As part of the reference material used by the Historic District Review Board for the latest draft of the Heritage and Specimen Trees portion of the Tree Conservation and Preservation Ordinance, these “guidelines” from from The International Society of Arboriculture were consulted. The HDRB continues to refine a draft that will help protect the town’s tree canopy.
As noted in our discussion of provision 31, individual trees may be considered important community resources because of unique or noteworthy characteristics or values. Such trees have been described in ordinances as heritage, historic, landmark, legacy, special interest, significant, or specimen trees or various permutations of these terms (e.g., heritage oak, exceptional specimen tree). In some ordinances, trees are simply labeled protected trees (i.e., trees afforded protection by the ordinance). Regardless of the term used, the concept is the same: trees with certain characteristics are singled out for special consideration in the ordinance. Most commonly, one or more of the following criteria are used to define a special status tree:
Size – Some component of tree size, most frequently trunk diameter, may be used to define a special status tree. Most commonly, a given diameter at 4.5 ft above grade (i.e., diameter at breast height or DBH) is used as the size standard. Additional rules are typically needed to handle trees that are multi-trunked or branch below4.5 ft. Because the relationships between DBH and canopy spread or DBH and tree age vary by species, different tree diameter standards may be applied to different species. Although a tree diameter standard is fairly objective, the threshold diameter is often set more or less arbitrarily. As such, management decisions based solely on a threshold diameter may not be particularly logical.
For example, if the threshold diameter for protecting a tree is 24 inches DBH, a tree with a diameter of 23.9 inches would be ignored, even though it might have a greater canopy spread than a tree with a larger DBH. Furthermore, the measurement of DBH with standard equipment such as diameter tapes or calipers is subject to errors related to trunk or bark irregularities and minor shifts in the location of the measuring device. A tree with a DBH measured as 24.2 inches by one observer could be measured at 23.5 inches by another observer. These problems are minimized when small threshold diameters (e.g., 3 inches) are used. Other components of tree size, such as maximum canopy spread or height, may also be considered independently or in conjunction with tree diameter. The National Register of Big Trees, maintained by American Forests, uses a point system to rate tree size. Points for each tree are calculated by summing trunk circumference (at 4.5 ft) in inches, tree height in feet, and one-quarter of the average crown spread in feet. This system is used to determine “champion” trees for each species. Some ordinances expressly confer special tree status on state or national champion trees. More local “champion” trees could be defined using the same methods.
Species – Special status may be conferred only to certain species of trees. Special status trees are often, but not always, important locally native species or trees that are associated with the character of a community. Certain species that are relatively rare in an area, whether native or not, may also be granted special status. In some cases, species is used to specifically exempt certain trees from special status regardless of size. For instance, weedy trees such as tree-of-heaven (Ailanthus altissima) or trees used for commercial purposes (e.g., fruit trees, plantation lumber or pulp trees) may be excluded from consideration as special status trees. Unless interspecific hybrids are present in an area or the taxonomy of a species changes, species is probably the most objective criterion used in defining special status trees.
Age – Especially old trees are a link to the past, so many definitions of special status trees include age as a criterion. In practice, tree age is fairly difficult to determine in standing trees unless documentation of tree age exists from historical accounts, photographs, or associations with historical structures. Tree age is sometimes inferred from tree size, especially DBH. However, the relationship between age and DBH varies with species, site quality, management history, and other factors, so DBH is usually only a crude estimator of tree age. Determining age by increment boring is theoretically possible, but is potentially damaging to the tree and is fraught with difficulties if trees are large, have very hard wood, or are decayed in the center.
Historic significance – A tree may be associated with a notable local or regional historical event, person, structure, or landscape. Almost every tree that has been around for a while has some historical significance, whether it is recognized or not. Determining whether the historical significance of a given tree is sufficiently notable is therefore a subjective matter. Historic tree status is typically granted by a governing (e.g., city council) or advisory body (e.g., tree commission). Some ordinances automatically confer historic status on trees designated as historical landmarks by certain other organizations (e.g., historical societies). In addition, ordinances may assign special status to trees dedicated or planted as public memorials.
Ecological value – All trees serve a variety of ecological functions. Certain trees or groups of trees may have especially high ecological value because of their location, size, species, and/or condition. For example, a given tree may be an important roost, nesting site, or food source for certain wildlife species; it may be situated in a site where it plays a critical role in stabilizing soil or providing shade needed by other plant or animal species; it may be an important genetic resource for a local tree population or the species as a whole. Input from trained biologists and ecologists may be necessary to document particular ecological values that may not be obvious to the general public.
Aesthetics – Since beauty is in the eye of the beholder, assigning special status on the basis of aesthetics is always highly subjective. A tree may have special aesthetic value due to its form, whether it is especially perfect and symmetrical or notably craggy and idiosyncratic. Also, the function that a tree serves in a landscape may be sufficient to justify special status; for example, a landmark pair of trees that frame an entrance. In the absence of other noteworthy characteristics, it may be contentious to base special status upon aesthetics alone.
Location – Trees in particular locations may be accorded special status in recognition of the important aesthetic or ecological functions that they serve. Proximity to a thoroughfare can be used to classify a tree as a street tree, which may be accorded special status whether or not it is in the public right-of-way or is under public or private care. Trees located along or within a set distance from watercourses may also be give special status due to their importance in stabilizing stream banks or providing shaded riverine habitat. In some cases, the location of a tree is considered in conjunction with size or species parameters.
Required plantings and retained trees – If trees are have been preserved or planted as a requirement of development, the community has a vested interest to ensure that the trees are protected. The purpose of planting and tree retention is to develop mature tree canopy, and this cannot occur if the subject trees are eliminated, ruined by topping or other poor maintenance practices, or replaced frequently with young trees. By explicitly providing special status to such trees in the ordinance, a jurisdiction may be able to provide a higher level of regulatory protection to such trees and increase the penalties associated with unauthorized damage to or removal of the tree.
Other unique characteristics – This grab-bag term may be added to the list of criteria used to designate special status trees because it is difficult to anticipate all possible situations of significance. For example, a given tree may become a local or regional cultural icon due to an event or apparition that is associated with it. This criterion will again be subjective and typically may be invoked through the approval of a governing body.