Maintaining Old Trees
Since the beginning, trees have furnished us with life essentials. To our ancestors, trees were part of their subsistence and economy. Over time, our uses and our knowledge of their benefits increased. Regardless of the time period trees are an integral part of life.
Our planet's largest and oldest trees provide the greatest environmental benefits. (Fig. 1). Therefore, as the trees growing in our urban environment reach maturity, developing strategies for their preservation and avoiding their unnecessary removal is of vital importance. The solution is conservation arboriculture: an ecosystembased management strategy that addresses a tree's biology, life stage, and risk for the purpose of the tree's preservation.
The Roots of Conservation Arboriculture
Though the term "conservation arboriculture" is relatively new, the practice dates back to at least the 15th century: a time when demand for timber was intense and forest management became necessary. Through practices such as pollarding or coppicing, trees functioned as "working trees" to produce fuel wood, building materials, and other benefits. As society evolved, people migrated to nearby cities and abandoned those once meticulously maintained trees. As a result, the trees flourished. They grew very large and old, some exceeding 1000 years in age. Such longevity was made possible through past management practices, which encouraged canopy rejuvenation, maintained the root mass, and reduced static loading on trunks and branches, even when they were decayed.
These veteran trees are often the sites of complex interactions with other organisms some that require characteristics only found on mature trees, such as large parts of dead wood and decay. Research has revealed that these trees are veritable arks of biodiversity, with species richness comparable to or exceeding that found in natural forests.
Fortunately, modern arboriculture has progressed greatly especially in the field of tree risk assessment. Today, advanced assessment methodologies such as resistograph readings, sonic and electric impedance tomography, static load testing, and other technologies are widely available. These tools contribute to evidence-based tree risk assessment, and can therefore help arborists determine whether (and how) to invest in the retention of veteran trees. The care of mature trees is also supported by an ever-growing body of research and knowledge that offers new insights into tree biomechanics particularly the effects of internal decay on strength loss and stability. In doing so, these methods provide precedents for arboricultural prescriptions designed to maintain old trees in reasonably safe condition or provide real justification when removal is deemed necessary.
Conservation arboriculture is an evolving discipline. The keys to its success will include experience, innovation, experimentation, knowledge transfer, and adaptation. Preserving our veteran trees for the many important benefits they provide will require a change in the way we think about tree aesthetics as well as an adoption of more innovative management techniques. If we can accomplish this, future generations will reap the rewards of these living monuments to cultural and natural heritage. By their nature, veteran trees are survivors and can considerably outlive our life spans; but only if we give them that chance.
Philip van Wassenaer, B.Sc., MFC, ISA Certified Arborist and principalconsultant for Urban Forest Innovations Inc. contributed to this article. Philip is a world renowned expert on Conservation Arboriculture.