Factors influencing pine engraver ( Ips pini Say) colonization of ponderosa pine ( Pinus ponderosa Dougl. ex. Laws.) slash in Northern Arizona

Abstract

Thinning projects have increased in recent years to reduce standing fuels and fire hazard within the Wildland Urban Interface (WUI) and to improve general forest conditions through forest health restoration treatments. As a consequence, large volumes of thinning slash have been generated, increasing the potential for growth of bark beetle ( Ips spp.) populations. Because slash can initiate outbreaks in standing trees, a critical examination of slash management guidelines is needed to minimize bark beetle impacts associated with thinning treatments. Specifically, we examined effects of season of slash cutting, sunlight intensity (stand density) and log size on pine engraver ( Ips pini Say) reproduction in North-central Arizona, USA. Ponderosa pine ( Pinus ponderosa Dougl ex. Laws.) trees were felled and cut to 16 sizes (diameters of 10, 15, 20, and 25 cm and lengths of 15, 30, 45, and 60 cm) in October, January, April and July, for 2 years. The 16 logs of differing size were placed in groups in overstory stand densities of 5, 14, and 27.5 m 2/ha (representing high, partial, and low sunlight levels). Response variables included attack density, emergence density, and beetle population growth rate. Log length had a positive relationship with both attack and emergence density. Log diameter had a significant effect on attack and emergence density, with a preference shown for intermediate diameter bolts. During a year with above average winter precipitation, cutting date and the interaction of cutting date and light level had significant effects on attack density. Fewer beetles attacked both bolts that had been exposed for longer periods of time and logs exposed to high sunlight levels. Cutting date, and the interaction of cutting date and light level, influenced emergence density, but light level alone did not. Emergence density was lowest in spring-cut logs in both years. Cutting date also had an effect on beetle population growth rate, with fall-cut logs having the highest population growth, while light level had no effect. Extended time-since-cutting (e.g. fall cutting), high light levels and small log diameter did not consistently cause reductions in beetle attack and reproductive performance as was hypothesized. Implications of these results for slash management guidelines are discussed.