THE WORLD IS OUR GARDEN
- Urban forestry is multidisciplinary, requiring knowledge of plant science, horticulture, ecology, planning and design, community laws and standards, and local history and culture.
- But Urban Forestry also demands skills and practical experience in equipment use and safety, management of materials, logistics, and simple maintenance and repair.
- Understanding tree growth and structure, biology and development, and ecology and health will inform all areas for those who plant, maintain, and oversee urban forests.
Students in my urban forestry classes typically have experience in pruning and managing equipment, but find that getting inside a tree botanically, giving attention to soil-root biology, and learning about plant identification and classification create wholly new insights and areas of interest. The discussion below includes points I make when it is a one-shot deal, when the lecture is part of a single day workshop that includes use of microscopes and examination of wood specimens.
The Short Lecture Transcribed: Trees hold a special place in cultural memory as well as in ecosystems, which is confounded by the fact that “tree” has no truly rigorous scientific definition. Indeed the on-going flexibility in thought as to what constitutes a tree speaks to the reality that “tree” is a statement of cultural value – if a plant has enough presence, permanence, and importance, it is likely to be a tree. This means that what we consider a tree is personal, varying from one region to another, and with the circumstances.
Books tell us that a tree is a woody, long-lived plant with a main trunk, which I have always simplified to the idea that a tree is a long-lived, woody plant under which you can walk. My definition of “tree” eliminates the banana, but includes many palms. And in my world, a bonsai specimen, though marooned in a small pot, remains a tree, even though horticulturally it is as much a shrub as anything.
Communities value and regulate trees differently from other plants. A homeowner can plant a geranium and an oak seedling, taking care to keep both alive for twenty years. By the end of that time, an ordinance might prohibit any alteration or removal of the oak, but give little note of guidance concerning the geranium.
Cities love and hate trees. Aesthetic appeal and quality of life often bear direct relationship to the health, composition, and beauty of the urban canopy. But trees are implicated in many malicious deeds, such as lifting sidewalks, invading sewer lines, contributing to allergies, dropping messy leaves, flowers, and fruit, and creating hazards. And maintaining trees takes more work than people want to imagine, from planting, protecting, and pruning to watering, restoring after windstorms, and responding to attack by pests and disease.
I’m contacted regularly by homeowners and merchants who have grave concerns about the future of a given tree or shady grove. Usually this relates to fear of losing a wonderful plant that creates a lovely and pleasant surround. But sometimes a plea is more political, related to a completely different issue residents aim to derail, such as new construction, a change in zoning, or some conditional use permit. In these cases people are hoping the emotional freight of potential harm to a specimen tree will stop the rocket. Trees are personalities. As an urban arborist, you will find your duties wide-ranging, including far-flung community services we normally delegate to bartenders and undertakers – listening, giving comfort, and doing the necessary deeds.
In every case, it is the longevity and majesty of specimen trees that underpins reality and emotional impact. These are living objects. Arboriculture (and silviculture) are the applied fields of botany that document and manipulate the real natural and physical phenomena that, when understood, make trees yet more beautiful.
Establishment: Trees start life humble enough, like other plants. Take a quintessential tree seed, an acorn, and watch it grow. Compare that to a peanut. Though the oak and a peanut are unrelated, germination is hardly any different for one as compared to the other. A primary root anchors quickly into the soil and a growing tip rises straight upward, leaving behind the seed leaves that provide the chemical energy powering the oak’s rapid establishment.
The growing tip follows a plan, node by node making segments that build the primary stem. Each segment includes a length of clear stem (the internode) that is complete with a leaf and a side bud (a lateral bud) that constitute the branching zone called the node. All along, the growing tip seamlessly initiates upcoming segments. Indeed if you examine microscopic details of the growing tip, primordia already exist for the growing season’s leaves and nodes.
If we were observing peanuts, branching would begin with some of the earliest lateral buds. But this is an oak, so the lateral buds remain dormant, meaning the stem is unbranched, and the season’s growth winds down with a straight unbranched twig topped with a terminal bud amidst a set of handsome, tightly-clustered lateral buds. These clustered buds and their scale leaves represent tightly stacked nodes without normal green leaves, structures that will persist through winter. In spring, strong leafy primary growth will resume from the terminal bud, creating the next season’s “scaffolding”.
Had this been a peanut, lateral buds would have grown out at many nodes, the plant would have flowered from numerous leaf axils, fruit would have formed, and the herbaceous stems would have perished with fruiting. But the oak did not flower or fruit – that will be delayed for years, perhaps even decades. The immediate task is to establish a permanent, woody presence, so something very different begins to happen.
Stem and Trunk: All of the growth that yielded the peanut and the oak seedling had been what botanists call primary growth (1º) – which means the shoot, leaves and lateral buds were produced by the shoot growing tip. The existing root was produced by the root growing tip. But in a new season, the shoot will grow taller and the root system will become more extensive, so the frame that supports them will have to become stronger and more permanent. This is where secondary growth (2º) comes into the story.
In both roots and stems, 2º growth means creating an increasing core of wood, which we call xylem, In an oak, xylem is the permanent record, accumulating year after year as annual rings, each ring telling a story of fat and lean seasons, each helping to build the remarkable resource we call wood. The cells that make these rings are generated by a layer called the vascular cambium.
I think of the cambium as an ultra-thin cylinder that enrobes the entire trunk (and branch system) of a woody plant. Picture it like a latex glove covering your hand, your palm and fingers being the trunk and branches, and it produced essentially all of the tissue that is inside.
If that internal tissue (your hand) is like wood, then it has complex layered character. If your hand grew like a tree, then during the next growing season the glove would create a new complete layer of flesh to the inside, and would stretch to cover that new layer. At the same time the glove would generate a new layer of bark to the outside. After a few seasons, your hand would be made of layers of flesh (in a tree, wood) that are interior to the glove, and it would be covered with layers of bark on the outside. Because the bark sheds constantly, the only permanent components are the layers of wood inside.
Getting back to trees and wood for the moment, we will stick with the prevailing wisdom that wood gives support and creates the conduits for water and nutrients from the soil to enter the root system and support the life of the tree to the very last cells on the surfaces of leaves at the farthest branch tips. Wood is protected and preserved.
Physiologically, wood is half of the story. The unsung hero of tree life and 2º growth is the phloem, which is a tale of sacrifice. The same layer (the cambium) that produces rings of wood also produces rings of phloem along the exterior surface, a process which becomes quickly implausible. Something has to give, and it does.
Baked into this implausibility is functional perfection, because there are many issues that need to be resolved in the life of a plant. Sure, water and nutrients must rise from the soil to the ultimate tips of all branches and leaves. But the leaves are busy photosynthesizing sugars that are required for growth throughout. Sugars have to be translocated throughout a plant, and that is the function of phloem. At their destinations, sugars are dismantled to yield ready energy, and they provide building blocks. All of that cellulose invested in the woody core is nothing more than macromolecules constructed of thousands of glucose molecules (β-1,4-linked). If you disrupt the connectivity of phloem that connects leaves to roots, the roots are starved of sugars and cannot continue to grow or live. Pioneers long ago understood the significance of this when girdling trunks to kill large trees while clearing land.
Phloem differs in absolute ways from xylem. It is mostly living tissue, while mature xylem is (in large part) fiber and vessel cells, which are dead when fully developed. Phloem requires energy to translocate dissolved sugars (photosynthate), while xylem is more like pure conduit. Phloem contains cells that with another season will differentiate into special tissues that create bark; xylem is fixed. So 2º phloem, which is an active and complex tissue in its first season or two changes course and generates the tree’s bark. That is the reason I think of 2º phloem as sacrificial tissue. It is the well-spring of bark which is constantly sloughed off and must be renewed yearly. This becomes obvious when you observe and compare the bark of different kinds of trees. It is always layered, layers that are comparable to the annual rings permanently documented in the wood. In every large tree, over time there have been as many layers of bark as there have been annual rings of wood, but the layers do not persist to the same degree. Bark sheds in differing patterns, and ranges from continuous (think Sycamore and Eucalyptus) to totally ridged or flakey (think Oak and Elm) to structurally sinister (think of prickles on the trunk of Chorisia).
From outside to inside, then, a woody branch or trunk is made of layers: Periderm (Outer Bark), Phloem, Cambium, Xylem, 1º core (just a few cells thick, perhaps not even persistent.) I left a short-lived layer out of this sequence, but it is important. Cells in older (outer) layers of phloem become active and produce actively dividing cells that form the layers we call cork. These active cells differ in pattern from one kind of tree to another, and are called Cork Cambium (or Phellogen). Unlike the Cambium itself, they do not persist. Rather, these active zones are seasonally regenerated from within older layers of phloem, produce a layer of cork (bark) cells, then die – as do the cork cells they produced. The outermost bark, as dead tissue, protects the tree and serves as the mechanism to shed phloem as the woody core expands.
The fallout of this great activity is trunks and branches that create the natural architecture that characterizes the canopies of natural communities. Most pointedly, this knowledge reveals the significant truth that when we look at a wonderful tree, a stately Oak or towering Redwood, the great mass of tissue that makes those trunks and branches is made of dead cells. The only living tissues in trunks and large branches are the thinnest layers of phloem and the cambium – a precious and fragile cylinder below the outer bark, just a few cells deep.
The most evident life of a tree is present in the countless young stems and leaves that constitute the green canopy. Our discussion headed straight to 2º growth because that is such an important and characteristic aspect of what it is to be a tree. But it is the 1º growth that happens first, creating the basic architecture and setting patterns (the same point growth that characterizes all plants.) Let’s return thought to the germinating oak seedling. The first year a growing tip formed a stem, which consists of repeating units – an internode topped by a node that hosts a leaf and a lateral bud.
The leaves are important as sites of photosynthesis and transpiration, but they are temporary organs. Next year there will be a new set of leaves and last year’s leaves are toast. But the lateral buds are subsidiary growth tips, which means they can take off and do things, such as make a branch, create a female flower that can form an acorn, or generate a catkin adorned with pollen-producing male flowers.
In oaks, the future of certain lateral buds is established when they are produced. Take the branch of a healthy, actively growing oak and give it your undivided attention. All buds are not created equal, and what you see will depend on the season of your inquiry (check out Oak Growth Patterns on WWW).
When the year’s growth begins, several things happen. The terminal bud and one or more of the large lateral buds that formed in the terminal cluster begin active growth. Oaks flower early, before the leaf canopy is fully formed, so pre-ordained lateral buds on the new side branches quickly expand and generate catkins or female flowers. By the end of the growing season, a new strong extension of the main branch has been produced by the terminal bud, while a cluster (almost a whorl) of side branches marks the site of last year’s terminal bud cluster. Curiously, most of the lateral buds down stem (the buds that were tucked in axils of normal leaves) remain dormant – unless the main stem is cut. If the growing tip is eliminated, then one or more of the side branches will take off. Botanists say the bud is “released” because dormancy of lateral buds is enforced by hormones produced in the apex – a control we call apical dominance.
Apical dominance determines growth behavior. Storm and insect damage, or physical alteration such as pruning, interrupt norm normal patterns of growth and force the plant to respond through growth of lateral buds. Arborists must understand this process if we wish to ensure trees develop strong, healthy, and expected branch scaffolding
Point growth (1º growth) due to the apical meristem in the terminal buds is linear, conquering new three dimensional space. After laying the pattern with a year’s primary growth the stem that has been formed shifts to a pattern of wood development, just as last year’s stems, and those of preceding years back to the seedling stage.
On a casual basis, people sometimes suspect that tree trunks continue to grow. Of course they do, if we are talking of thickening (or lateral) 2º growth. But 2º growth alters nothing about the vertical position of stems and branches. Primary growth stems, once mature, do not lengthen. The point of attachment of a branch formed five feet from the soil surface will never rise any higher. A child who pounds a nail into a tree trunk will find that nail in the same position seventy years later, should both the tree and child survive so long.
There is plenty of change however – inside the young stem. Primary growth transitions to secondary growth through an intricate and marvelous suite of “differentiations” of cells that are in perfectly coordinated locations. Everything relates to the veins (the plant’s vascular system), stacks of cells that assumed critical functions as the stem formed. The organization inside each vein and the functions are the same as veins in leaves, which are so readily visible and with which the veins in stems are contiguous.
So what is a plant vein? As the term suggests, it is something that conducts vital fluids. Though we use the word “vein”, do not confuse plant and human veins and vasculature. In plants, individual cells are the conduits – materials move through cells, from one to another. In humans, veins are tubes (like tiny hoses) with walls formed by millions of impenetrable cells (we call them endothelial cells, which means they are similar to our epidermis).
Plant veins consist of two parallel kinds of tissue – living phloem cells that transport dissolved sugars and dead, hollow xylem elements through which water moves. There are no pumps, nothing like a heart. Plant veins have no layers of smooth muscle (as in our arteries) that keep volumes of fluid moving. Plants do not make muscle. And there is not a nervous system to coordinate the activity of muscles either.
In lab classes we examine prepared slides of stems that show stems in both long and cross section. Prepared slides have been stained with red Safranin (that is specifically stains the hardened and thickened cell walls of fibers and vessels) and Fast Green (which stains normal cells light green and phloem cells a slightly more intense blue green.) You can also check out textbooks and WWW resources to see photographs of such teaching slides. The stains make prepared specimens more useful, but re-member the colors are “artifacts” we made – they are not natural.
Despite the fact that the only similarity between plant veins and human veins is movement of fluids, it has become common for modern biology books to treat plant and animal vasculature together as a single topic. I fear the current trend creates deep-seated confusion for students. This may be confusion that can never be resolved because students quickly move on to specialty studies and seldom learn more about both plants and animals, so there is never the opportunity to resolve the unfortunate comparisons. Biology can be confusing when taught this way.
So get your head around the fact that a plant vein has no similarity or relationship to veins in animals. Think of a plant vein as a bundle of multi-strands of organized and connected cells that move water in one strand of cells and solutes
(dissolved sugars) in another. In the young Oak stem, the veins are imbedded in lother less-specialized iving cells that make up the bulk of the stem, beautifully laid out in a ring (as you see them in a stem cross section). The veins and surrounding cells are basically colorless, so they are distinguished mainly by size and textural differences.
Before we move headlong into a description of the plant vascular system, it’s useful to have some mental model for the internal structure of a young stem that was just outlined. Imagine a concrete block silo, about 40 feet tall. Mine has an aluminum-silver dome and dark tan concrete blocks. Inside the silo (I think of it as a hollow cylinder, at least 40 feet tall) picture a ring, near the outer wall, in which someone assembled about twenty stacks of small pipes, all fixed together such that each stack almost reaches the top of the silo. Each pipe stack has two sorts of pipe segments that marry to their own kind, creating two major strands
Together the segmented strands make parallel conduits – one strand of conduits that that can move water up as far as the top of the silo, and another strand that circulates material down to the foundation. Along the outer side of each stack there is a third strand, built of rebar (steel reinforcing rods) that protect and give strength. Each combined pipe stack is equivalent to a plant vein, and the stacks are spaced inside the silo so as to form the ring – just inside the outer wall. The remainder is filled with clear beach balls.
Every individual element of the silo represents some kind of cell. The concrete blocks are epidermal cells. The stacks, which we called veins, are truly called vascular bundles. Within the vascular bundles, pipe segments represent special cells of the phloem and xylem, and the sections of rebar constitute a bundle of fibers that runs along (and sometimes around) each bundle. Finally, the main volume of the stem (our silo) is filled with simple parenchyma cells (our clear beach balls.)
Toward the top, inside the silo dome is a growing point. It makes the cells we just described. And miraculously, it can keep going. This silo self-assembles because its growing point (the apical meristem) creates new generic cells – cells that will either differentiate to become something specialized, or will hang out as beach balls. As the growing tip makes new cells, the stem adds length, at the tip. The vascular bundles must keep up with this new stem tissue, so just down from the tip, new cells that are near the ends of existing bundles will automatically differentiate, becoming xylem, phloem, or fibers based on their position. If this were our Oak seedling, then next year there would be a new silo stacked on top. In fifty years, there would be fifty silos stacked one atop the other, plus silos that branches out to the side from the top of last year’s growth.
You can see this quickly becomes untenable. You cannot stack all of these silos, one on top of another, without some added strength and conduits for circulating water, nutrients, and sugars. The solution lies in the very organization of the three tissues that make a vein; this arrangement predicts the future origin of secondary growth.
But remember, in each vascular bundle there are only a few living cells. Running up along the outside is the tough strand made of fiber cells, which are hardened and dead when mature. This strand strengthens the stem and protects the vascular bundle (vein). Running up the inside of each bundle (closest to the center of the silo) is the strand of cells (vessel elements and tracheids) that make up the xylem. Vessels are also dead when mature and functional, because they become hollow tubes.
Sandwiched in each bundle, between the outer fiber bundle and the inner xylem, is the strand of cells that make up phloem. Thus, the ring of bundles in a stem (and in our silo) forms an internal cylinder with living phloem that is the source of future secondary growth.
But for secondary growth to begin, a new growth zone must originate and take control of structure inside the stem. The cylinder of phloem tissue is part of a pre-ordained continuous cylinder of living cells that will materialize among the living cells (the beach balls) that fill the stem, uniting the vascular bundles into a complete cylinder. This now-apparent, cell-thick layer is the active zone of cell division in the stem. We call it the vascular cambium, and it is the source of all cells that will build the annual rings and new phloem – which means it is the ultimate source of the bark.
In summary, the vascular bundles that formed as part of primary growth of stems coalesce to generate a vascular cambium – which builds the wood that makes the trunk, season after season, ring by ring. This makes infinite sense when we then understand that the ring of veins in a new stem will coalesce to initiate secondary growth. The vascular cambium is that glove-like layer described earlier, so crucial to the generation of both xylem and phloem.
It is normal to think of a tree in cross-section. We can easily see the rings and imagine the many growth seasons that have passed. But arborists and wood anatomists think in three dimensions, and imagine wood in three planes: cross-section, radial-section, and tangential-section. You will need to get your mind around these planes, because it is only possible to understand the way specialists describe wood when imagined in these sections.
We mentioned cross-section already. This is what you see when you fell a tree. But I guarantee you that the fine detail is not discernible until someone has planed or finished the surface. When properly cut or prepared, you can see the growth layers beautifully, you can even see into the lumens of larger cells in the wood (the xylem). Under a simple dissection microscope, or using a handlens, you see the microscopic tubular construction of vessel elements. This is most particularly true for the oak, which can produce huge vessel elements, particularly in the first growth of the year.
Oak is, therefore, great wood to examine if you wonder why people get so excited about spring vs. summer growth. Examining oak cross-sections, it is pretty easy to pick out the difference You might argue that any idiot can tell the difference, because the spring growth is bound to be the inner layer of each ring. But in a dendrology or botany class, you will probably have a chance to examine prepared slides of mature wood. The thin sections that are stained and mounted are about 1/2” by 1/2” – which means that if the section was taken from outer rings of a large tree, you might not be able to pick up enough of the arc in each ring to figure out which side was closest to the center of the tree. But in many trees you can determine that by understanding that cell sizes will become smaller as the growth season progresses. In the oak, particularly, you will see that the large vessel elements which formed later in the growth cycle are smaller than those formed in the spring.
In this prepared oak section, you would be looking at just a few kinds of cells – vessel elements, tracheids, fibers, and rays. Of those cell types, only the ray cells are alive in a trunk.
To follow rays and get a bottom-to-top look at the water-conducting vessels and tracheids, you want to move to a normal long-section. This will be called a radial section, because it is a slice of wood made by cutting along a line that approximates the radius for the stem. It is like slicing a tree in half, from top to bottom, and looking straight into the wood along that cut surface. You will see annual rings with the same varying thicknesses that were obvious in the cross-section. But you now see the full length and connectivity of the vessel elements and tracheids.
At this point you can confirm, for personal edification, the absolute, complete, total, and undeniable difference between the vascular system of a tree and that of a human. You are looking at thousands, even tens of thousands of cells. Water (containing dissolve nutrients) moves up the tree trunk through these tiny cells, through a nearly inexplicable combination of capillarity (cohesion of water to itself and adhesion of water to surfaces), root pressure, and the pull of transpiration from leaves. That is only possible due to the microscopically small size of cells, and is not how blood passes through human arteries and veins. It is not even close.
It is in this radial section that you also see the continuity of rays. These are bands of living cells running horizontally from the tree core to the circumference. They come into being when secondary growth starts, and persist as the tree gains girth. Rays provide the only living connection into the wood of the tree, and the only source of regenerative cells inside the trunk. Everything else is dead.
Perhaps some of the most important points to make relate to the long-term functionality of wood (xylem). Alex Shigo famously conducted autopsies on tree that had experienced problems, or even branch failures. He explained to the professional community that trees have methods of compartmentalizing areas that have been impacted. Cells can be plugged with chemicals such that disease is less likely to spread throughout the wood. That is easier to understand when we consider the difference between heartwood and sapwood. As trees expand in girth, older wood at the core of the tree is plugged with insoluble compounds and no longer serves for water movement. This heartwood is harder, more darkly colored, and more resilient. It is valued as lumber, but we humans are heedless, and much beautiful heartwood has been wasted over the centuries.
The wetter, softer and less intensely-colored wood in outer regions of the trunk remains functionally useful for water movement. This is the sapwood. If you wish to examine wood that is still active, then this is the zone from which the third section must be cut. This third, tangential section, is cut along a tangent – that is, this is the slice that is perpendicular to a radius. Looking at a tangential section is the view you’d have if you could look right into the wood of a standing tree. It’s the Superman view. The tangential section is the only view from which you can get a sense of how the rays are constructed. And there are a lot of rays, more than you’d suspect from study of the cross-section, and more than were present when the branch was younger. (There must be a system for branching, or multiplication of rays over time, though I do not know of diagrams or studies that help with this understanding.)
This is the diagnostic anatomical view for oak wood, in that oaks are characterized by their massive “multi-seriate” rays. Any botanist or research arborist would immediately recognize oak with a glance at its tangential section. The pattern is unmistakable.
Other kinds of trees have characteristic wood anatomies also, such that specialists can often identify a tree to genus, and sometimes to species, through examining a wood sample. Archaeologists rely on this specificity because wood is sufficiently durable as to be a common artifact in their digs.
What’s more, researchers have dedicated great effort to comparing varying widths of tree annual rings. Patterns of exuberant ring growth reflect periods of rapid growth, while patterns of poor growth reflect epochs of drought or poor weather. These patterns can be especially useful when studying large specimens that span hundreds of years. Correlating patterns from specimen to specimen, we can make assumptions about climates over time. Studying fossil and living trees in the Southwest, scientists have been able to document these assumptions over the last several thousand years. This is one way we have to estimate historical rainfall patterns.
The totality of wood anatomy is complex and significant. The wood anatomy of each different kind of tree determines special characteristics that make it valuable for certain uses. Quarter-sawn oak is a fancy cut that highlights the tangential view with its long runs of multi-seriate rays, giving the wood flecked or shimmering patterns that some designers adore and others despise. Birch is valued for its hardness and resistance to splintering – thus it is a preferred material for manufacture of toothpicks and hockey sticks.
The wood of conifers does not generally produce vessel elements, just tracheids – which are more regular in size. Spruce has such homogeneous anatomy that it is a preferred wood for sounding boards in string instruments. California’s Incense Cedar has very regular wood, which led to its fame as wood for pencil manufacture.
If you want to mold natural foliage patterns in concrete, look for larger leaves that lay flat, have strong abax-ial veins, and are covered with hairs. They will leave the better imprint in a concrete surface.
The Canopy: Earlier in the discussion, I said that the life of a tree is up in the canopy. This is where primary growth reigns, for the shoot and root tips directly make everything that is the work of a tree. With plants that have a strong dormant period, one could almost think of each growing season as a repeat performance. Primary growth in shoot tips manufactures a new canopy – stem, leaves and buds, and the shoots generate special buds that will produce the year’s crop of flowers and fruit.
The growing tip of each branch both produces cells for seasonal growth and organizes the future of those cells such that leaves and buds develop in exactly the right places to the proper shape and size. Inside the stem, as has been discussed, the appropriate vascular system develops.
Leaves merit some attention. Botanists have come to realize that growing tips only make new stem and temporary appendages, such as leaves, bracts, petals, and stamens. All of those temporary appendages are initiated in the same manner, so we think of them, generally, as leaves. Thus a
“leaf” can be and do many things for a plant, but it is the green photosynthetic leaves make planet Earth habitable.
Green leaves seem so thin, how is it that they can have enough internal structure and process to have occupied thousands of botanists for entire lifetimes of study – and still harbor biochemical mysteries and unexplored variation. They appear so simple, but leaves are constructed of cells (millions of cells in some cases) that are differentiated into layers and networks. And leaves have sides. There is an “upper” side, which botanists call the adaxial surface, and a “lower” side that is called the abaxial surface. We have to be cautious when using the terms upper and lower because sometimes it is not clear what is up and what is down. There is a bit more morphological precision in the words adaxial and abaxial. The adaxial side is the surface that forms closest, or adjacent to the growing tip of the stem, while the adaxial side is “away” from that stem and tip. You will also read about “dorsal” and “ventral” surfaces, with dorsal typically being adaxial, because that is the side that typically faces upward. But in leaves, dorsal and ventral can also be imprecise. For simplicity, we are going to use “upper” for adaxial and “lower” for abaxial surfaces. Just understand that the upper leaf surface might actually face down, and the lower surface could face upward.
The reason surfaces matter is that the adaxial leaf epidermis usually differs considerably from that of the abaxial surface. It is often waxier, and has a different texture as compared to the abaxial epidermis. It may show as greener, because the chlorophyllous cells are usually closer to the upper (adaxial) surface – but do not be tricked. There are many instances in which things are different.
The lower (abaxial) surface is usually replete with stomata, special cell-framed openings through which plants exchange gases with the atmosphere. Of course, for arborists and horticulturists, the most relevant aspect of this structure is the evaporation of water from the lower leaf surface. This is termed transpiration, and we say that plants transpire, but most gardeners ruefully refer to this as water loss, as though it were a complete tragedy. The more complex reality is that without transpiration, water would fail to move through the plant. And lacking evaporative cooling, our world would be less livable. There is a great difference between the shade of a shelter and the cooling shade of foliage from which water is evaporating, perhaps as much as 20 ºF.
In many plants (especially those from xeric regions) the lower epidermis is densely covered with trichomes. Those trichomes (you can also call them hairs) protect leaf surfaces from damage, and can significantly reduce surface evaporation by limiting air movement. And it is usually this lower surface where the texture of veins is more expressed, so examination provides many clues and characters.
Between the lower and upper epidermis, we typically find two additional organized cell layers, plus the entire network of veins that include xylem which keeps the leaf supplied with water and nutrients, and phloem, the tissue through which sugars (photosynthates) are exported. One layer, usually closer to the upper surface is filled with cells thats contain chloroplast – where light energy is captured. The other layer, which is typically loosely organized, creates air chambers that funnel gases to and from the stomata, which are adjacent. Using backlight to examine a leaf with a handlens or dissection microscope, you can confirm that vascular bundles continue to branch such that every small patch of cells is close to one of these conduits.
The reticulate (or parallel) branching of veins reminds me of plot maps that show every street and taxable property in a community. The network of streets and services making each home and business functional is physically analogous to the geography of a leaf. Organizationally, it is as though every leaf on a tree is a 1:500,000 scale model of Los Angeles County. All of this happens in the thickness of a leaf blade.
These are the green leaves – and chlorophyll is the reason. This is not the place to wander deeply into physiology, but the green of chlorophyll changes our world in many ways. First, it is chlorophyll that absorbs red and blue light, capturing energy the plant uses to manufacture sugar. As part of this energy capture, water is split, releasing oxygen. Because the leaves and canopy are absorbing red and blue light predominantly, the leaves are green and the color of light in the shade of trees is different from pure sunlight. This color difference impacts life and growth characteristics of the understory.
But as was mentioned earlier, plants make other kinds of leaves. Bracts and scales are common. We may not give it much consideration, but all dormant buds are covered and protected by tiny brown scales (which are miniature leaves.) Pine branches are completely encased in scales – like some kind of skin. Many Junipers make needle-like leaves when juvenile and green scale leaves at maturity.
When something foliar is too large to be called a scale, and too unusual to be considered a leaf, we end up calling it a bract. Those also turn up in many roles. The showy, white bracts of Dogwood deceive us into considering them as flower petals. Palm inflorescences usually begin life encased in large, sometimes woody-textured bracts that linger in the crown, contributing to the debris piles generated during windstorms.
Even more elaborately detailed are the appendages that make up cone scales in conifers and flower parts (sepals, petals, stamens, pistils) in what arborists call the “broadleaf” trees.
Among the many broadleaf trees are the oaks, though most people do not even think of them as flowering plants. Oak flowers are objects of beauty to naturalists, but mostly annoying to everyone else. These are among many kinds of trees that are monoecious, which tells us each oak tree bears two distinct flower types – female flowers (which mature as a fruit called an acorn) and male flowers that make only pollen.
Since oaks are wind pollinated trees, they typically bloom in late winter to early spring, and are well-known for their profligate pollen production.
Moreover, wind pollinated flowers share a suite of characteristics. They stereotypically do not have showy flower parts. The copious pollen is formed in stamens from which it can be easily shed with light air movement. This syndrome plays out perfectly for oaks, which produce their pollen in staminate flowers trailing down in inflorescences we call tassels, or catkins. We are all fairly aware of pollen production because it seems nobody is immune to the allergic effects of tree pollen.
But the female flowers of wind-pollinated plants gain little attention (until they mature as fruit). In oaks, the female flowers consist of demure naked pistils, appearing as simple nubs on spring growth – trolling the air with oversized stigmas. In oaks, the female flowers only become obvious when they mature as the acorn crop, a harvest so important to forest animals that we even have a unique term for the annual yield – oak mast.
Any botanical skill will prove valuable to the arborist. Tree identification, for example, is of great importance. In this Reader, that skill is covered by the discussion on Botany for the Naturalist. And time could be devoted to botanical information that helps explain disease activity and response. But we wrap up this discussion with thoughts about planting, pruning, and training trees. Tree biology and ecology suggests there are overarching concepts:
Deal with Soil Compaction: Trees and all plants require oxygen. We are so concerned with gas exchange that involves water loss (transpiration) and the use of carbon dioxide for constructing sugars that it is too easy to ignore a plant’s need for respiration, which requires oxygen. Plants are net producers of oxygen, but it is not produced in roots, where this can become a problem. Ideal soils have great porosity, for both drainage and air exchange, meaning that soil compaction is a major issue for trees, especially in the urban setting. You will find this a nearly intractable issue, in that contractors and property managers seldom wish to invest time, planning, and resources needed to mitigate compaction for urban treescapes.
Compaction creates other related problems. It is not uncommon for major soil preparation to precede planting of landscape and street trees. When planting occurs late in the project, industrial-strength equipment (like back hoes) will be used to dig a planting hole. Container-grown trees are dropped in and set up with watering dams and even moisture sensors.
Underground, evil reigns. Poor drainage in the surrounding soil and the differential in texture between the native soil and the container mix can create perverse circumstances. Water could be corralled in the root ball, which remains waterlogged – or water might wick into the surrounding soil, leaving the roots dry, regardless of reasonable irrigation. Moreover, if the containerized roots are already encircling and care was not given to shaving roots during planting, you might find that roots will not establish successfully in the native soil, regardless as to other issues. Inspection of trees is needed to ensure potential for establishment, and aggressive oversight during planting is needed to mitigate problems.
Understand the impact of Pruning and Training: Given what we know about tree growth and architecture, there are real lessons to keep in mind. Once a tree has been pruned, the natural growth sequence is interrupted and that tree may always require intervention. Minimally, inspection and preventive pruning should be on-going, even if the goal is to return the tree to natural growth patterns as completely as possible. If you plan from the start to shape or train the tree, intervene early, when wounds are small and readily healed by new growth.
From observing the ways trees respond to storm damage and harsh pruning techniques (such as lopping off branches without regard to branching patterns), we know that regrowth can involve clustered shoots with problematic attachments. Years of follow-up attention are required to restore natural taper and promote strong and safe structure.
Appreciate that Selection is a best fit compromise between tree Nature/Nurture and Site Issues: I hear the phrase “right tree, right spot” often. And it is likely that the most common questions we receive from home gardeners relate to which tree they should consider for their own properties. When responding I find people really want everything – a tree that grows quickly, but then stops at the right size for their situation, one that never causes root-related problems, a sort of tree that has the desired leaves and flowers, pretty much all of the time, and a tree that does not drop messy leaves or fruit.
Not so easy. The selected tree can only do what is within its genetic capacity – structure and phenology (seasonal patterns of growth, such as leafing out, flowering, fruiting, etc.) will be predictable for your area. S0 my common suggestion is to walk or drive around the local neighborhoods to determine if there is a tree that seems right for the circumstances. Take a photo, or even stop and ask the residents for their feedback as to what seem to be good trees. Get an solid identification. Gardens and nurseries can usually provide that level of assistance.
Plant trees as small as you have patience. A smaller tree is easier to plant, less expensive, and has the best chance for successful establishment. The easiest tree to plant is a seed; the next easiest is bare root nursery stock.
A few years ago, I was told by Zsolt Debreczy, lead author of Conifers Around the World, that the proper way to begin a tree collection is to collect seed of the target species, prepare the ground, and plant several seed in a small area. Once they have germinated, you cull out all but the strongest tree. Zsolt’s system has great merit, in that the strongest root system is one that develops in place.
Though the Gardens take a long-term approach, there are times when we plant trees as temporary occupants of a space that we know will be altered in the twenty year timeframe. Trees can be perfect for this because they simplify the planting scheme, require relatively modest attention, and occupy the site handsomely. But we are particularly careful with selection in these situations. We will either select trees that we plan to relocate and use ($$$) as part of the future project, or trees that are shrubby, multi-trunked, and grow quickly – which means they will not cause as much heartache when we come to the time that we must remove them. In some projects we have learned that tree moving companies will trade work for the Gardens in exchange for specimens they can sell to other clients.
Understand the Site and Impact: Step back from your immediate objectives for a few minutes and imagine the trees and site two to three decades into the future (and even longer). What benefits and problems are you planting? Does the landscape design include sufficient space for the mature specimens – especially for root establishment that will not become problematic? Are projected tree canopies improving livability (shade for activities, sun blocking to alleviate intense southern and western exposures, ambience), aesthetics
(framing and texture, color, seasonality), and sustainability (self-mulching, water conserving, community building)?
And there are totally functional issues. What are the slope and soil depth? What kind of infrastructure (pipes, electrical lines, natural gas, sewer, structures and foundations) passes through the potential rootball? How close are walks and roads? Will branches overhanging roadways cause problems for large trucks? Are there power lines? What hazards should be analyzed and mitigated? Is the planting long-term or short-term? Responding to those issues could save someone great agony down the road; that someone could be you. Because Huntington Botanical Gardens will always be on our same footprint, any such concerns remain live issues for us.
Communities should take the same approach. It is an unusual thing for a town to relocate. And it is common for the beauty and quality of life of a community to benefit from a handsome canopy of trees, along streets, in parks and public spaces, and on private properties. Citizens of every community should have shared goals concerning the greening of public and private lands. Understanding the biology of trees is crucial to that goal.