Microclimate: an Alternative 

 to Tree Vigor as a Basis for 

 Mountain Pine Beetle 

 Infestations 



Dale L. Bartos 

 Gene D. Amman 



mXRODUCTION 



Temperature is an important factor in the ecology of 

 insects. It affects the physical conditions of habitats and 

 the insects themselves (Wellington 1950). In the case of 

 mountain pine beetle (MPB) {Dendroctonus ponderosae 

 Hopkins [Coleoptera: Scolytidae]), observations were 

 made on the effects of extremely high (Patterson 1930) 

 and low temperatures (Somme 1964; Yuill 1941). Be- 

 tween the extremes is an optimum zone of temperature 

 that may be modified by other microclimatic factors 

 (Rudinsky 1962). 



Thinning forests causes subtle changes not only in tree 

 physiology (Nebeker and Hodges 1983) but also in inci- 

 dent radiation, temperature, light (Reifsnyder and Lull 

 1965), and windspeed. These changes brought about by 

 thinning lodgepole pine (LPP) ( Pinus contorta var. latifo- 

 lia Engelm.) forests have profound effects on MPB activity 

 (Schmitz and others, in press) resulting in reduced tree 

 mortality in thinned stands (McGregor and others 1987). 



The silvicultural practice of thinning has been used in 

 the past as a way of increasing tree vigor (Graham and 

 Knight 1965; Keen 1958), which in turn should make the 

 residual trees better able to resist attacks by MPB. The 

 removal of large-diameter lodgepole pine, which are pre- 

 ferred by MPB, can also result in reductions in tree loss 

 during epidemics (McGregor and others 1987). Reduced 

 infestation by MPB occurs immediately following partial 

 cutting of LPP stands (McGregor and others 1987) and 

 before residual trees could express resistance due to in- 

 creases in growth and vigor as measured by periodic 

 growth ratios and growth efficiency (Amman and others 

 1988). This phenomenon suggests that factors other than 

 vigor may be responsible for reduced MPB infestation. 



Subtle changes in chemistry of residual trees following 

 thinning of stands is one possibility. Biochemical re- 

 sponses detected by the beetles could cause them to avoid 

 trees in thinnings. Nebeker and Hodges (1983) studied 

 the effects of thinning and thinning-related injury to lob- 

 lolly pine (P. taeda L.) by measuring oleoresin exudation 

 pressure, electrical resistance, resin flow rate, total flow, 

 and relative viscosity of resin. They found total flow and 

 relative viscosity were significantly different by month 

 and treatment, with the greatest increase in trees receiv- 

 ing only basal wounding. Matson and others (1987), also 

 studying effects of thinning loblolly pine, found resin flow 



rates, starch concentration in phloem tissue, relative 

 growth, and bark and phloem thickness were greater in 

 trees in thinned plots than trees in control or fertilized 

 plots, 6 years after treatments were applied. Raffa and 

 Berryman (1982) studied physiological differences be- 

 tween lodgepole pines resistant and susceptible to MPB 

 and associated microorganisms. They found no relation- 

 ship between resistance and daily rate of resin flow, rate 

 of resin crystallization, monoterpene content, monoter- 

 pene composition, or current growth rate. Resistant trees 

 responded to artificial inoculation of fungi vectored by 

 MPB by forming greater quantities of resin than suscep- 

 tible trees. However, Peterman (1977), in a field test of 

 the fungal inoculation method of distinguishing resistant 

 and nonresistant trees, found the method ineffective. 

 Therefore, we chose to explore the microclimate of the 

 altered stand because (1) microclimate changes immedi- 

 ately following tree removal from the stand and (2) resid- 

 ual trees in the stands were slow to increase growth. The 

 objective of this paper is to present microclimatic differ- 

 ences we observed between thinned and unthinned LPP 

 stands and to discuss beetle behavior in response to these 

 differences. 



METHODS AND MATERIALS 



The study site is south of Mountain View, WY, on the 

 north slope of the Uinta Mountains in northeastern Utah, 

 at an elevation of 2,865 m. A thinned and an adjacent 

 unthinned stand of LPP, both of which had current beetle 

 infestation,were selected for study of microclimatic differ- 

 ences. An additional thinned and an adjacent unthinned 

 stand of LPP, both of which had no beetle infestation, 

 were selected for study of beetle response to pheromone- 

 baited traps. 



Stand Characteristics 



Characteristics of the thinned and unthinned LPP 

 stands studied for microclimatic differences were deter- 

 mined through variable plot (10 BAF) cruising. Plots 

 were 50 m apart in a grid pattern in each stand. Trees on 

 the plot were tallied as live or dead by cause of death and 

 were measured for diameter at breast height (d.b.h.). The 

 dominant or codominant tree closest to plot center was 



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