Trees are an important resource to the people of San Francisco and the varied wildlife species that utilize the urban forests within the City, and in an urban setting, methods such as thinning is necessary to ensure a healthy environment for tree growth.
As important of a resource as the trees are, the species that have been planted throughout the Natural Areas are almost entirely non-native and most are also invasive. These include eucalyptus (Eucalyptus globulus, Eucalyptus sp.), Monterey cypress (Cupressus macrocarpa), Monterey pine (Pinus radiata), acacia (Acacia longifolia, Acacia melanoxylon), plume acacia (Albizia lophantha), and myoporum (Myoporum laetum). While some of these species are native to California, none of them are native to San Francisco.
In many Natural Areas, trees capture moisture from the coastal fog. This moisture drips onto the ground creating artificially wetter than normal conditions which favor invasive weed species. Eucalyptus in particular do not provide a substantial source of food and the oils in their leaves inhibit the germination of plants in the forest understory. This creates a relatively open forest floor that severely limits the habitat value of eucalyptus forests. Many native birds, mammals, and reptiles require relatively dense scrub cover for nesting and roosting. Most urban forests provide minimal habitat for these species. In other words, Eucalyptus trees are fire hazard, toxic to soil, invasive to other plant growth. The invasive forests within the Natural Areas are predominantly eucalyptus, although cypress, pine and acacia also occur.
All of these species are subject to removal within the Significant Natural Resource Areas Management Plan. Overall, there are an estimated 118,000 trees within the Natural Areas System. Of these, approximately 54,000 are at Sharp Park, leaving 64,000 within San Francisco. Total removals within the City are estimated at 3,424 trees, which will be implemented over 20 year-period.
Basal area: A measure, typically in square-feet per acre, of the area covered by trees within a given urban forest. Basal area is used as an index of tree production. For example, a basal area of 25 square feet per acre could equate to 11 trees with diameters of 20 inches, or 45 trees with diameters of 10 inches, in a single acre.
Seedling or Sapling: Young trees that are less than 15 feet tall. Seedlings and saplings are not included in the calculation of the number of trees within the Natural Areas, trees to be removed, or basal area because they are not considered trees.
Selection Silviculture: The removal of individual tree(s), or small groups of trees within the stands or along its edges.
Tree: A plant having one dominant vertical trunk that is over 15 feet tall.
Tree Stand or Stand: A unit of trees that is relatively homogeneous in age, structure, composition, and physical environment. Urban Forest: Any significant stand of non-indigenous trees.
SYSTEM-WIDE FOREST MANAGEMENT PRACTICES
The following forest management practices could apply to any tree removal in Natural Areas, including urban forests. The choice of forest management technique to be applied depends on the location and number of trees to be removed as well as the end goal of the management action. In general, any removal of trees over 6 inches in diameter at breast height (dbh) requires coordination with, and evaluation by SFRPD’s Arborist. In addition, prior to any tree removal, individual trees measuring 6 inches dbh or greater must be posted for 30 days.
Group selection results in the removal of a number of trees within a specified area. It is often used to create openings or plots within existing dense forests. For example, many of the urban forests are very dense with many trees per acre. Application of group selection to small areas between 0.25 and 0.5 acres in size, would create the desired openings, but with the surrounding forest remaining intact.
Thinning activities typically occur over large areas (several acres) and result in the removal of smaller trees and saplings, effectively opening up the understory. Some overstory trees may also be removed; however, the overstory trees to be removed will be individually selected. Application of thinning management results in an increase in the average diameter of the residual trees, promotes the growth of those trees, and improves forest health through the removal of suppressed trees.
SLASH AND DEBRIS REMOVAL
Unless it can be used to create wildlife habitat (see Section 5, GR-9) all large woody debris will be chipped on site, stockpiled in visually hidden places or removed manually off-site. Some of the chips may be used to deter understory invasive vegetation in the stand, or could be used as beneficial mulch on other revegetation projects in Natural Areas. Some large trunks may be left on site if they provide habitat value. Large trunks may also be moved to appropriate locations within the Natural Area and used for recreational or maintenance purposes.
REGENERATION AND INVASIVE SPECIES CONTROL
During the first few years after tree removal activities, remaining trees within the stand may suffer shock from changing environmental conditions. Once the initial shock has passed, tree growth may accelerate. Growth of understory non-native brush and shrub species will likely be promoted as a result of the opening of the canopy created by the removal of trees. The understory vegetation should be managed as appropriate for that particular Natural Area in accordance with site-specific management recommendations.
The potential for erosion resulting from tree removal is expected to be minimal because tree removal activities will be done almost entirely by crews using hand tools and chainsaws. Use of heavy equipment will be minimized within Natural Areas, thereby minimizing ground disturbance and potential soil erosion. To the fullest extent possible and with consideration given to topography, lean of the trees, landings (staging areas), utility lines, local obstructions and safety factors, trees will be cut to fall away from any creeks or ponds.
Specific removal operations will be planned to avoid the potential for tree removal to result in slope instability and potential mass wasting (landslide or large erosion) events. In some areas it may be necessary to phase tree removal over several years to allow for vegetation regeneration and minimize erosion risks. The removal of large trees will be evaluated by the SFRPD Arborist prior to implementation. Site-specific erosion control best management practices should be applied as necessary to minimize erosion.
As the stands naturally age, some of the larger overstory trees in the stand will decline in health. Snags (dead trees) which do not pose any public hazards and do not harbor infestations of deleterious insects or diseases should be retained for wildlife function. Snags within 100 feet of structures maintained for human habitation should be removed, consistent with CDF regulations for snag retention.
The following general standards for protection of sensitive species shall apply to tree removal activities in the Natural Areas. These standards are similar to those applied by CDF to timber harvest operations:
- During tree removal activities, nest tree(s), designated perch trees(s), and screening trees(s) shall be left standing and unharmed.
- Tree removal activities shall be planned and operated to commence as far as possible from occupied nest trees.
- When an occupied nest site of a sensitive bird species is discovered during tree removal activities, staff will protect the nest tree, screening trees, and perch trees, and establish a 150-foot buffer zones around the trees to minimize disturbance to the nesting birds. No power tools shall be used within this buffer, although hand weeding may occur within 50 feet of the nest.
FOREST PRESERVATION: BEFORE & AFTER
Photos of successful example of forestry preservation through the process of restoration at Glen Canyon Park: