and allows the passage of water into cells. Forest trees are very well designed for photosysnthesis. Leaves are displayed in an orientation and pattern that maximizes energy absorption and penetration to deeper layers. The leaves possess ventilation conduits to permit release of oxygen and water vapor and the inclusion of carbon dioxide from the atmosphere. Water is transported from the roots through cellulose and lignin fortified pipelines to the leaves where it is used to coat cell surfaces to facilitate gas exchange and for photosynthesis. Minerals and other elemental nutrient are dissolved and transported in the water stream. Solar energy drives the movement of water through the plant. Trees are giant biotic sponges and as such have a high surface area relative to their volume. Fine root hairs near the tips of growing roots greatly increase the surface area to mine the soil for water. Broad, thin leaves have great surface area for light absorption and gas exchange. Like all great sponges, this surface area facilitates absorption and exchange with the environment. Heat is lost and leaves cooled through high leaf surface area. Oxygen gas which is toxic to photosynthesis is quickly released through the high surface area provided by leaves. Leaves capture carbon dioxide quickly and efficient from the atmosphere. Almost 50% of the dry mass of an organism is carbon captured by photosynthesis. Oxygen and hydrogen are the other primary elements in biomass. To answer Van Helmont’s question, plants eat carbon dioxide and water. The number of ways carbon, hydrogen, and oxygen can be rearranged and built into large and more complex molecules is almost endless. Plants make the four primary categories of organic molecules: proteins, lipids, carboydrates, and nucleic acids. Plants also make a wide range of other organic molecules, anthocyanins, carotenoids, terpenes, and alkaloids to name a few. These secondary molecules are not essential to living, but they make life easier in a variety of ways for plants by advertising fruits and flowers and deterring attacks by insects, bacteria, and fungi. When non-photosynthetic organisms ingest food they are absorbing, utilizing, and rearranging the organic molecules manufactured in plants. The muscle tissue of your body, the proteins in your blood, the hair on your head, and the collagen holding your skin in place are all made using carbon captured from the atmosphere and energized by photosynthesis in green plants. In recent decades, forest researchers have developed allometric equations for estimating the biomass of trees of different size classes (Figure Tree Biomass). The details of these equations are determined by collecting every plant part of many trees, separating them by plant organ (stems, trunk, leaves, and roots), drying and then weigh what is left over. All we need to know to make the equations work is the diameter of the tree trunk at 4.5 feet from the ground. In general, root mass is 20% of total stem mass and carbon content is 50% of dry mass. These relationships aren’t perfect and there are many assumptions when they are used. Trees behave and grow differently in different environments or regions throughout their range. It is also too difficult to harvest, dry, and weigh trees of great size, thus allometric equations are best used for small and medium sized trees. Nevertheless, the allometric equations provide a better estimate of biomass than nothing at all. A simple allometric growth curve for above ground tree biomass provides many opportunities for interesting calculations. One can estimate the total mass, and below ground biomass of the tree. The mass of carbon and its equivalent in terms of carbon dioxide can be released. Every gram of combusted of decomposed organic carbon will generate 3.66 grams of carbon dioxide. Take for example an eastern hemlock with a DBH of 40 cm, approximately 16 inches. In central NY, a tree of this size may be 30 years old. Its total biomass will be 718 kg (1,579 pounds) and total carbon content of 359 kg. Upon decomposition or combustion of this tree it will release 1314 kg (2890 pounds) of carbon dioxide. This is roughly the same carbon dioxide emissions from driving a car 30,000 miles at 16 miles per gallon mileage. This exercise of calculating biomass and carbon content illustrates two amazing aspects of tree life. First, trees store a large amount of energy in their organic structure. Imagine a 30 year old eastern hemlock liberating nearly the same energy content as 1875 gallons of gasoline (energy from ancient sunlight). Second, trees eat an awful lot of carbon dioxide from the atmosphere. This 30 year old hemlock ate an average of 96 pounds of carbon dioxide per year. All of this carbon dioxide passed through tiny ventilation holes on the needles and was fixed onto organic molecules. Trees and forests are carbon sinks. They remove and store more carbon each year than they emit. Additional good news is that they produce more oxygen gas than what they use in cellular respiration. Broad leaved deciduous trees do shed their leaves annually. The organic molecules in leaves are cycled through a rich ecosystem of saprobic organisms such as bacteria, fungi, and insects. Carbon dioxide is gradually released from the detritus on the forest floor as these organisms work, but mature forests accumulate more organic matter than can be decomposed in a year. When William Bartram made his first treks across the forests of the southern Appalachians he encountered thick forest detritus and humus that swallowed his legs. Deforestation and farming of the last three centuries has altered the decomposition rate and allowed much of the organic debris to decompose to carbon dioxide. Nitrogen, phosphorus, and many minor elements followed a similar fate and were recycled through organisms and some leached from the forest community. Forest Water Cycle. Water is a precious commodity in the social struggle of forest plants. Water is required for growth, mineral movement and transport, translocation and movement of sugars, hormones, and nutrients, and cooling of forest trees. The humid feel to the forest environment is due to evaporation of water from leaf surfaces through the process of transpiration. Transpiration is both a curse and a blessing for forest plants. The water stream within the vascular tissue of the plant is responsible for mineral transport, sugar movement from leaves, photosynthesis and growth. More than 90% of the water absorbed by tree roots is transported to leaves and evaporated from leaf surfaces. Sunlight powers the evaporation of water from cell surfaces within leaves. This water vapor moves out of the leaf through pores on the underside of deciduous tree leaves to drier air. The evaporation of water is critical for plant survival. It allows gases, mainly carbon dioxide and oxygen, to diffuse across cell walls and membranes into leaf cells and allows oxygen to travel in a reverse direction. The leaf stomates facilitate this exchange with the atmosphere. Autonomic control of stomatal opening allows the plant to slow the process of transpiration during water stress. Because energy is required to break hydrogen bonds and evaporate water, transpiration has a tremendous cooling effect on the leaves, trees, and forest community. In fact evaporation cooling is a widely used and efficient means of cooling production greenhouses world-wide. The effects of high transpiration rates on growth are sometimes evident in nature. Trees that are extremely tall (giant redwoods) or in high wind areas are exposed to different forms of water stress. The forces of gravity and the stress on the continuous column of water up 300+ feet slow the movement of water to upper most needles to a trickle. Leaves on the highest reaches of the tree grow less and remain smaller than needles produced closer to the ground. Similarly, trees on mountain tops already have lower water availability due to the absence of ground water. Combine the low water table with persistent winds and trees have little water left over to expand cells and allow for growth. Needles or leaves on the windward side (i.e., flag tree) of the tree are usually reduced in size. Forest humidity is important for the survival and reproduction of many plants and animals. Non-vascular plants such as mosses and liverworts, ferns, clubmosses, and horsetails require a watery film across the soil and plant surfaces for the free-swimming sperm to unite with egg cells on gametophyte generation. Furthermore, the thin unspecialized leaves of mosses and liverworts require sufficient soil moisture and humidity to remain viable and capable of photosynthesis. Forest tree roots may grow to sufficient depths to utilize the continuously moving water table below ground. Herbaceous forest perennials are unable to capitalize on this resource just feet below them but may use the tree as an escalator for water to the surface level. This process of hydraulic lift was demonstrated in a forest near Ithaca NY where it was demonstrated that tree roots and stems have the remarkable ability of redistributing some ground water to surface layers. Thus, some spring wildflowers such as trillium, blue cohosh, and wild ginger take advantage of hydrolyic lift when they grow close to tree trunks. Leaf Colors. Summer leaves are usually associated with the darkness of green chlorophyll, but hidden amongst the pigment centers of chloroplasts are the bright yellows and oranges of carotenoids. Like chlorophyll, these pigments are imbedded in membranes because of their fat soluble nature. Carotenoids act as accessory pigments to chlorophyll by expanding the range of light wavelengths that can be utilized by photosynthesis. Additionally, the highly stable nature of carotenoids may offer anti-oxidative protection to the photosynthetic membranes from intense sunlight and oxygen free radicals. Unlike carotenoids, chlorophyll is an unstable molecule and leaves constantly manufacture new chlorophyll throughout the summer until the shorter photoperiod of autumn days interrupt the process. At this time, the brightness of the carotenoid pigments shows off in the orange and yellow of autumn. Red and purple leaves appear on both ends of the growing season. These pigments are water soluble anthocyanins in the cell vacuole. Anthocyanins are abundant in flowers and fruits, leaf petioles, and leaves in the spring and autumn. Anthocyanins require sugar for production and as such the brightest and darkest reds and purples are produced when the plant part has access to sufficient carbohydrate. Thus leaves and fruits with abundant sunlight are more brightly colored where light does not limit photosynthesis. This analysis can be used to explain many color patterns in nature: why shade grown northern pitcher plants are green and those in full sunlight are red, why shade red apples are green, and why the most intense leaf fall color occurs on the side of the tree exposed to direct sunlight. In spring and early summer, the young tree foliage of many species in boreal, temperate, and tropical is red of various shades (Figure Leaf Anthocyanins). The red leaves are most evident on early leaves of basswood, black cherry, dogwood, hazel, quaking aspen, sumac, red maple, white ash, and willow to name a few. Why should anthocyanins be abundant in young leaves? Anthocyanins protect leaves by preventing photo degradation of some organic molecules, protect from oxyen free radical damage, may protect DNA from ultraviolet light damage, and protect developing pigment centers in the chloroplast. Laboratory plants grown in strong light manufacture more anthocyanins than those grown in less intense ight. These anthocyanin rich plants recover more quickly from photo-oxidative stress as compared to plants with lower levels of anthocyanin. Presumably anthocyanin rich leaves and fruits like blueberry, apple, grape and cherry are healthy because the anti-oxidant properties protect our cells. Many trees and shrubs increase anthocyanin content of leaves very early in the autumn and immediately prior to fruit maturation. Speculation among ecologist proposes that these colorful late summer leaves are foliar fruit flags to increase the advertisement of ripening fruits. Among the temperate forest plants producing foliar flags are sumac, hobblebush, Virginia creeper, sassafras, and some dogwoods. Foliar fruit flags are also evident on Indian cucumber which produces dark purple berries above three whorled leaves with reddish bases. Autumn colors are arguably best in deciduous forests of eastern North America. The combination of bright sunny days, cool nights, and the right mix of species provides optimal conditions for a rich display of red, purple, yellow and orange on the autumnal blue skies. Sunny days allow sugars production to continue in chloroplasts even as chlorophyll exits the leaf and carotenoids shine through. Cool nights keep rates of respiration low and allow sugars to accumulate in leaves and produce anthocyanins. Maples, sumac, northern red oak, and sassafras produce intense reds, oranges, and yellows. Although aspens and hickory lack anthocyanin rich leaves, these species provide the eastern forests with the brightest yellows and oranges. The environmental influence on fall coloration can be observed in a number of ways. Street lights often inhibit the formation of fall colors on part of the tree. Light supplementation lengths the active photoperiod and inhibits the formation of leaf abscission zones and the breakdown of chlorophyll. Color change is often delay on tree parts receiving supplemental light. The importance of sunlight in anthocyanin production can be observed by examining the intensity of red and purple along tree branches. Interior leaves that receive less sunlight are generally more yellow and less red. Occasionally one may find leaves that have had another leaf, needles, or other objects glue to the upper leaf surface by water adhesion. These objects filter sunlight from striking portions of the leave. When the object is removed its silhouette is outlined by anthocyanins. The area shaded by the object shows only the carotenoid accessory pigments and no anthocyanins. A keen observer can usually find many leaves or fruits with color imprints in maple and dogwood forests. References: Aber, J. 2004. Biogeochemistry: The physiology of ecosystems. Pages 32-40, Forests in Time: The Environmental Consequences of 1,000 years of change in New England, D.R. Foster and J. D. Aber, eds, Yale University Press, New Haven, CT. Gould, K. S. 2004. Nature’s Swiss Army Knife: The Diverse Protective Roles of Anthocynanins in Leaves. Journal of Biomedicine and Biotechnology 5:314-320. Niklas, K.J. 1996. Differences between Acer saccharum leaves from open and wind-protected sites. Annals of Botany 78:61-66. Raven, P.H., R. F. Evert, and S. E. Eichhorn. 2005. Biology of Plants. W.H. Freeman, NY, NY. Ter-Mikaelian, M.T. and M. D. Korzukhin. 1997. Biomass equations for sixty-five North American tree species. Forest Ecology and Management 97:1-24.