The higher temperatures on south sides of trees in 

 thinned stands could be a deterrent to MPB landing and 

 boring into the bark. In our study, south-side tempera- 

 tures between 10 a.m. and 2 p.m. averaged 2.3 °C higher 

 than those on north sides, with the maximum tempera- 

 ture 12 °C higher in the thinned than unthinned stand. 

 Powell (1967) reported subcortical temperatures were 

 occasionally 35 °C or higher on south sides. Beetles 

 emerge at a greater rate from south than north sides of 

 trees (Safranyik and Jahren 1970). North-side tempera- 

 tures in thinnings in our study would not have been a 

 deterrent to beetle infestation and offer more favorable 

 physical environment than south-side temperatures for 

 attacking MPB. Beetle attack densities are higher on 

 north sides (Reid 1963; Shepherd 1965), and when trees 

 are strip-attacked, the attacks usually occur on north and 

 east sides (Mitchell and others 1983). 



The effect of temperature could be more subtle than a 

 direct inhospitable physical environment on south sides of 

 trees. Mountain pine beetles may have evolved behavior 

 to avoid situations where their brood are not likely to 

 survive. In thinned stands, where tree temperatures are 

 a few degrees above those of trees in unthinned stands, 

 MPB may proceed too far in their development before 

 winter, thus entering winter in stages that are susceptible 

 to freezing — for example, the pupal stage as observed by 

 Reid (1963) and Amman (1973). 



Another way that MPB behavior may be affected by 

 thinning is through the disruption of the pheromone com- 

 munication system. More sunlight penetrated the canopy 

 in the thinned than unthinned stands, resulting in signifi- 

 cantly higher soil temperatures. The increased soil tem- 

 peratures, which averaged 4.8 °C higher in the thinned 

 than in the unthinned stand in this study, increase con- 

 vection currents (Rosenberg and others 1983) and air 

 turbulence that could disrupt pheromone plumes and re- 

 sultant MPB communication. In addition, windspeed is 

 greater in thinned than unthinned stands, thus possibly 

 further compHcating pheromone communications by MPB. 

 Beetle response to pheromones is more predictable at 

 windspeeds under 5 km/h, but a few beetles flew at 

 7.5 km/h (Gray and others 1972). Twice as many males as 

 females flew at windspeeds in excess of 4 km/h. 



In dense stands sunlight is absorbed by the upper levels 

 of the tree canopy that in turn heats the surrounding air, 

 creating instability in the air within the upper canopy. 

 This creates an inversion in the stem zone that is charac- 

 terized by more stable air (Chapman 1967; Fares and 

 others 1980). Inversions tend to be more pronounced in 

 dense stands than in sparse ones (Fares and others 1980; 

 Fritschen 1984). Aerosol movement below a dense canopy 

 on a sunny day is trapped beneath the canopy until it 

 flows to a point where the canopy is less dense or has an 

 opening (Fares and others 1980). Solar energy penetrat- 

 ing through canopy openings to the forest floor heats the 

 ground and adjacent air, which becomes buoyant and 

 rises through the canopy opening, carrying the aerosol 

 with it (Fares and others 1980). The aerosol or phero- 

 mone plume would be torn apart in the faster, more tur- 

 bulent air currents above the canopy. Therefore, when 

 MPB infest a tree in a recently thinned stand, canopy 

 density usually is insufficient to trap the pheromone and 



move it horizontally to attract other beetles. Rather, the 

 pheromone rises through the canopy on convection cur- 

 rents and is dispersed above the canopy. Schmitz and 

 others (in press) concluded that most MPB fly in the bole 

 area beneath the canopy where the pheromone communi- 

 cation system would be most effective. 



When MPB do infest a tree in a thinned stand of lodge- 

 pole pine, usually only the single tree is infested, and 

 occasionally a nearby tree when spacing is not main- 

 tained. The openness of the stand causes convection cur- 

 rents created by solar insolation to transport the phero- 

 mone plume around infested trees vertically out of the 

 stand rather than horizontally. Thus, the infestation of 

 other trees would be dependent on the degree of thinning. 

 Older thinnings will probably provide a suitable environ- 

 ment for MPB. As the thinned stand matures, shade will 

 increase and light and temperature decrease because of 

 increased crown size, development of a shrub and young 

 tree layer, and failure of limbs to prune. Therefore, stand 

 microclimate will likely become conducive to beetle infes- 

 tation before tree competition becomes severe. 



Beetle Response 



Mountain pine beetle response to baited funnel traps 

 was much less in an uninfested thinned than in an unin- 

 fested unthinned stand located 1 km from stands that 

 contained weather instruments, with only about 5 percent 

 of total beetles caught in the thinned stand. Beetles may 

 have passed through the thinned stand without detecting 

 a point source of aggregative pheromone. Beetle abun- 

 dance was also reflected in the percentage of trees killed, 

 which was much less (2 percent) in the thinned stand 

 than in the unthinned stand (16 percent) where weather 

 instruments were located. Schmitz and others (in press) 

 caught fewer beetles in passive traps in heavily thinned 

 than in lightly thinned and check stands in Montana, and 

 McGregor and others (1987) found significantly fewer 

 infested trees in heavily thinned than in lightly thinned 

 and check stands. Because air temperatures in thinned 

 and unthinned stands are about the same, beetles may 

 sense the difference in light intensity or the greater air 

 turbulence in thinned stands and avoid the open stand. 

 Light could serve as an integrator of other micrometeorol- 

 ogical features such as temperature, humidity, and air 

 turbulence. Shepherd (1966) showed in laboratory studies 

 that MPB increased attempts to fly as light intensity and 

 temperature increased. 



Our observations suggest microclimate could play a 

 significant role in MPB behavior in lodgepole pine stands. 

 Infestation risk of managed LPP stands possibly could be 

 assessed by monitoring stand microclimate, specifically 

 light. If microclimate changes are responsible for keeping 

 beetles at low levels or completely out of thinned stands, 

 forest managers could use such information to write stand 

 management prescriptions to minimize beetle infestation. 

 As crown closure begins to occur in partial cut or thinned 

 stands, a favorable microclimate may occur and invite 

 beetle attack, regardless of tree vigor. Additional studies 

 are needed of MPB infestation in thinned stands to deter- 

 mine more definitively microclimatic thresholds of MPB 

 infestation and the association of thresholds with tree 

 vigor levels. 



9 



