China-Firs

China-firs (Cunninghamia ssp.) are impressive trees, attaining heights of 150 feet in their native lands. Although they are native to Asia, they have been planted in the Unites States since the beginning of the 19th century. Fortunately, they do not exhibit invasive habits, so you can plant them in your yard without feeling guilty.

Taxonomic Topics

Despite their common name, China-firs are not true firs; the lineage is actually more closely related to the redwoods. Botanists disagree whether the genus contains one or two species. While most recognize two species, some evidence suggests that both populations form a single species. Cunninghamia lanceolata occur on mainland China, Vietnam and Laos, while Cunninghamia konishii are indigenous to Taiwan, although some scientists recognize cloud-mountain populations from Vietnam and Laos as members of konishii.

Growth and Characteristics

Despite their tendency to grow into towering specimens in the wild, most China-firs planted outside of their native range grow to between 50 and 75 feet tall. China-firs sometimes produce multiple stems, although the majority exhibit very tall, columnar growth habits. The bark of mature specimens forms long strips that peel from the tree, providing interesting aesthetics for any landscape. Fortunately, the crown of these trees is usually rather open, allowing you to see the handsome bark. Dead leaves tend to persist on the tree for several years, until the entire branch is dropped. Some consider this a drawback for the species, while others find it to be part of the plant’s considerable charm.

Commercial Applications

China-fir wood has a number of valuable properties, including a pleasant aroma and extreme resistance to decay. It is an important commercial species in China, as it is frequently used in the construction of temples, coffins and other items for which a pleasant aroma is desirable. The primary limitation for commercial propagation is the minimum winter temperatures of a particular region, which may cause the tree stress during the late winter.

Awesome Ornamentals

Because it resists most insects and diseases, adapts well to a variety of soil conditions and can tolerate moderate shade, China-firs are great ornamental specimens. The primary limiting factor for their use is the space available. They are best planted as specimen trees on large properties. Nevertheless, the trees grow best in moist, well-drained soils. Avoid exposed sites that are difficult to keep from drying out, as the plants require significant soil moisture during establishment. However, once they have become established, they are very drought resistant – an important quality for our drought-stricken area. Deer reportedly find the needles unpalatable, and the slightly prickly leaves will help to keep mischievous children at bay. Because China-firs often tolerate urban conditions well, they are also used as street trees.

Notable Cultivars

Several China-fir cultivars are available, allowing landowners to select a form well suited for the space available. The ‘Glauca’ cultivar usually stays much smaller than some other forms, topping out at about 50 feet in height. The cultivar is also noteworthy for its pale blue needles, which may offer a unique color for landscapes. The ‘Samurai’ cultivar may remain even smaller, although its growth rate is quite rapid.

False Cypresses

The false cypresses (Chamaecyparis spp.) are a group of beautiful, evergreen trees native to portions of North America and Asia. Perhaps unfairly tarnished by the “false” moniker, it is important to understand botanists do not intend the common name as a pejorative; they simply use it to distinguish between two common lineages. False cypresses are not inherently superior or inferior to “true” cypresses — they are simply different trees.

Popular Ornamentals

False cedars are among the most popular ornamental conifers planted in the world. This is particularly true in Europe, where the trees grow well in the mild climate. Mature trees bear scale-like leaves, arranged in flat splays; but juvenile trees have longer, oval-shaped, needle-like leaves. The small female cones fall from the limbs shortly after releasing their seeds. False cypresses have long lifespans, and a few documented individuals are over 1700 years old.

Variety and Selection

In addition to the naturally occurring species, horticulturists have created hundreds of false cypress cultivars, giving property owners a number of different options for tree installations. If you intend to install false cypresses in your yard, research the available varieties to determine the best one for your yard (or simply contact your favorite ISA-certified arborist for assistance).

Hinoki false cypress (Chamaecyparis obtusa) – These delicate trees grow very slowly, perhaps adding 12 inches of height per year on good sites. The slow growth rate makes the species popular among Bonsai practitioners, although they will eventually reach 50 feet in height when planted in landscapes.

Golden Hinoki false cypress (Chamaecyparis obtusa ‘Aurea’) – A cultivar of the Hinoki false cypress that displays golden foliage. Relatively small trees, Golden Hinoki false cypresses only reach heights of 35 feet.

Alum Lawson cypress (Chamaecyparislawsoniana ‘Alumii’) – A compact, blue-needled form of the Lawson false cypress. Reaching modest heights of less than 30 feet, these trees are small enough to plant near utility lines.

Stewart golden cypress (Chamaecyparis lawsoniana ‘Stewartii’) – Another small version of the Lawson false cypress, the Stewart Golden cultivar produces splays of yellow foliage. These false cypresses are rather slow growing, but once mature, they can create a wonderful windscreen.

Atlantic white cedar (Chamaecyparis thyoides) – The only form native to the eastern United States, Atlantic white cedars have a discontinuous distribution, stretching from Alabama and Florida to Maine. A fast growing species relative to some of their congeneric relatives, Atlantic white cedars may add 24 inches of annual growth when planted on good sites. The Atlantic white cedar is more tolerant of moist soils than many of the other species in the genus.

Andeylensis white cedar (Chamaecyparis thyoides ‘Andeylensis’) – A dwarf form of the normally large Atlantic white cedar. Blessed with very dense foliage, these cultivars make excellent windbreaks.

Sawara false cypress (Chamaecyparis pisifera) – These Japanese natives are a small species, usually growing to about 30 feet in height. Because they are slow growing, reach only modest heights and lack the foliage density of many other false cypresses, they are better suited for ornamental use, rather than for windscreening or shading purposes.

Plume false cypresses (Chamaecyparis pisifera ‘Plumosa’) – These small cultivars produce compact crowns, relative to the other members of the genus. Preferring highly acidic to neutral soils, these relatively small trees are suitable for use under utility lines.

Sugi Trees

Known in its native Japan as Sugi, Cryptomeria japonica is a beautiful, evergreen conifer that is the sole member of its genus. It also goes by the names Japanese or oriental cedar, although this conifer is not that closely related to the true cedars (Cedrus spp.). Sugi have been cultivated in China for centuries, and now grow in a naturalized form. As the national tree of Japan, citizens often plant Sugi trees near temples and civic buildings.

Stately Specimens

Sugi are immense trees that, when planted in their native range, may reach heights in excess of 225 feet, and produce gargantuan trunks of more than 13 feet in diameter. However, most ornamental specimens planted outside of the native range fail to reach 60 feet. Additionally, horticulturists have produced a number of cultivars that only attain modest sizes – some people plant them in small gardens or use them for Bonsai purposes. The growth rate of these impressive conifers varies widely, depending on the local conditions and the cultivar in question. Although usually a slow-growing species, these trees grow more quickly when planted on good sites.

Reminiscent of Redwoods

Sugi trees are somewhat similar to California’s native sequoias (Sequoiadendron giganteum), save for their longer leaves and smaller cones. Another important difference is the lifespan of the trees, which may reach thousands of years for sequoias, while Sugi rarely reach 200 years of age. Cryptomeria also exhibit different regeneration patterns than redwoods do, which causes differences in the forests these trees create.

Living in the City

Sugi require fertile and moist – but well drained – soils to grow, but they are quite adaptable trees that may thrive in a variety of locations. They safely tolerate many of the indignities of city life, thereby making them excellent street trees. Neither pollution, nor soil compaction or drought is likely to kill healthy, established specimens. However, these trees grow best with full sun exposure, so care must be used when choosing a planting location. Sugi do not thrive in damp weather, which may lead to leaf blight. Avoid this by planting them in locations with ample airflow. Sugi are susceptible to relatively few insects or diseases, although they may suffer from leaf blight or leaf spot in some cases.

Popular Ornamentals

Sugi are quite popular ornamentals that have been propagated in Japan for over 500 years. Currently they are also grown in North America and Europe. Because of their relatively narrow crowns, they are suitable for moderate-sized locations, although the lower branches may require removal to permit access by pedestrians and vehicles. Fortunately, the wood is relatively strong, so limb drop is a rare phenomenon on healthy trees. Once mature, Sugi possess a beautiful, reddish colored, peeling bark.

Commercial Uses

In addition to those traits that make them excellent ornamental species, Sugi trees yield high quality wood that has a number of commercial applications. The fragrant wood is very resistant to rot, so it is used in extensively in construction and furniture making. To satisfy the demand for the useful timber, Sugi trees are grown on large plantations in Japan.

Incense Cedars (Calocedrus decurrens)

Named for the spicy aroma of the wood, incense cedars are interesting trees. Close relatives of the northern white cedars (Thuja spp.); incense cedars are relatively rare in the natural world, although they were formerly common. Considered at one time to be represented a single species, botanists now recognize several different species of incense cedars. The only North American native, California incense cedars (Calocedrus decurrens) range from California to northern Oregon, while C. formosana, C. macrolepis and C. rupestris grow in Taiwan, China and Vietnam, respectively. Additionally, paleobotanists have described an extinct species – C. huashanensis – that formerly grew in China.

Size and Shape

Incense cedars are long-lived trees that often reach ages in excess of 500 years. Other forest species often outcompete these trees, and retard their growth rate. However, over the course of their long lives, incense cedars often reach moderately large sizes. Typical specimens grow to between 60 and 80 feet, although some giants of the Sierra Nevada Mountains grow twice as tall.

Because they are drought resistant, relatively pest free natives of California, they can make a wonderful addition to your property. Their form and foliage make them aesthetically pleasing, while their extremely dense foliage makes them excellent for windscreens. These trees help support wildlife as the tiny seeds that emerge from the 1-inch-long cones feed a variety of songbirds and native rodents.

Can’t Stand the Heat?

Incense cedars are well adapted for hot, parched conditions; once established, they are remarkably drought tolerant. According to U.S. Forest Service, incense cedars tolerate dry conditions better than sugar pine, Douglas firs and grand firs, although ponderosa pines are better equipped to deal with drought in sandy areas than the cedars. (Oliver) While they often become canopy trees on southern or southwestern hillsides, other species outcompete incense cedars on moist sites, causing them to remain much smaller. Although high-intensity crown fires kill incense cedars, they often survive low-intensity ground fires.

Variation and Variety

Incense cedars vary slightly based on their location. Southern California specimens reach shorter heights than their northern relatives do. Additionally, these southern individuals often have shorter branches, and they tend to exhibit a more columnar growth form. Perhaps unsurprisingly, these southern varieties are more susceptible to frost damage than their northern counterparts are.

Commercial Uses

Incense cedars make wonderful ornamental trees, even well-outside their native range. They are popular in Europe and along the Atlantic seaboard of the United States. Incense cedars require partial to full sun, well-drained soils and protection from strong winds. Because they grow quite large, they are best planted on large properties where they have ample room. Fortunately, for those who share space with deer, the hoofed browsers rarely feed on incense cedars.


Because the wood of incense cedars resists decay — even in the presence of moisture — it is often used in outdoor construction. The wood also accepts paint well, leading many to use it for building picnic tables, exterior siding and fence posts. However, one of the most common uses of incense cedar is in the manufacture of pencils.

 

Threats

Good ‘ol root rot brings more incense cedars to their knees than any other cause, although pocket dry rot (Tyromyces amarus) is also a threat to these gorgeous conifers. This fungus causes significant internal decay, which ultimately compromises the tree’s structural integrity. Healthy bark provides an effective barrier against fungal colonization, but knots and damaged areas are vulnerable. Pocket dry rot primarily affects those trees that grow on favorable sites; for example, some stands in the Sierra Nevadas exhibit 100 percent colonization of the mature specimens.

Incense cedars may also serve as the host for incense-cedar mistletoe (Phoradendron juniperinum libocedri), but the parasitic epiphyte rarely causes tree death. A number of insects feed on these trees, but few cause serious problems. The only common foliage disease – Gymnosporangium libocedri – rarely causes the trees to die.

 

References

Oliver, R. F. (n.d.). Incense-Cedar. U.S. Forest Service.

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Kauri Conifers (Agathis spp.)

Kauri trees are a group of about 21 species that form the genus Agathis. Native to the South Pacific region, various representatives of the genus occur in Australia, New Zealand, Indonesia, Philippines and New Guinea. The biodiversity of the genus reaches its peak on the island of Borneo, where up to five species may occur in the same area.

Most kauris are large trees, but some are truly massive, with nearly as much volume as the largest redwoods. The largest species within the genus are emergent trees that push through the forest canopy to reach the sunlight above.

Ancient Trees

Kauris are ancient trees, who, along with the other members of the family Araucariaceae, were quite common during the Jurassic Period. However, angiosperms (flowering plants), which began taking center stage in the Cretaceous Period, eventually outcompeted most conifer lineages. Nevertheless, a few members of the family managed to survive the angiosperm onslaught, and retain their role in their native habitats. Many of these continue to serve important roles in their native ecosystems. Currently, most members of the family – including the Kauris – are restricted to the southern hemisphere.

Survival Strategies

Most kauris are residents of lowland rainforests, although a few grow in other habitats, such as cloud forests. To survive in such competitive habitats, kauris have developed a number of important adaptations. For example, many species possess smooth, gray, peeling bark, which helps them to shed parasitic plants and fungi before they can gain a foothold. Additionally, as the trees grow, they jettison their lowest branches, which helps prevent vines from colonizing the limbs, and ultimately stressing the trees.

Kauri trees also engage in chemical warfare with their competitors. The acidic leaves and shed bark alter the soil chemistry near the base of the area. As rain falls on the debris, the nutrients in the soil percolate down to deeper layers of soil, making them unavailable to seedlings that may compete with the kauri tree.

Despite their climax-species-like lifespans, these trees produce seeds like a pioneer species. Many begin producing seeds at a relatively young 50 years of age. Upon reaching maturity, the female cones release the winged seeds, which disperse across the area, buoyed by gentle air currents.

Commercial Use

Historically, kauri conifers were important timber trees, often used in marine applications, courtesy of the wood’s natural resistance to rot. However, timber harvests are greatly restricted in the modern world, and the tree is of lesser commercial importance.

Currently, most of the harvested lumber is ultimately utilized in niche markets. For example, many of the wood’s characteristics – including an attractive grain pattern and very light weight – make it an attractive material for the construction of guitars, flutes, drums and other musical instruments.

California Cultivation

Many kauri conifers are poorly known and unavailable commercially; additionally, many are threatened species, whose future is in doubt. In fact, scientists suspect that only 1,000 or so mature Agathis flavescens exist within the remote Malaysian wilderness. Additionally, as most members of the genus quickly grow into truly massive trees, they are simply inappropriate for most lots.

Nevertheless, a few species are occasionally planted successfully in southern California. While most members of the genus are vulnerable to frost damage and require very warm climates, Queensland kauris (Agathis robusta) are suitable for much of the West Coast. Hardy down to about 20 degrees Fahrenheit, Queensland kauris usually grow to about 80 feet in height, although wild specimens may exceed 150 feet. Additionally, bull kauris (Agathis microstachya), from the Atherton Tableland region of Queensland, may thrive in Southern California.

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Tasmanian Cedars (Athrotaxis spp.)

Tasmanian cedars are attractive conifers, native to the temperate rainforests of west Tasmania. Even though these trees originate on the opposite side of the planet and inhabit very different ecosystems, they resemble a few North American natives.

The Family Tree

Although Athrotaxis species often bear common names including terms like “pine” or “cedar,” they are neither; instead, Athrotaxis are in the cypress family (Cupressaaceae). Accordingly, Tasmanian cedars are closely aligned with other members of the cypress family, such as Atlantic white cedars (Chamaecyparis thyoides), bald cypresses (Taxodium distichum) and arborvitaes (Thuja occidentalis).

Species Specifics

Taxonomists disagree on the number of species present within the genus; some evidence suggests the genus contains two species and a hybrid of the two, while other evidence is consistent with the existence of three distinct species.

Besides growth form and habitat preference, the three species are very similar in general biology. They all exhibit roughly similar life cycles and reproductive strategies. All Tasmanian cedars are monoecious trees, meaning that individual trees bear both male and female cones. About six months after pollination, the quarter-inch-long cones open and release scads of tiny seeds.

Pencil Pines (Athrotaxis cupressoides)

Pencil pines are medium-sized trees, averaging about 20 to 30 feet in height. However, occasional specimens exceed these heights, climbing up to 60 feet above the ground. Pencil pines have scaly leaves, which lie against the tree’s branches. Pencil pines often occur at the top of the timberline, at higher elevations than Eucalyptus trees grow. This helps to explain why pencil pines are the most cold tolerant of the Tasmanian cedars; but curiously, all three forms exhibit similar drought- and flood-tolerance, despite preferring different elevations.

King Billy Pines (Athrotaxis selaginoides)

King Billy pines are the largest of the three forms, occasionally reaching heights of 90 feet. The leaves are much different from those of the pencil pines; they are elongate, pointed and claw-like. Formerly an important timber species, the wood of King Billy pines is soft, lightweight and rot resistant. Unfortunately, the useful wood is unsuitable for commercial harvest, given its slow growth rate and bush-fire-induced decline.

Summit Cedars (Athrotaxis laxifolia)

Summit cedars share some similarities with pencil pines and others with King Billy pines, and they appear like a cross between the two species. A number of clues suggest that summit cedars are the naturally occurring hybrid offspring of pencil and King Billy pines. For example, the cones and leaves of summit cedars are intermediate between those of the other species. Additionally summit cedars only grow in habitats that contain both other species.

Cultivation

None of the three species is common outside its native range, although all three thrive in many botanical gardens of northwestern Europe, where the mild climate and ample rain suit the trees well. Athrotaxis prefer cool summers, mild winters and abundant rainfall, and they are rated for USDA Hardiness Zones 7b – 10b. While not commonly grown in the United States or seen in nurseries, Tasmanian cedars may thrive in cool Californian microclimates. Seeds are occasionally available from commercial sources; summit cedars are more commonly seen than the other two species.

Lifespan and Growth Rate

All three species of Athrotaxis live long lives. Several living pencil pines are more than 1,000 years old, and scientists know of at least on living specimen — a King Billy pine – that is nearly 800 years old. All species are relatively slow growing; for example, a King Billy pine planted at the Osborne House – a seaside palace on the Isle of Wight – is over 100 years old, yet has only grown to a height of 25 feet.

Araucaria araucana playa

Monkey-Puzzle Trees (Araucaria araucana)

Monkey-puzzle trees are unusual-looking and long-lived conifers, with lifespans exceeding 1,000 years in some cases. With ancestors dating back to the Jurassic Period, it is easy to imagine long-necked dinosaurs munching happily on the tree’s strange foliage and large seeds. The species draws its common name from an offhand quote uttered by an owner of a specimen, who suggested that attempting to climb the tree would “puzzle a monkey.”

General Description and Information

Native to Chile and Argentina, monkey-puzzle trees are large evergreens that occasionally reach heights of 130 feet. Hailing from mountainous areas with frequent volcanic activity, monkey-puzzle trees have evolved the ability to cope with frequent fires. Large monkey-puzzle trees are more likely to survive fires than small trees are, but most surviving specimens sprout vigorously after fire.

The tree’s sharp, triangular leaves often persist for extremely long periods of time – up to 15 years in some cases. Monkey-puzzle trees exhibit a symmetrical growth habit, and they resemble Christmas trees while young. However, they shed their lower branches as they age, eventually becoming little more than long, bare trunked trees, topped by sprawling clusters of branches and leaves. By the time these trees mature, they may reach up to 120 feet in height.

Monkey-puzzle trees produce incredibly large cones that may represent a hazard to those walking beneath the tree. Pollinated by the wind, the cones mature in the autumn, about 1 ½ years after pollination takes place. Most specimens are dioecious, meaning that male and female cones are produced on separate trees. However, this is not always the case, as a few of the trees bear cones of both sexes.

Monkey-puzzle trees were an important timber species historically, but they are no longer legal to log in their natural range. Currently, monkey puzzle trees are endangered in the wild and listed on Appendix I, by the Convention on the International Trade of Endangered Species (CITES). They play an important role in local indigenous cultures, and are recognized as the national tree of Chile.

Edible Seeds

Interestingly, the seeds of monkey-puzzle trees are edible; they are even collected commercially in Chile. Unfortunately, the trees take about 30 to 40 years to reach maturity and begin producing seeds, thus requiring the investment of significant amounts of time to yield a return. However, once the trees reach maturity they produce an incredible quantity of the seeds. Additionally, because the cones fall to the ground upon maturity, harvesting the seeds is relatively easy.

The seeds taste similar to pine nuts, yet they are soft like cashews. Not only appealing to people, many wildlife species, including birds and rodents, subsist in part on the high-calorie treats. This helps the trees disperse their seeds throughout the habitat, ultimately allowing the species to persist.

Californian Cultivation

Because of their unusual appearance, monkey-puzzle trees are popular specimens in gardens, backyards and parks. Monkey-puzzle trees require well-drained, slightly acidic soil, although they are adaptable to a variety of soil chemistries, as long as the drainage is adequate. They thrive best with plentiful sunshine, but will adapt well to shaded habitats.

Monkey-puzzle trees are incredibly hardy, and grow well along the Californian coast. They are rated for USDA Hardiness zones 7b through 20 according to most authorities, but some suggest they are suitable in locations rated up to USDA Hardiness Zone 11.

While the salt spray coming off the ocean will not negatively affect monkey-puzzle trees, they are rather intolerant of pollution. Accordingly, they are not ideal street trees for dense cities. However, they are resistant to most common pests.

Callitris_glaucophylla

Cypress-Pines (Callitris spp.)

Cypress-pines are small- to medium-sized trees, although a few species reach heights in excess of 100 feet. Although they resemble junipers (Juniperus spp.) in general appearance and by virtue of possessing two different types of leaves, cypress-pines bear both needle-like and scaly leaves throughout their lives.

Name and Classification

The genus name of cypress-pines comes from the Greek words meaning “beautiful” and “three,” which refers to the thrice-whorled leaves and cones that adorn the trees.

Although they are somewhat similar to pine trees, cypress-pines have no special allegiance to the pine tree clade (Pinus spp.). Instead, they are firmly nested within the cypress family (Cupressaaceae). Accordingly, many tree professionals are beginning to drop the “pine” portion of their name, instead simply referring to them as cypresses.

Fifteen species comprise the genus Callitris; two are native to New Caledonia, while the remaining species inhabit Australia. They are relatively rare outside of the South Pacific, although they have established themselves in some portions of Florida, where they now grow wild.

Reproduction and Ecology

Cypress-pines produce small, woody cones that bear about 30 to 40 seeds. As monoecious plants, cypress-pines produce both male and female cones occur on the same tree. All but one species grow in arid habitats; the outlier — Callitris macleayana — is native to Australia’s eastern rainforests.

Although most cypress-pine species grow in areas plagued by fire, they exhibit very little fire tolerance. After fires pass through an area and kill the majority of the mature plants, the next generation occurs as seeds, buried in the ground. Like some native California conifers, cypress-pine cones remain closed until a fire heats them enough to cause them to open and release their seeds. This way, the seeds usually land on a bare patch of ground, where they will experience limited – if any – competition.

Commercial Uses and Cultivation

The hard, compact bark of cypress-pines covers and protects the lightweight, aromatic wood underneath, which is the hardest wood among all softwood species. The wood has many useful commercial properties, and the wood of many species resists both termites and decay, even when not chemically treated. This makes the timber useful for outdoor applications, such as landscape timbers and playground equipment.

Gardeners often plant cypress-pines as ornamental plants, but they are extremely susceptible to brush fires. They prefer sandy soils and full sun, although the young will continue to grow – albeit slowly – when they sprout under the forest canopy. Most of the cypress-pines are more frost tolerant than other conifers of the southern hemisphere.

Notable Species

South Esk pines (Callitris oblonga) are the only members of the genus adapted for riparian habitats. Consequently, they are perhaps the most flood-tolerant members of the genus, although even though they strongly prefer well-drained soils. South Esk pines usually grow as large shrubs, but occasional specimens take on a tree-like form and reach 30 feet in height. Like most other representatives of the genus, South Esk pines are rarely cultivated in the United States, however a few botanical parks in California maintain specimens, and you may be able to find seedlings or seeds via commercial avenues.

White cypress pines (Callitris glaucophylla) are handsome trees that grow to about 90 feet in height. These plants are allelopathic, meaning they release chemicals into the soil that inhibit the growth of competitors. Accordingly, these trees rarely form pure stands. These trees are rather drought and frost tolerant, and thrive best in sandy, well-drained soil. Although the tree produces very shallow roots (which make it unsuitable for growing near other plants) it makes an excellent windbreak and helps to stop substrate erosion.

White cypress-pines are the most commercially important members of the genus, as they are harvested for their timber, as well as resin, oils and tannins. Northern cypress-pines also go by the scientific name Callitris glaucophylla.

The scrubby cypress-pine (Callitris canescens) is a drought-tolerant species, suitable for USDA Hardiness Zones 9a through 11. They are moderately drought tolerant, making them suitable for planting in parched environments. The largest member of the species, the stringy bark cypress-pine (Callitris macleayana) grows to incredible heights – occasionally exceeding 150 feet, making them largely unsuitable for private cultivation.

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Street Trees: Separating the Superlative from the Second-Rate

While few disagree with the need to increase the number of trees lining the nation’s cities and streets, it is important that such advances proceed deliberately, in accordance with well-conceived plans. While it is true that most trees provide tangible, quantifiable benefits, they also require resources to maintain.

Selecting the wrong trees for a given location not only fails to maximize the potential benefits — which a better species could provide — but it increases the likelihood of negative consequences. If a city plants labor-intensive trees, maintenance costs chip away at the realized savings; likewise, a city that plants trees with invasive root systems must allocate sufficient funds to repair damaged sidewalks and other components of the infrastructure.

The ideal trees for a given location vary greatly, so policy makers are wise to consult with an industry expert before designing a tree plan and selecting the species, which will make up the planting. Different species thrive in different hardiness zones and under different hydrological conditions.

Nevertheless, most street trees have a few common characteristics. None of these traits is singly essential, but the more of the characteristics present in a tree, the more effective it will perform as a street tree.

Reasonable Roots

Tree roots are opportunistic, and they can exacerbate any faults or flaws present in the surrounding hardscape. Given enough time, even relatively modest-sized trees can raise sidewalks or compromise foundations. Accordingly, it is important to plant trees that have manageable root systems. Avoid installing species that produce large surface roots, such as Blackwood acacia (Acacia melanoxylon), Norway maples (Acer platanoides) and camphor trees (Cinnamomumcamphora) in areas adjacent to concrete features. Instead, opt for species that are unlikely to cause such problems, such as Pacific wax myrtles (Myrica californica), Pacific dogwoods (Cornus nuttallii) and madrones (Arbutus menziesii).

Suitable Stature

One of the most important considerations regarding street tree selection is the mature height and spread of the crown. Trees that outgrow their allotted space may end up rubbing against buildings, blocking the right-of-way and growing dangerously close to utility lines. The pruning necessary to mitigate these problems increases maintenance costs, thus reducing the economic impact of the tree. Several small species, including western redbuds (Cercis occidentalis) and Macnab cypresses (Hesperocyparis macnabiana), have many of the attributes one would want in a street tree, while usually remaining under about 25 feet or so in height. Of course, there is nothing wrong with planting tall species in areas without overhead obstructions; however, the crown’s spread – horizontal growth – is still an important consideration for street plantings. Trees with a columnar growth habit are often helpful in this regard, such as Columnar Sargent Cherries (Prunus sargentii ‘Columnaris’), which grow up to 35 feet high, but usually have a crown spread of less than 15 feet.

Limited Labor Liabilities

While the cost benefit ratio of many street trees is a net positive, selecting species that require frequent maintenance increases the odds of ending up in the black. For example, wild-type mulberries (Morus spp.) and sweetgums (Liquidambar styraciflua) produce copious fruit, which often necessitates frequent attention. Either select cultivars that produce no fruit – such as “Rotundiloba” sweetgums (Liquidambar styraciflua ‘Rotundiloba’) – or select lower-maintenance species entirely. Additionally, drought tolerance is an important consideration for all southern California tree installations. Some species with exceptional drought tolerance include scrub oaks (Quercus dumosa) and “Shademaster” honeylocusts (Gleditsia triacanthos var. inermis ‘Shademaster’), which although unsuitable for use near sidewalks, require little maintenance and allow enough light penetration that grass grows well underneath them.

Superb Solar Solutions

Some deciduous trees provide two different solar-related benefits at opposite ends of the calendar. During the summer, they provide dense shade, but in the winter, when their leaves carpet the ground below, these trees allow the warm rays of the sun to penetrate to ground level. The maidenhair tree (Gingko biloba) – especially the Autumn Gold cultivar (Gingko biloba ‘Autumn Gold’) – provides a great example of this. In fact, gingko trees are not only “solar friendly” and capable of surviving the indignities of urban living, their leaves turn marvelously yellow in the fall. Unlike normal (female) maidenhair trees, the Autumn Gold cultivar does not produce the smelly fruit, characteristic of the species.

Pollution-Proof

Whether by runoff water, soil contamination or through the air, street trees exist under the constant assault of pollutants. These substances stress many trees, causing a reduction in vigor, often preventing them from thriving. Ideally, trees planted along streets should be very tolerant of pollution – especially air pollution, which constantly bathes the trees in harmful chemicals. Hedge maples (Acer campestre) are particularly well suited for urban environments, although they thrive best in moist, cool locations. Hedge maples not only tolerate pollution well, but they rarely produce roots that cause problems, nor do they struggle with compacted soils. Additionally, hedge maple leaves are often brilliant gold in the fall, making them a gorgeous addition to any street.

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The Bark Barrier

While a few traits characterize the bark of all trees, these protective coverings are remarkably diverse. As with other tree components, such as leaves, branches and roots, every bark is adapted to suit its environment and the life history of the species.

Although bark hardly seems as exciting or fascinating as so many other aspects of trees, this complex tissue, comprised of many layers of both living and dead cells, tells an intriguing tale, to any curious enough to listen.

Catch-All Definition

Bark is actually an informal term that refers to several of the outer layers of trees and other woody plants. Specifically, it includes all layers occurring outside of the cambium – namely, the phloem, phelloderm, cork cambium and cork. This outer layer of dead cork cells comprises most of what people think of as “bark.”

Whereas the cambium is a lateral meristem (area of cell division and growth) that produces the xylem (wood) and phloem, and it is largely responsible for the tree’s increase in girth, the cork cambium is a secondary lateral meristem, which produces the cork and phelloderm.

The inner layers of bark – specifically the phloem – are responsible for transporting chemicals and calories through the tree. Accordingly, damage to this layer can cause great stress for a tree. This is one of the reasons trees are easily killed by “girdling” – a technique used for deliberately killing trees in which a wide swath of bark is removed around the trunk’s circumference.

Slipping into a Stronger Skin

Bark production is resource intensive, meaning that there is a very good reason trees produce it. After all, plenty of green plants thrive in the modern landscape, bereft of bark.

One key distinction is that trees are perennial plants whose lifecycle depends on lasting for many years – sometimes many hundreds of years. By contrast, many herbaceous plants, grasses and shrubs are annuals that die off each year – producing such a robust outer covering is a frivolous use of precious resources for these ephemeral species.

Bark protects the interior and vulnerable portions of the tree, just as skin or scales protect animals. It reduces water loss from the wood and deters some predators and pests. Some species have even developed spines and thorns that confer additional protection.

Bark Reflects Habitat History and Evolutionary Patterns

Many trees have bark that clearly reflects the specie’s survival strategy. For example, the deeply fissured bark of black oaks (Quercus velutina), tuliptrees (Liriodendron tulipifera) and other species that inhabit areas with cold winters is an adaptation that provides protection from freezing temperatures.

However, the bark of some trees reflects the evolutionary history of the species more than it serves a current need. For example, beech trees (Fagus grandifolia) arose from within a largely tropical lineage, native to habitats with saturated atmospheres. Epiphytic plants abound in such locations, where they have grown on the trunks of trees for millennia. To prevent the plants from colonizing the trunk, the ancestors of beech trees developed smooth, thin bark, making it hard for the plants to obtain a secure “grip.”

Animal Assaults

Bark provides a number of important resources for various animals. Porcupines, beavers and many other mammals munch on the delicate inner bark and cambium of aspens (Populus tremuloides), beech and basswood (Tilia sp.) trees. Countless insects and other arthropods take shelter under the bark of trees, using it to not only shield themselves from predators, but to provide them with a thermally appropriate microclimate.

Humans also derive resources from tree bark. Aspirin is derived from chemicals present in willow (Salix sp.) bark, while quina trees (Cinchona sp.) gave humanity quinine – an important malarial medication.

Incredible Examples

A number of species produce truly unique bark. Check out the following links to learn about some rather spectacular species and the bark that helps them survive.