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gradual decrease in biomass occurred due to the periodic mortality of individual trees that is common in mature climax communities. The second type of pe~ turbation occurred at the 3500m location on Missionary Ridge where ,a severe loss in standing crop is shown for 1972; this loss can be attr buted to a high wind- storm that caused severe wind throw on that high eleva- tion site. Ovington (1962) stated that in mature climax forests, the biomass is fairly constant from year to year, as a result of an overall equilibrium between production and decompositionj this type of biomass pattern can be seen at the lower elevational plots at both locations. Kira et al. (1967) concluded that mortality' rates in forests-of-Cool climates fluctuate from year to year and that in certain years, several tre~s are lost in one year due to environ- mental cata~trophes, such as storms. Thus, the biomass values of such forests will fluctuate over time and the equilibrium value of climax can only be recognized on a long-term basis. These subalpine forests can therefore be best described as a "disturbance climax"; the influences of snow manipu- lation on such a system are difficult to separate from the other environmental influences and could only be studied over a long time period. Influences of snow on the spruce-fir association. The direct influences of snow on the spruce-fir association can be organized into two categories; first, its effect on the length of the growing season, and second, the physical effects of snow on the trees themselves. The snow melt pattern on the intensive study sites varied considerably between the years 1972 and 1973 (Figure 4). In 1972, the plots were free from snow almost a month earlier than the following year when Snow persisted beyond the date of cambial activation as estimated by Blaue (1973). In general, snow remains much later in the season on the Wolf Creek P~ss locations than on Missionary Ridge and, accept- ing these data, cambial initiation may often occur while snow is still on the ground. Blaue (op cit) found that the length of the growing season is set primarily by photoperiod, with variables such as t~mperature having a minor effect. From these data, it could be concluded that the length of the growing season is largely independent of the direct effects of persisting snow. D~rect physical influences of snow on individual trees is negligible on trees of the intermediate class or greater; however, saplings and seedlings can be adversely affected by extended periods of snow cover- age which increase the probability of snow breakage, snow mold infection, or simply elimination of sun- light. These conditions were observed to increase definitely on the biomass plots in the heavy snow year of 1973, compared to the other years of the study. Considering the small stature and hence, small wood production, of trees of these size classes, the wood production of the stand as a whole is not significantly affected. Such events should be noted, however, because these smaller trees represent the f9ture of these stands and any such damage could reduce the number of trees available for replacing those lost in the overstory. ;: .... 4.0 >. 4.0 " .. ~ ~ ~ X ., X ~ 3,0 5 .3.0 .s z z ~ ~ 2,0 ~ 2,0 ~ u u x= 3.10 ~ 5(;2.96 ~ 0 0 1,0 5= 0.03 0 1,0 5=0.02 0 CV=0.01.'. '" CV=0.01.'. '" .. .. 73 71 69 67' 65 63 61 59 57 55 TIME (yrs) MISSIONARY RIDGE 3200m 't 4.0 ~ . . ] 3.0 z o i= 2.0 U ~ o 01.0 '" .. X X =2.48 5 =0.02 CV:O.01.1. 73 71 69 67 65 63 61 59 57 55 TIME (yrsl WOLF 'CREEK PASS 3100m 13 71 69 67 65 63 61 59 57 55 TlMECyrs) ,MISSIONARY RIDGE 3500m '24.0 ~ . ~ :;; 3.0 oS z 02.0 ~ u ~ g 1.0 '" .. x X:2.38 5:0.02 CV:0.01.'. 73 71 69 67 65 63 61 59 57 55 'TIME (YN!i) WOLF CREEK PASS 3300m Figure 2. Stand bolewood production by sampling location 21 I I I I I I I I I I I I I I I I I I .'