Figure 1 — Tower and surrounding instrumentation of 

 21X as it appeared in the field during the summer of 

 1986. 



measured for height and crown length. Stand density was 

 determined using basal area and number of trees per 

 hectare. 



Monitoring Microclimate 



An automatic recording device (21X micrologger, 

 Campbell Scientific) was used to measure the microcli- 

 matic factors (fig. 1) of temperature, incident solar radia- 

 tion, windspeed, and wind direction for a 23-day period 

 starting July 19, 1986. This time encompassed the peak 

 MPB flight period, which occurred in the area on August 

 3, 1986 (Julian day 215) (Schmitz 1987). Equipment 

 limitations dictated we could monitor a full complement of 

 factors on only two trees — one in the thinned and one in 

 the unthinned stand. Both trees were similar in height, 

 crown length, and d.b.h. (about 23 cm). Parameters were 

 recorded every 15 minutes, with averages calculated 

 hourly. 



Temperature — Thermocouple psychrometers were 

 connected to the micrologger to measure temperature at 

 the following points on or near the sample trees: 



1. At breast height, 1.4 m above ground, on the bark 

 surface and immediately below the surface for both the 

 north and south sides of the trees. The below-bark sur- 

 face probe was positioned to measure temperature of the 

 tree phloem. Phloem is the substrata in which MPB 

 adults mine and lay eggs and also serves as the food 

 source of developing larvae. 



2. In the lower third of the crown on the bark surface, 

 both on the north and south sides of the trees. 



3. On the tower (about 3 m above ground) where the 

 micrologger was situated. This sample area was to repre- 

 sent air temperature of the interspace of the stand. 



Incident Solar Radiation — LI-COR quantum sensors 

 were used to measure incident solar radiation. These 

 sensors measure photosynthetically active radiation in 

 the 400- to 700-nm waveband. Values reported were for 

 the average energy flux density for the sampled period 

 and are given in units of 1,000 \i einsteins/mVs. For both 

 treatments, these solar sensors were placed in three loca- 

 tions: one on the tower, one at breast height on each 

 sample tree, and one in the lower third of the crown. So- 

 lar sensors attached to the tree were placed on the north 

 side. 



Windspeed — Windspeed sensors were standard 3-cup 

 anemometers, which were connected to the micrologger. 

 Data were reported in units of kilometers per hour. Two 

 sensors were placed in both the thinned and unthinned 

 stands. One sensor was placed on the tower (about 3 m 

 above ground), the other in the lower third of the crown of 

 the sample tree. 



Wind Direction — The lightweight wind direction sen- 

 sor we used is an air-foil vane and a potentiometer, which 

 produced an output that varies proportionally to the wind 

 direction. Output was recorded in degrees and varied 

 between and 360°. This sensor was placed on the tower, 

 which would allow minimum interference from the trees 

 in each of the stands. 



Stand Temperatures— Temperatures in the thinned 

 and unthinned stands subjected to microclimate measure- 

 ments were surveyed to obtain a random sample of tem- 

 peratures between 10 a.m. and 2 p.m. on August 6, 1986. 

 This period was selected to record maximum solar pene- 

 tration into the stand. The survey was conducted on 10 

 transect lines 20 m apart; observations were made at 

 20-m intervals along these lines. Temperatures of the 

 ground and the north and south sides of the living tree 

 nearest to plot center were obtained with an infrared 

 thermometer (Wahl Digital Heat Spy Model DHS-14). 

 Tree temperatures were taken at breast height. Each 

 survey line passed through both thinned and unthinned 

 stands to avoid any large time lag (approximately 20 

 minutes used per line) that would have occurred if one 

 stand had been completely surveyed before starting the 

 survey of the other stand. 



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