Chapter 2: Trees and Shrubs Trees and Shrubs: Overview Trees and shrubs are among our landscapes' most valuable assets. (photo II-Oa: IMG_0002B ) They often represent a large financial investment on our part, and they definitely represent a major investment on nature's part, in terms of time, energy and nutrients expended to create them. Beyond the fact that they delight us aesthetically, trees and shrubs play many other roles in our lives, as they : Cool buildings, helping us save energy Form the structure and framework of our outdoor spaces Screen undesirable views, create privacy, and frame favored views Increase property values Heal the mind and spirit Mark the seasons Provide a sense of place, belonging and familiarity Help slow or reverse climate change by removing CO2 from the atmosphere and storing it in their trunks, branches, roots, and the soil where they grow [Start quote] "A tree's beauty is functional, like the beauty of a great machine that performs swiftly and quietly. Or like the grandeur and dignity of a ship where every line is in harmony with the law of its being, and every accessory fits perfectly into the situation which demands exactly that thing. We can justly admire man-made structures like a machine shop or milk pasteurizing plant or a rolling mill. These are, after all, only adaptations of fundamental laws. But we can be astonished at a tree which is an original expression of those laws, mysteriously compounded out of the elements." -- Rutherford Platt, This Green World [End quote] Forests constitute one of the two primary types of natural vegetation (after grasslands) in the natural world. In addition to - and perhaps more important than - what they give to us humans, trees and shrubs also provide these vital services to the natural world: Provide diverse and essential habitat; trees offer nesting places, cover, and abundant insect food; shrubs offer nesting places, pollen, nectar, fruit and cover. Produce oxygen, as a byproduct of photosynthesis. Cool the air and the ground. Absorb polluting gases and trap microscopic polluting particles in their foliage . Pull moisture from the soil and release it into the atmosphere. Break the impact of rainfall, which can help reduce soil erosion Hold soil on hillsides and slopes, and help absorb rainwater like a sponge. [Start box] A closer look at trees... How many of these facts surprise you? A tree can absorb close to 50 pounds of carbon dioxide per year. By the time it's 40 years old, a tree can sequester 1 ton of carbon dioxide. One large tree can lift up to 100 gallons of water out of the ground and discharge it into the air in a day. One large tree can provide a day's supply of oxygen for up to four people. Almost 98% (by weight) of a tree is made up of six elements: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Trees do not grow beyond their ability to support themselves. During periods of stress they shed leaves, flowers, fruit and/or branches. Different parts of the tree grow at different times of the year. A typical pattern is for most of the foliage growth to occur in the spring, followed by trunk growth in the summer and root growth in the fall and winter. [i] [End Box] What about shrubs? In this section of the book, much of the information might seem to relate more directly to trees than to shrubs. This is not to slight shrubs, but rather it reflects society's greater scientific knowledge about trees, the larger and more obvious impact of trees in our landscapes, and their greater capacity for carbon sequestration and effect on climate issues. However, it's important to note that although shrubs might seem to hold less carbon in their biomass than do trees, and for a shorter time, shrubs actually do contribute greatly to the amount of carbon held in soil ecosystems (for more information, see the Primer on Soil Carbon and Ecosystems on page ??). In addition, shrubs play a significant role in the work of nature, providing a huge variety of essential pollen, nectar, seeds, fruit, nest sites and protection from predators. (photo II-Ob: IMG_2485 ) In this Section For all of these reasons, trees and shrubs are vitally important in both our individual daily lives and the larger realm of civilization and humanity. This section of the book presents ideas for helping us get the most from - and give the most to - the woody plants in our care. Action Topics include: II-1: Take good care of trees II-2: Choose plants to tolerate new/challenging conditions II-3: Maximize carbon storage II-4: Get the most cooling benefit from trees [Start quote] "It is increasingly clear that much of our wildlife will not be able to survive unless food, shelter, and nest sites can be found in suburban habitats. And because it is we who decide which plants will grow in our gardens, the responsibility for our nation's biodiversity lies largely with us. Which animals will make it and which will not? We help make this decision every time we plant or remove something from our yards." Doug Tallamy, Bringing Nature Home, [End quote] * * * Action Topic II-1: Take good care of trees Why this matters Trees provide vital carbon storage and wildlife habitat for the natural world, along with all the delight and pleasure they give to human beings, so it just makes sense for us to do everything possible to protect and preserve them. And when it comes to taking care of trees, there are two things we need to pay attention to: the aboveground parts we can see, and the belowground parts we generally can't see and don't notice. We often think of trees as mighty and strong. And it's true: trees can grow amazingly large. They can hold their great weight upright against relentless gravity, and withstand the forces of wind, deluge, blizzard, heat, and cold. But still, tree trunks and branches are not indestructible. (photo II-1a: IMG_0322) We also tend to imagine that tree roots are similarly strong, with their ability to support a massive structure overhead, and their apparent power to crack sidewalks and penetrate foundations. But although the large roots of trees are pretty tough, a tree's "feeder" roots, which actually keep the tree alive, are quite sensitive. Taking care of a tree's roots is a little harder than protecting the tree itself, because: The roots are invisible to us, so we tend to forget about them. We can't see or know exactly how our actions are affecting them. The care they need might be ecological and chemical, and this is tricky to understand and get right. When roots are suffering, sometimes the only evidence might appear in the uppermost branch tips, which we hardly notice until the foliage of whole branches starts to look discolored, droopy, tattered or stressed. And by then the damage is usually irreversible. This Action Topic covers both the above- and below-ground aspects of tree care. Keep in mind that when the root zone of trees is protected and left undisturbed, this also minimizes the release of soil carbon back into the air. [Start box] Interesting facts about tree roots The spread of a tree's branches (when grown in the open) generally ranges from 65-100% of the tree's height. In contrast, the root system often forms a root zone with a diameter of one, two or three times the height of the tree, and sometimes much more. In a healthy forest, some tree roots will reach hundreds of feet beyond the trunk; and this is true to a lesser extent even in lawn. A tree's roots rarely are evenly distributed; they might be thousands of medium-sized roots and root hairs connected to the tree via a single large transport root. Tree roots don't grow very deep. Most tree roots are in the top 12 inches of soil, although in rich, deep loam roots may extend deeper. The majority of roots grow in a wide, flat "pancake" of soil. Roots store more starch (carbohydrates and sugars) than the trunk. About 60% of a tree's biomass is in the trunk. The tender feeder roots of a tree make up about 5% of the plant's biomass, and larger transport roots comprise another 15%. This corresponds almost exactly to the aboveground proportion of 5% foliage and 15% branches and twigs. I n other words, only 10% of a tree is used to produce/gather the nutrients needed for growth. And half of this vital function is performed below ground! This is why it's so important to protect tree roots. [ii] [End box] Actions The actions detailed in this Section fall into three categories: before construction, during construction and after construction. Before Construction Consult with a certified arborist whenever a project involves cutting into any tree's root zone. Never tear or rip roots with a backhoe, but instead to cut all roots cleanly. And avoid cutting any roots over 4 inches (10 cm) in diameter, as these roots are likely to be essential support roots. Don't cut any support roots on the opposite side from a nearby building, which could cause the tree to fall toward that building. Maintain and/or restore the essential soil fungi on which trees' roots depend for optimal uptake of nutrients. Fungi grow naturally in most healthy, moist and/or mulched soil. Myccorhizal amendments can also be added to the soil. Avoid cutting roots that would make trees vulnerable to falling toward a building during strong storms. This is especially important in coastal regions, where hurricanes may become stronger and more frequent. Add a temporary construction fence around the outer root zone area of a single or stand of trees. The most critical area to protect is a zone that extends from the trunk outward a distance that is either one foot (0.3 meters) of distance for every one inch (2.5 cm) of trunk diameter OR to the drip-line of the canopy, whichever is larger. Relocate trees that will be fatally damaged by construction, if they small enough to be transplanted. (photo II-1-b: IMG_0045) Don't save just isolated, spindly trees, which will be most vulnerable to wind damage, disease, or infestation. Perform basic tree maintenance for all trees near the building, such as mulching (see V-3 for more about mulching), watering, and careful pruning (no topping or lollipopping). Consider assigning a financial penalty for harming any valuable tree on a job site. Identify these trees, and bring them to the contractor's attention. If feasible, save a dead or failing tree as a standing snag, for wildlife habitat. In situations where vehicles must drive over tree roots, lay over the root zone a thick layer of spongy mulch covered with either plywood or some other traction material, to spread out the weight of vehicle tires and stored material. In vulnerable or constrained sites, require that the storage area be as small as possible, to minimize soil compaction in nearby root zones. During Construction Although different tree species have differing tolerance for harm to their roots, it's best to follow these guidelines. Within the canopy area of any tree: Don't pile construction materials, soil or mulch, and don't park or drive any vehicles. For underground utilities or irrigation avoid using trenches, which necessarily sever all roots. Tunnels are preferable, at least 18 inches below the soil surface to protect most roots. Don't wash equipment, rinse concrete chutes, etc. Don't break or tear branches overhead, with vehicles and heavy equipment. [Start box] Changing the grade around trees Adding or removing soil near a tree may seriously disturb the roots and their ability to obtain nutrients, moisture and oxygen from the soil. If no alternatives exist, get the advice of a certified arborist. And at the very least follow these guidelines: Raising the existing grade (photo II-1c): IMG_1990 Adding fill on top the existing ground level can smother and kill roots. Never place any fill or organic materials directly against the trunk. Limit fill to a maximum of 2-4 inches depth, and use fill with a coarser texture than the native soil, to maximize movement of air and water to the roots. Don't spread fill with equipment that compacts existing soil. Lowering the Existing Grade Lowering the grade can be as harmful as raising it if the work involves cutting any large tree roots or any more than about 25% of the feeder roots area. Several months before construction, irrigate the tree to build up root growth. Then cleanly cut roots in the area where the grade is to be lowered. This work is best done by a professional. If possible, use retaining walls beyond the canopy spread to create the maximum area of unchanged grade around a tree. Mulch the exposed root area to prevent soil erosion and moisture loss, and to keep temperatures low, but don't let mulch touch the trunk. [End box] After construction Evaluate and restore trees to their best possible condition. This is especially important for trees that provide valuable shade that cools the house (See IV-3 for more about maximizing the cooling effects of trees), or that form an integral part of wildlife habitat or wildlife corridors in your region, or if you just love them! In every case, and especially if a tree is standing within twenty feet of a house, the ideal approach will be to get the advice of a certified arborist. Last Thoughts Both trees and shrubs play valuable roles on the planet. Both provide essential habitat, both build soil ecosystems, and together they create the rich mosaic of vegetation that constitutes the working of nature across much of the earth. In our residential and managed landscapes as well, both trees and shrubs can have great significance. Trees might seem more important because they represent a large investment and take so long to mature, their trunks and branches sequester tons of carbon, and their overhead canopy adds so much to our lives. But we should remember that shrubs are equally vital: their cover, nesting sites, flowers and fruit invite countless beautiful birds and pollinators to live close to where we can see and appreciate them. Start quote] "We should forever bear in mind that the beautiful world our species inherited took the biosphere 3.8 billion years to build. The intricacy of its species we know only in part, and the way they work together to create a sustainable balance we have only recently begun to grasp. Like it or not, and prepared or not, we are the mind and stewards of the living world. E.O. Wilson , Half-Earth, Liveright edition 2016 [End quote] * * * Chapter 10: Materials Materials: Overview We can purposely design and build our landscapes to incorporate climate-wise materials, which are characterized by: Durability and potential for long lifetime A small carbon footprint (see the Climate Change Primer for more on this) Strong potential to be recycled/reused/renewed Local availability Minimal impact on wildlife habitat and corridors High capacity for carbon sequestration Important note about this Section: Unlike the rest of the book, here we do not provide "What to do" lists. Every landscape construction situation is unique, and each one involves its own particular mix of options, costs, traditions, local materials, and regional expectations. So here we present information to help you understand and evaluate options, from the perspective of shrinking our carbon footprint and doing well for the planet. We do not recommend one material over another. (photo X-Oa: Austin crop.jpg) Also note: The guidelines in the Section focus on design choices, and do not cover specific construction techniques. General advice about choosing materials Obtaining firm data for every product or material under consideration can be difficult or impossible in some cases. Yet even in the absence of exact figures we can easily reduce our climate impact by following these suggestions: Choose recycled or "repurposed" materials. Give priority to locally made materials, to both reduce transportation distances and support local economies. Be on the lookout for usable onsite materials, either within your own project or in nearby or neighboring construction projects. All of these choices will help reduce manufacturing costs and emissions, minimize waste going to landfills, reduce strain on ecosystems (potentially including old-growth forests), and generally be good for the planet. [Start Box] Life Cycle Assessment (LCA) Life-cycle assessments determine the environmental impact of a process or product by measuring the energy consumed during: Raw material extraction and procurement Production and manufacture Transport and distribution Use, repair, and maintenance End-of-life recycling or disposal LCA information can be quite difficult to obtain. It requires examining data from diverse, scattered worldwide sources, about processes that can be complex and sometimes not well documented at the source. Inevitable uncertainties, disagreements and gaps must be resolved. And then the data must be translated into uniform terms of energy consumption (usually in Joules) and CO2 and pollution emissions (usually in kilograms). As a result of this complexity, data simply don't exist for most landscaping processes and products. One fact is true even without the support of studies and data: today in our western society, we tend to use more of the materials with large carbon footprints and environmental impact, (concrete, steel and asphalt) than those with smaller impact (wood, earthen and do-it-yourself products). It would be ideal if we could try to re-balance or even reverse that equation, either by using less of the materials with a large climate footprint, or by encouraging manufacturers to shrink the footprint of their products, and buying from those that do. [End box] In this Section The following Action Topics explore the various attributes of the five materials most commonly used in landscape construction projects: X-1: Stone X-2: Wood X-3: Metal X-4: Concrete X-5: Earthen Materials [i] Source: https://projects.ncsu.edu/project/treesofstrength/treefact.htm By Erv Evans, North Carolina State University [ii] Tree Roots: Facts and Fallacies, by Thomas O. Perry Journal of Arboriculture 8: 197-211, 1982 Excerpted from Climate-Wise Landscaping: Practical Actions for a Sustainable Future by Sue Reed, Ginny Stibolt All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.
