Assessing the Urban Forest Effects:The Greater Kansas City Region https://www.nrs.fs.fed.us/pubs/rb/rb_nrs75.pdf
Below is only partial text extracted from the Kansas City report (pg53) that relates to Forecast methods - see the actual document for full text with cited references.
APPENDIX VIII. i-TREE FORECAST PROTOTYPE MODEL METHODS AND RESULTS
The i-Tree Forecast Prototype Model was built to simulate future forest structure (e.g., number of trees and sizes) and various ecosystem services based on annual projections of the current forest structure data. There are three main components of the model:
- Tree growth—simulates tree growth to annually project tree diameter, crown size, and leaf area for each tree
- Tree mortality—annually removes trees from the projections based on user defined mortality rates
- Tree establishment—annually adds new trees to the projection. These inputs can be used to illustrate the effect of the new trees or determine how many new trees need to be added annually to sustain a certain level of tree cover or benefits.
Annual tree diameter growth is estimated for the region based on: 1) the length of growing season; 2) species average growth rates; 3) tree competition; 4) tree condition; and 5) current tree height relative to maximum tree height.
To determine a base growth rate based on length of growing season, urban street tree, park tree, and forest growth measurements were standardized to growth rates for 153 frost-free days based on: Standardized growth = measured growth × (number of frost-free days of measurement/153).3 Growth rates of trees of the same species or genera were also compared to determine the average difference between standardized street tree growth and standardized park tree and forest tree growth rates. Park growth averaged 1.78 times less than street trees, and forest growth averaged 2.26 times less than street tree growth.
For this study, average standardized growth rates for open-grown trees was input as 0.26 in/yr for slow growing species, 0.39 in/yr for moderate growing species and 0.52 in/yr for fast growing species. Th ere are limited measured data on urban tree growth for slow, moderate or fast-growing tree species, so the growth rates used here are estimates. Th ese growth rates by species growth-rate class were estimated such that the entire population average growth rate was comparable to the measured growth rates for trees standardized to the number of frost free days.
Crown light exposure (CLE) measurements of 0-1 were used to represent forest growth conditions; 2-3 for park conditions; and 4-5 for open-grown conditions. Th us, for: CLE 0-1: Base growth = Standardized growth (SG) / 2.26; CLE 2-3: Base growth = SG / 1.78; and CLE 4-5: Base growth = SG. However, as the percent canopy cover increased or decreased, the CLE correction factors were adjusted proportionally to the amount of available greenspace (i.e., as tree cover decreased and available greenspace increased—the CLE adjustment factor decreased; as tree cover increased and available greenspace decreased—the CLE adjustment factor increased).
Base growth rates are also adjusted based on tree condition. Th ese adjustments factors are based on percent crown dieback and the assumption that less than 25 percent crown dieback had a limited effect on diameter growth rates. For trees in fair to excellent condition (less than 25 percent dieback), base 54 growth rates are multiplied by 1 (no adjustment), trees in poor condition (crown dieback of 26-50 percent) by 0.76, critical trees (crown dieback of 51-75 percent) by 0.42, dying trees (crown dieback of 76-99 percent) by 0.15, and dead trees (crown dieback of 100 percent) by 0.
As trees approach their estimated maximum height, growth rates are reduced. Th us the species growth rates as described above were adjusted based on the ratio between the current height of the tree and the average height at maturity for the species. When a tree’s height is over 80 percent of its average height at maturity, the annual diameter growth is proportionally reduced from full growth at 80 percent of height to one-half growth rate at height at maturity. The growth rate is maintained at one-half growth until the tree is 125 percent past maximum height, when the growth rate is then reduced to 0.
Tree height, crown width, crown height, and leaf area were then estimated based on tree diameter each year. Height, crown height, and crown width are calculated using species, genus, order, and family specific equations that were derived from measurements from urban tree data (unpublished equations). If there is no equation for a particular species, then the genus equation is used, followed by the family and order equations, if necessary. If no order equation is available, one average equation for all trees is used to estimate these parameters. Leaf area was calculated from the crown height, tree height, and crown width estimates based on i-Tree methods.
Total canopy cover was calculated by summing the two-dimensional crown area of each tree in the population. Th is calculated estimate of crown area was adjusted to attain the actual tree cover of the study area based on photo-interpretation. Trees often have overlapping crowns, so the sum of the crown areas will often over estimate total tree cover as determined by aerial estimates. Th us the crown overlap can be determined by comparing the two estimates:
% crown overlap = (sum of crown area – actual tree cover area)/sum of crown area
When future projections predicted an increase in percent tree cover, the percent crown overlap was held constant However, when 100 percent tree cover was attained all new canopy added was considered as overlapping canopy. When there was a projected decrease in percent tree cover, the percent crown overlap decreased in proportion to the increase in the amount of available greenspace (i.e., as tree cover dropped and available greenspace increased – the crown overlap decreased).
Tree Mortality Rate
Canopy dieback is the first determinant for tree mortality. Trees with 50-75 percent crown dieback having an annual mortality rate of 13.1 percent; trees with 76-99 percent dieback have a 50 percent annual mortality rate, and trees with 100% dieback have a 100 percent annual mortality rate.36 Trees with less than 50 percent dieback have a user defined mortality rate that is adjusted based on the tree size class and diameter.
Trees are assigned to species size classes: small trees have an average height at maturity of less than or equal to 40 ft (maximum diameter class = 20+ inches); medium trees have mature tree height of 41- 60 ft (maximum diameter = 30+ inches); large trees have a mature height of greater than 60 ft (maximum diameter = 40+ inches). Each size class has a unique set of seven diameter ranges to which base mortality rates area assigned based on measured tree mortality by diameter class (Fig. 40). The same distribution of mortality by diameter class was used for all tree size classes, but the diameter range of the classes differed by size class. The actual mortality rate for each diameter class was adjusted so that the overall average mortality rate for the base population equaled the mortality rates assigned by user. That is, the relative curve of mortality stayed the same among diameter classes, but the actual values would change based on the user-defined overall average rate.