Needles and Litter Beds 



The needles used in all tests were first-year cast, collected in September and 

 October. The needles were sorted to remove broken needles and those separated from 

 the fascicle. This tended to eliminate differences in moisture responses because of 

 physical defects. The general physical properties of these ponderosa pine needles are 

 (Brown 1970) : 



Particle density 

 Surface area/volume 

 Average thickness 

 Shape factor 



31.8 Ib/ft^ (0.51 g/cc ±0.046) 

 1,755 ft^/ft^ (57.57 cm^/cm^ ±6.81) 

 0.027 in (0.0695 cm) 

 1.3 



The litter beds of the needles for the EMC tests were placed in two wire screen 

 containers to occupy a bulk density of 0.94 Ib/ft^ (0.015 g/cc). This bulk density is 

 the medium value found by Brown (1970); the values for the light, medium, and heavy 

 bulk densities were, respectively, 0.31, 0.94, 2.81 lb/ft3 (0.005, 0.015, and 0.045 

 g/cc) and were used for the tests of bulk density effects on moisture response times. 

 The needles were loaded to a depth of 2 cm for each bulk density. Litter beds for the 

 EMC tests were preconditioned at a high and at a low relative humidity until stabilized 

 with their environment, then transferred to a final conditioning cabinet at a controlled 

 humidity and allowed to stabilize. Stabilized conditions were considered to be achieved 

 when consecutive moisture content determinations were within ±1 percent moisture content. 



Pine needles for the response time tests were preconditioned at a high humidity, 90 

 percent relative humidity at 75° F (24° C). When the needles were stabilized in moisture 

 content at approximately 23 percent moisture content ovendry weight, a precalculated quan- 

 tity of needles was weighed and transferred to the programable environmental chamber. 

 In the chamber, with conditions at 90 percent relative humidity and 80° F (27° C) , the 

 litter beds were made to the desired bulk density on special aluminum weighing trays 

 with solid bottoms and sides to minimize airflow within the litter bed (fig. 1). 



The chamber and litter beds were conditioned as shown in figure 2. Adsorption test 

 time was 24 to 48 hours, after which the litter beds were removed, weighed, and moisture 

 content determined to provide a check of the weighing system's recorded weight change. 

 These needles were discarded and another quantity of conditioned needles used for the 

 next test. 



The environmental chamber controls and air conditioning equipment are capable of 

 making the change for 90 to 20 percent relative humidity in 60 minutes and from 20 to 

 90 percent relative humidity in 5 minutes. Some error in response time measurements 

 can be caused by the response time of the chamber. Because pine needles have long 

 response times (Simard 1968b; Van Wagner 1969; Fosberg 1975), greater than 4 hours, 

 errors should be small. 



The influence of solar heating upon the moisture response was investigated by 

 running a second series of sorption tests using the solar heating capability of the 

 environmental chamber. Nine overhead solar lamps provided the radiant heat simulating 

 solar heating and were controlled by a pyroheliometer sensor and an electropneumatic 

 recorder controller. A 3-mil Cu-Cn thermocouple was attached to the surface of a 

 needle to monitor surface temperature on a strip-chart recorder. Solar heating was 

 started at the beginning of the desorption run and maintained until the start of the 

 adsorption run, when it was turned off. An intensity of about 0.6 solar constant 

 (1.2 cal/cm^-min) was used. These tests provided information on how response time was 

 influenced by the additional temperature stress of solar heating. 



3 



