Fact Sheet #25
Alternate Planning for small forest landowners in eastern Washington under the Forest and Fish Rules

Figure 1:

BA distribution post harvest for riparian inner zones in low elevation case study sites under the FFR.

Outcomes depicted in Figure 1 suggest that AP that alter stand density and thus mitigate insect dynamics while still meeting economic and riparian functional requirements are needed. In determining appropriate thresholds, AP to address MPB risk must consider "stockability". Stockability refers to the inherent biological carrying capacity of the site which can be inferred from the plant association or habitat type (Cochran et al. 1994, Fiedler et al. 1988). Stockability measures are derived relative to 'normal' or full stocking using site index (SI), maximum SDI, and growth basal area (GBA) (Cochran et al. 1994). The types of relationships between inherent carrying capacity of low productivity sites and insect attack identified in the figures and literature would indicate that AP on low productivity sites should look at maintaining lower basal areas than those that occur under the FFR criteria, given current stand conditions. In addition, linking harvest timing to mortality indices and basal area increment decline rather than FFR upper basal area limits, could reduce the stand stress that is a precursor to MPB infestation. Under the FFR, riparian function is maintained using 5 key leave tree requirements: minimum BA, TPA and DBH, upper BA limit, and largest tree requirements. Collectively, these variables can be described using stand density index or SDI, which forms a baseline metric for comparison of disparate stand conditions to the riparian functional requirement.

Stand density index (SDI), as given by the equation SDI = TPA(DBHq/10)^1.6, gives a relative density measure of the number of 10" trees/acre of land (Reineke 1933). Calculating a SDI for the FFR riparian requirements of 50 TPA> 10" and a basal area of 60 square feet/acre gives a SDI value of 95. For low elevation stands, the minimum acceptable target SDI is 95 with only trees >10" dbh considered acceptable in meeting riparian functional requirements. With alternate planning, the options for meeting the SDI requirements can be expanded by considering the dual metrics of stand density (TPA) and quadratic mean diameter (DBHq). Table 1 outlines the relationship between these metrics in meeting the SDI goal of 95. It should be noted that in all cases the basal area for a SDI of 95 is below the threshold for MPB infestation for either ponderosa or lodgepole pine. If we use the SDI of 95 as an acceptable measure of riparian functionality and treat the riparian zone in case study 7, a site in Okanogan County that continues to experience mortality from mountain pine beetle, the metrics are given in Table 2. If riparian function is given by a SDI of 95, then these riparian stands largely meet or exceed that goal. The stands also maintain a basal area below that of the susceptibility level for MPB. Table 3 compares the economic outcomes relative to the baseline under FFR and the AP developed to address riparian functional requirements while addressing bark beetle risk for Case 7.

Mortality and stand basal area growth rate declines were apparent in the simulation results for Case 7 once the stands exceed 80-90 square feet of basal area/acre. This decline is consistent with the literature which indicates that growth rates, as determined by basal area increment, decline at these stocking levels (Fiedler et al.1988, Larsson et al. 1983). Alternate plan strategies for Case 7 used the simulated growth rate decline as an indicator for riparian stand entry which meant that stand entry occurred prior to meeting the required upper basal area limit. At entry, a SDI of approximately 95 with approximately 50-60 square feet of basal area and 50 trees > 10" dbh were retained. Limits on leaving the 21 largest trees were ignored to effect basal area reduction without excessive loss of tree cover necessary to meet shade requirements. A comparison of the outcomes for FFR and the AP are given in Figure 2. Figure 2 indicates that the average stand condition over 90 years under the FFR scenario is on the isoline where mortality from MPB can be expected. Under a no management scenario, as would occur in the core zone, stands fall below the MPB risk zone in only 1 of 9 decades. In contrast, the AP scenario maintains an average basal area below the MPB risk level in all decades. For case study 7, the alternate plan is much more likely to address a currently existing MPB infestation than the FFR. The alternate planning approach used in this example could be applied for many situations where stocking control is desirable to avoid the unintended consequences of MPB infestation within riparian zones. Given the range of options in the look-up table (Table 1) for low elevation sites, optimal solutions regarding the number and size of leave trees can be varied to address specific LWD or shade requirements on a specific stream reach.

Conclusions

The stated intent of the rules in eastern Washington is to provide for restoration of riparian function while allowing activities that can ameliorate risks associated with disturbance agents within riparian zones. The development of AP that address susceptibility factors associated with MPB infestation while meeting riparian habitat requirements provide a potential solution. Adoption of a multi-metric approach that considers the equivalency of SDI across acceptable tree diameter classes can provide a mechanism to achieve risk reduction for multiple resource values, while improving economic consequences. Sensitivity analysis indicated that the growth models used in this analysis responded to site and habitat type parameters; this information can be used to tailor AP by habitat type to better reflect biological limits and stressors. In the long term, an eastside dynamic range model (EDR) might be a more comprehensive approach than risk avoidance approaches.