PREDICTING PINUS-PONDEROSA MORTALITY FROM DORMANT SEASON AND GROWING-SEASON FIRE INJURY

Understory prescribed burning was conducted in an immature Pinus ponde rosa (ponderosa pine) stand in southwestern Colorado during three seas ons, late spring, midsummer, and autumn. Tree mortality from various l evels of crown scorch was compared for the different seasons of injury . A total of 526 trees of different sizes, with crown scorch ranging f rom 20 to 100%, were monitored annually for 10 years. Over 80% of the 10-year mortality from injury in all three seasons had occurred by yea r 3, with over 90% occurring by year 4. Mortality of trees scorched in the spring and summer was about 2.5 times greater than that in the au tumn for similar crown damage. Most trees larger than 18 cm in diamete r survived autumn injury, even with greater than 90% scorching. Follow ing spring and summer injury, trees smaller than 10 cm in diameter die d readily with greater than 50% scorching, but about 90% crown scorch was required by large trees to be lethal. A logistic regression model was developed to predict the probability of mortality given tree size, scorch class, and season of injury. Because mortality was similar wit hin scorch classes less than 90%, they were combined into a single cla ss. Scorch thresholds with large increases in mortality occurred at 90 % and 100% crown scorch. The season variable includes two groups, dorm ant (autumn) and growing (spring and summer). Use of this model to pre dict mortality of immature P. ponderosa is appropriate where stand, fu el, and fire conditions resemble those of this study.