Alex Shigo: The Father of Modern Arboriculture

No matter the field, subject or industry in question, it is always important to understand the role important predecessors played in shaping the world modern world.

For example, horn players should learn about Miles Davis; baseball players should learn about Mickey Mantle and Babe Ruth;and artists should learn about Picasso, Da Vinci and Dali.

But if you are an arborist, tree-care professional or simply someone who loves to learn about trees, you should familiarize yourself with the work of Alex ShigoPh. D. – better known as the father of modern arboriculture.

The Early Years

Alex Shigo was born in Duquesne, Pennsylvania in 1930. He was a talented and dedicated musician from an early age, who played clarinet for the official United States Air Force Band as part of his service during the Korean War. After the war was over, he enrolled in Waynesburg College, where he eventually obtained a Bachelor of Science degree in biology. Soon after, he was admitted to West Virginia University, where he obtained both a Master’s and Doctoral Degree in plant pathology.

Career Path

In 1960, Dr. Shigo was hired to perform tree pathology work for the U.S. Forest Service. Dr. Shigo stayed with the forest service for 25 years, before he retired and began working as an author and lecturer, sharing his knowledge of trees with all who cared to listen or read. A prolific writer, Dr. Shigo authored and co-authored 160 different books, academic papers and popular pieces during this time.

Major Contributions to Arboriculture

A pioneering thinker in the world of tree science, Dr. Shigo turned many time-honored tenants of tree care on their head. Dr. Shigo’s contributions to the world of arboriculture could fill volumes, but a few of his most important lessons and breakthroughs include:

  • In the 1950s, Dr. Shigo began using a newly available technology – the one-person chainsaw – to learn more about trees. During this time, Dr. Shigo dissected more than 15,000 trees to better appreciate their biology. Researchers had already been carrying out tree dissections for many decades, but most of these dissections were carried out on in a crosscut-manner. By contrast, Dr. Shigo began dissecting trees along their vertical axis, which revolutionized our understanding to the spread of decay through a tree.


  • Thanks in part to these dissections, Dr. Shigo re-wrote our understanding of tree wounds. Specifically, he argued that trees do not heal – they seal up wounds and grow around them. He coined the term CODIT (Compartmentalization of Decay in Trees) to help explain this process and allow tree-care professionals to benefit from the knowledge.


  • Shigo studied the way trees in the forest shed their branches, and compared this with what he learned from his study of decay. In doing so, he realized that the flush-cutting technique preferred by arborists at the time was not the best way to prune trees. Instead, he championed the importance of leaving the branch collar intact, and invented the tree-cut pruning technique, which prevents stripped bark and damaged branch collars.

Dr.Shigo continued to learn about trees and share his considerable knowledge about them for the rest of his life. He died unexpectedly after suffering a fall on October 6, 2006, at the age of 76.

Try to keep Dr. Shigo’s contributions to arboriculture in mind the next time you find yourself looking at a beautiful urban forest. Thanks to his work, those trees will feature full, vibrant canopies and properly trimmed stubs – and they’ll be healthier and more attractive for it.



Yuccas (Yucca) are iconic perennials, found across a wide geographic area, stretching from Canada to South America. The fifty-odd species that comprise the genus inhabit a variety of different climate zones across this range, but most are native to arid habitats.

Because they provide the raw materials necessary for making many basic necessities, yuccas have been important to people for hundreds of years; while they are not commercially important in the modern world, several species are very popular, drought-tolerant ornamentals.

Basic Biology

Most yuccas possess a few key traits that characterize the entire genus. These include the presence of long, pointed leaves that arise from a dense rosette, and tall, “floating” inflorescences. Some forms remain close to the ground and superficially resemble a tuft of giant grass, while others (especially some of the most famous members of the group) produce a tree-like growth form.

Arid Adaptation

Taken as a whole, the genus Yucca displays a diverse array of adaptations that help them survive in arid habitats. Many of these traits make these plants particularly well suited for ornamental use in Southern California. Yucca plants thrive best in full sun exposure, but they can thrive in most well drained soils, and require very little (if any) supplemental irrigation once established.

Most of the species possess thick leaves and many feature a thick wax layer on the surface of the leaves; both adaptations help slow the rate of transpiration, and therefore, reduce the amount of water the plants lose. Some species shed their leaves and become dormant during dry, hot summers. In doing so, the plants typically shed their leaves to halt transpiration altogether. Additionally, the shed leaves provide some protection against the sun’s harsh rays.

Yucca Pollination

While a few yuccas are capable of self-pollination, most require crosspollination to produce seeds. Accordingly, most yucca species relies on insects – specifically, a group of lepidopterans called yucca moths – for pollination.

While feeding on the nectar produced by yucca flowers, the moths ferry pollen from male flowers to females, thus ensuring crosspollination. Because yucca moths also deposit their eggs on the female flowers, the hatching larvae enjoy a ready food source: the developing seeds.

Notable Species

Several yucca species are particularly interesting. Some of the most notable species include:

Joshua trees (Yucca brevifolia) are often mistaken for true trees, as they produce tall, thick trunks that branch in multiple locations. Found throughout the Mojave Desert, Joshua trees actually serve as an indicator species for the Mojave community. Joshua trees were important to the Cahuilla people (historic inhabitants of the Mojave region), who used the tough leaves of the plants to make sandals and other durable goods, and collected the flower buds and seeds as a food source.

Like Joshua trees, soaptree yuccas (Yucca elata) are native to the American southwest, but they inhabit the Sonoran and Chihuahua Deserts, rather than the Mojave. Also like Joshua trees, soaptrees provided a variety of resources for Native Americans living alongside them. In addition to stiff plant fibers used to produce baskets, sandals, cordage and similar items, a soapy substance found within the plant’s trunk and roots was harvested for use as a soap or shampoo.

Spineless yuccas (Yucca elephantipes) are quite tree-like, and they often produce several different “trunks.” Perhaps unsurprisingly, the spineless yucca is the tallest member of the genus Yucca, and it occasionally reaches 30 feet in height.

Spineless yuccas are popular ornamentals in many warm regions, including not only California and Arizona, but also the relatively humid south Florida region. Unlike many other yuccas, including the aptly named needle-palm (Yucca filamentosa) and Spanish dagger (Yucca gloriosa), which possess stiff terminal spines that probably serve to discourage predators; spineless yuccas lack hazardous terminal spines.

Thanks to their striking pale blue leaves, beaked yuccas (Yucca rostrata) are popular ornamentals. These yuccas also grow like small trees, and often reach heights of 15 feet or more.


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.