Agenda - 09-29-1999 - 9e - Laserfiche WebLink

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CONTEXTUAL RATING (CR) OFATRACT OF FOREST The value of a forest also depends on its context in a landscape and especially on possible connections and corridors to other tracts of forest. We need a way to evaluate the contribution of the context of a forest to its value for wildlife. A simple procedure is to consider several zones at different distances from the "target" tract. To calculate a Contextual Rating for a particular target tract of forest, we chose zones extending from 0 to 0.5 miles, 0.5 to 1.0 miles, and 1.0 to 2.0 miles around the tract. We then calculated the average Intrinsic Rating for each acre in each of these doughnut - shaped zones (some details of this calculation are explained in Appendix II). The average IR/acre for each zone' (AVIR1, AVIR2, and AVIR3) tells us what an individual animal migrating outward from the target tract can expect to find in each of the zones around its origin. A dispersing individual is more likely to move a shorter distance than a longer one (if only because an animal must first cross the nearer zone before reaching the farther one). Consequently, in calculating the Contextual Rating for the target tract, we gave the nearer zones heavier weight. The average IR/acre for the closest zone (AVIRI) was multiplied by 4, that for the middle zone (AVIR2) by 2, and that for the farthest zone (AVIR3) by 1. We added the three resulting numbers together to obtain a Contextual Rating (CR) for the target tract: CR = (AVIRi x 4) + (AVIR2 x 2) + (AVIR3 x 1) This rating indicates the chance that an animal dispersing from the target tract will find suit- able habitat nearby. As emphasized above, little is known about the ways animals move between patches of habitat. Of course, different species have very different chances of moving to any particular distance. It is only necessary to think about a Box Turtle and a Pileated Woodpecker to realize these differences between species. Nevertheless, any animal is more likely to move a shorter distance than a longer distance. In addition, a dispersing animal is more likely to find suitable habitat when there is on average more suitable habitat in the surrounding zones. Thus this Contextual Rating incorporates the basic features that might influence these movements. The Contextual Rating is incomplete near the boundaries of the County, because parts of the three zones fall outside the County. The County's maps of forests in Orange County extend far enough over the boundaries to allevi- ate this problem. Regional efforts to plan for preservation of forest would eliminate this problem altogether. Contextual Ratings for tracts of forest in Orange County in 1988 appear in Map 2. COMBINING INTRINSIC RATING AND CONTEXTUAL RATING INTO A TOTAL RATING (TR) The Contextual and Intrinsic Ratings for a tract of land can be combined into a Total Rating by adding 1 to the Contextual Rating and then multiplying the two ratings together: Total Rating = IR x (CR + 1) The Contextual Rating, after all three zones are combined, is a weighted average rating per acre. Like the maturity of a forest or its degree of "edginess ", the context of a forest multiplies the overall value of the tract for wildlife. 13