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Assess environmental factors
The environment affects what trees you should plant, how well they will grow, and how they should be cared for. The plan should discuss the environmental conditions that occur in the plan area.
If the plan area is small, the environment may not vary across the site. In larger or more diverse areas, soil, microclimate, wind exposure, or other factors may vary across the site. If so, it may be useful to identify zones for tree selection and maintenance.
Some factors, such as rainfall, are clearly beyond the control of the urban forest manager. Other factors, such as soil conditions, can be modified. Some environmental factors to consider are noted below.
- Soil conditions. A wide variety of soil properties can affect tree selection and performance. These include:
soil texture (sand/silt/clay balance)
soil compaction
rootable soil depth
soil water holding capacity
soil drainage characteristics
depth to water table
soil salinity
concentrations of specific toxic ionSoil survey data can provide soil information, but where soils have been altered through cut and fill or imported top soil, the soil survey data will no longer apply.
Planting sites in urban areas vary with respect to the amount of rootable soil volume, impervious soil cover, presence of underground utilities, irrigation, and other factors that affect tree growth. Some of these factors may be noted in existing tree inventories.
- Climate and microclimate. In hilly areas or near the coast, climate characteristics may vary across the plan area. Factors to consider include minimum and maximum temperatures, fog influence, and wind exposure. Significant differences in microclimate may occur between planting sites due to slope and aspect, shade from buildings or landforms, wind patterns, or reflected heat from buildings and pavement.
Overall climate conditions tend to vary over the years due to cyclical weather patterns such as the El Nino/Southern Oscillation. Record heat or cold, droughts, floods, or severe storms can have large impacts on tree populations that are geared toward average conditions. The possibility of extreme conditions may need to be considered in long-term planning as global climate change occurs.
- Fire risk. In parts of California, wildfire has become an increasing concern as communities spread into fire-prone vegetation types. Management of vegetation at the wildland/urban interface to reduce fire risk has become an important planning issue.
CalFire has developed a rating of wildland fire threat for the entire state. The rating is based on potential fire behavior (derived from weather, terrain and vegetative-fuel data) and expected fire frequency (derived from 50 years of fire-history data). Areas are assigned one of four fire threat ratings: moderate, high, very high and extreme.
The California State Public Resources Code section 4291-4299 has requirements for creating defensible space for structures in lands covered by flammable vegetation. The guidelines created by CalFire to help landowners interpret these rules state: In general, fuel reduction means arranging the trees, shrubs and other fuels sources in a way that makes it difficult for fire to transfer from one fuel source to another. It does not mean cutting down all trees and shrubs, or creating a bare ring of earth across the property.
Urban Forest Management Plans for areas with elevated threat ratings should consider how the plan will interact with other documents and plans that already address fire risk. The plan may need to include elements related to fire risk. Although vegetation management can play a role in reducing fire hazard, other factors, such as building construction and materials, play an important role in minimizing fire risk. Cities and county general plans include a safety element. This element should comprehensively address wildfire threats.
Example
The first step in management of any resource is an inventory to determine the extent, condition, and needs of, in this case, the urban forest. Since all life is rooted in the soil, geologic and soil survey information was examined to determine the physical characteristics of the soils in Lacey. The information pertinent to management of trees includes: general fertility levels, drainage, depth to root restrictions, organic matter contents, plant available water capacity, and windthrow potential. The suitability for wildlife, construction and engineering properties was also examined. This information was then considered when recommendations were made with regard to tree protection areas, tree species selection, and planting designs.
City of Lacey Washington, Urban Forest Management Plan, April 2005, Executive Summary:background, p.8
Soils and Site Information
The City of Lacey occurs entirely with the Puget Sound Trough which extends the entire length of Washington from the Canadian border to Oregon. the soils were formed during the most recent Vashon glaciation epoch. the terminal moraine of the Vashon glaciation is found in southeast Thurston County.
The topography of Lacy is undulating to rolling on uplands. The soils are predominantly formed in glacial drift deposited by the most recent of several continent-sized glacial iced sheets. The soils generally consist of compact basal till covered by a thin, discontinuous layer of ablation till. The predominant gravelly soil types formed from this material include the Algerwood gravelly sandy loam, Everett very gravelly sandy loam, and the Spanaway gravely sandy loam. Also commonly found are the Indianola loamy sand, Nisqually loamy fine sand, Skipoppa silt loam, and Yelm fine sandy loam. these sandy soil types are formed from outwash material and are found along the major stream courses and broad flats.
The windthrow potential for all of the soil types found in Lacey is slight (except the Alderwood which is moderate)/ meaning that blowdown of healthy trees is less likely during normal winter storm events. The moderate windthrow risk indicates a higher windthrow potential, especially during periods of wet weather and high winds. Perched water table above the Alderwood hardpan contribute to this increased hazard. Inclusions of poorly drained soils such as the McKenna gravelly loam (small areas that generally occur within wetlands) also have a moderate to high potential for windthrow. The productively of these upland soils is moderate to high, and competition for new seedlings is moderate to high. weed control is usually necessary during the first 3 years after planting seedlings in forested areas or gaps in the canopy. The soils found on the outwash materials are slightly drier, but have no barriers to root penetration and a slight windthrow potential.
City of Lacey Washington, Urban Forest Management Plan, April 2005, Appendix 1 Background and Baseline information, Section B, p.31-32
Planning questions
- What are the important environmental factors that affect tree selection or maintenance in the plan area?
- Do these factors change in different portions of the plan area?
- How can these factors be taken into account in the urban forest plan (e.g., zone maps)?
Work plan
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Use the form to indicate topic areas to be included in the management plan. Check those of interest
Environmental factor
Data source
By whom?
By when?
Rainfall
Temperatures
Climate zones
Soil
Native vegetation
Fire Other-specify Outline
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