Appendix D. Habitat capability index for red tree vole (Arborimus longicaudus).
Developers: Michael T. McGrath and Brenda C. McComb
Reviewers: John Hayes, Keith Aubry
Red tree voles occur in forests from 25 yr of average stand age through old-growth forests and are considered to be more numerous in mature and old-growth stands (Maser 1966, Zentner 1977, Corn and Bury 1986, Carey 1991, Hayes 1996, Meiselman and Doyle 1996). Typically, red tree voles inhabit Douglas-fir (Pseudotsuga menziesii) dominated stands, but will also occur in grand fir (Abies grandis), Sitka spruce (Picea sitchensis), and western hemlock (Tsuga heterophylla) stands to a lesser extent (Jewett 1920, Walker 1930, Maser 1966), and specializes on conifer needles for forage (preferring needles of the Douglas-fir; Wight 1925, Maser 1966).
Red tree vole nests are located within 1.8 and 49 m of the ground, often in contact with the tree trunk, but also occur in branches away from the trunk, in cavities, or in dead tops of trees (Taylor 1915, Wight 1925, Maser 1966, Gillesberg and Carey 1991, Hayes 1996). Nests are typically located near a source of live needles and are disproportionately common in large-diameter trees (Maser 1966, Gillesberg and Carey 1991, Hayes 1996). In order to procure water, the red tree vole may require more complex stand structures or moist sites (Maser 1966, Huff et al. 1992). Moisture in the form of precipitation, condensation, or fog drip may be required for red tree voles because they often obtain water by licking it off needles and have avoided pools of water in captivity (Maser 1966).
Because the red tree vole is primarily associated with Douglas-fir trees and prefers their needles over other coniferous needles for food (Maser 1966), the prevalence of Douglas-fir within a stand is the first screen for red tree vole habitat suitability. Stands which are predominately composed of Douglas-fir will be optimum conditions for the red tree vole, and stands composed predominately of the other species will be of an equal, sub-optimum suitability level.
All metrics for this index are calculated for a focal pixel at the center of a 3×3 “moving window.” This moving window of pixels averages conditions for the 0.5625 ha surrounding the “focal” pixel (i.e., 3×3 pixels). The averaging is done to: (1) smooth inter-pixel variation and (2) provide a “patch” or “stand” level summary, which is consistent with the scale of the stand modeling and stand inventory data. The moving window is assumed to encompass ≥1 red tree vole territory. The index is calculated as:
where HCI = habitat capability index, f = focal pixel, i = pixel, and NCI = nesting capability index.
Nesting capability index
Because each component of the model provides valuable resources for the red tree vole (i.e., food, habitat structure, and dispersal capabilities), the model assumes equal weighting for each component. Thus, the index scores are averaged within each pixel contained within the window. Although Douglas-fir needles are preferred by red tree voles (Wight 1925, Maser 1966), other needles of other conifers are also consumed (Jewett 1920, Walker 1930, Maser 1966). Therefore, the food component of this index does not receive additional weight in the equation. The nesting score is limited to the minimum of S2 and S3 to compensate for quadratic mean diameter abnormalities in the CLAMS simulation model. The index is calculated from the following equation:
where NCI = nesting capability index, S1 = Douglas-fir composition index (Eq. 3), S2 = quadratic mean diameter index (Eq. 4), S3 = canopy closure index (Eq. 5), S4 = canopy heterogeneity index (Eq. 6), and f = focal pixel.
Douglas-fir composition index
The Douglas-fir composition index (Fig. D1) is a proportional measure of the species within the stand and is indicative of the relative abundance of the red tree vole’s preferred food. Although red tree voles occur in trees of lesser diameter (Maser 1966), the diameter classes utilized in this index are limitations imposed upon the model by the CLAMS database. The index is calculated from the equation:
where S1 = Douglas-fir composition index, f = focal pixel, DF50 = density (trees per ha) of Douglas-fir, dbh >50 cm, and TPH50 = density of all trees >50 cm per ha.
|FIG. D1. Relationship of index to proportion of total stand basal area in Douglas-fir.|
Quadratic mean diameter index
Red tree vole nests are found in a diverse array of stand conditions and have been found in trees as small as 10 cm dbh (Maser 1966). Nests occur in young forest stands (Jewett 1920, Brown 1985, Maser 1966, Corn and Bury 1986) but are thought to be more numerous in mature or old forest stands (Corn and Bury 1986, Gilbert and Allwine 1991). Thus, the quadratic mean diameter index (Fig. D2) was designed to reflect higher habitat capability based on the size of trees within the stand, with optimum potential to support the species occurring at QMD >50 cm (i.e., old forest structure; Spies and Franklin 1991). The index is calculated using the following equation:
where S2 = quadratic mean diameter index, f = focal pixel, and QMDp = quadratic mean diameter (cm) of all trees for pixel (p).
|FIG. D2. Relationship of index to quadratic mean diameter of stand.|
Canopy closure index
The canopy closure index (Fig. D3) is designed as an index for the degree of connectivity among tree crowns within the stand to facilitate movement and dispersal of red tree voles. It is assumed that habitat capability will increase more rapidly at the mid range than at the extremes of canopy closure. Thus, a logistic response function has been assumed as the functional response between red tree vole habitat capability and percent canopy closure. The index is calculated from the equation:
where S3 = canopy closure index, f = focal pixel, e = Euler’s constant, and CC = percent canopy closure.
FIG. D3. Relationship of index to percent canopy closure.
Canopy heterogeneity index
The canopy heterogeneity index (Fig. D4) is designed to indicate the influence vertical complexity within a stand has on habitat capability for the red tree vole. Red tree voles have been located in forests of all ages greater than 25 yr old (Maser 1966). However, many researchers believe the red tree vole to be more numerous in mature and old-growth stands (Zentner 1977, Corn and Bury 1986, Carey 1991, Meiselman and Doyle 1996). It is also thought that the vertical complexity within a stand increases that amount of habitat capable of supporting red tree voles (Carey 1991). Thus, as a measure of the canopy heterogeneity present within a stand, the diameter diversity index will be used for correlation with habitat capability. Stands with DDI <3.0 are hypothesized not to be suitable for red tree voles and corresponds with stands <25 yr old. Due to the vertical complexity present within “old” forest stands, DDI scores >8.0 are considered to be “optimum” habitat. “Old” forest habitat corresponds with stands >160 yr old (Spies, unpublished data). Between very young stands (i.e., <25 yr old) and old stands (i.e., >160 yr old), it is hypothesized that red tree vole habitat capability increases logistically in relation to increases in the diameter diversity index. The index is calculated from the equation:
|If DDI ≤ 3.0 then S4f = 0;||
|If DDI > 7.926 then S4f = 1.0;|
|else S4f = 2.37*(Log10(1/3*DDI))|
where S4 = canopy heterogeneity index, f = focal pixel, and DDI = diameter diversity index.
|FIG. D4. Relationship of index to diameter diversity index.|
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