Understory plants as indicators of grand fir mortality due to the fir engraver

Mortality of grand fir trees, caused by the fir engraver, Scolytus ventralis, was monitored during 3 years on ten 0.1-acre (0.04 ha) circular plots in each of nine stands in northern Idaho. Understory vegetation was sampled on each plot on the basis of circular subplots of 0.03 acre (0.012 ha). Analyses showed four species to be strongly correlated with high and two with low tree mortality. The interaction between these groups of plant species provided a variable that increased as the proportion of high- to low-hazard plants increased. Various linear and nonlinear expressions were tested between the two plant groups and their interaction regressed against killed trees per acre. The plant group interaction term accounted for the most variation (r2=0.914) and produced the lowest standard error of the estimate (1.55). The equation for this variable took the form Y = 2.291 + 0.111ex. where X=plant group interaction. This equation provides an indication of the susceptibility of grand fir stands to mortality caused by the fir engraver.