Van Gunst, K.J. 2012. Forest Mortality in Lake Tahoe Basin from ...

18spatial spread of mortality agents among proximate conspecifics, and due to changes inspectral reflectance as needles become chlorotic and fall from tree.Therefore we used only the first incidence of mortality within a four-year period (asdefined by the dNDWI threshold), in a previously-healthy pixel. We used the temporaltrajectories of canopy fade by major bark beetle types within the Basin (Table 1-2) to seta mortality period at four years to allow for the longest sequence of bark beetle attackexpressed as canopy foliage change. Once a pixel reached the dNDWI threshold of atleast five percent loss of green canopy, that pixel was considered “available” formodeling, provided it had at least 6% green canopy.Using thresholded values and the four year window, we derived annual mortality foreach forest type as a quantification of the amount of new mortality seen on the landscapein each year. For the years 1986-1988, we excluded years that had not experiencedmortality in the preceding 1-3 years. Thus, “mortality” as used in this study equates to theonset of a significant canopy dieback event occurring within a 30m x 30m pixel.Mortality ValidationWe validated the five and ten percent loss of green canopy thresholds in 776 pixels,using 2010 high resolution imagery to select areas where gray and red canopies wereeither present (“mortality” pixels, n=277) or absent (“healthy”, n=499). We used the2010-2009 dNDWI values for the healthy pixels and the 4-year minimum of 2007-2006,2008-2007, 2009-2008, or 2010-2009 dNDWI for the mortality pixels. The majority ofmortality pixels were digitized in the High Meadows mountain pine beetle outbreak inthe southern portion of the LTB (Plate 2). Nearly all pixels were a mix of both red and