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gradual decrease in, biomsss occurred due to the periodic mortality of individual trees that is common in mature climax communities. The second type of per- turbation occurred at the 3500m location on Missionary Ridge where a severe loss in standing crop is shown for 197~; this loss can be attributed 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 decomposition; this type of biomass pattern can be seen at the lower elevationa1 plots at both locations. Kira ~ a1. (1967) concluded that mortality rates in forests of cool climates fluctuate from year to year and that in certain years, several trees are lost in one year due to environ- mental catastrophes, such 8S 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 400 E 350 ~ 300 X ~ 250 . W 200 a ~ 150 0 Z 100 ~ 50 ##ffoo,o, I ## 0,0,. ## O'o,......~ ..................# ~~ '. .. '. DEe JAN FEB MAR APR MAY JUNE JULY , " I Pass locations than on Missionary Ridge and, accepting 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 temperature having a'minor effect. From these data, it could be conciluded that the length of the growing season is largely independent of the direct effects of persist- ing : snow. I I Dir4ct 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 sunlight. 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 pr04uction, of trees of these size classes, the wood production of the stand as a whole is not significant- ly affected. Such events sho~ld be noted, however, because these smaller trees represent the future of these stands and any such damage could reduce the number of trees available for replacing those lost in the overs tory. I I I Ii If l .t' , i ~ I, I I I~ i c '1 Indirect effects of snow on wood production are those influences on the forest microclimate including modification of the soil temperature and soil water regimes during the early part ~f the growing season. The:influences of these facto~s on the physiology of trees during the growing season are being analyzed by the Tree Moisture Stress P~oject and are included in the report of the next sec~ion. Regardless of influences of snow within a given growing season, the bolewood production of individual trees of the spruce-fir association has been shown to remain relatively const8nt'~ver the past 20 years 400 E 360 ~ 300 X ~ 250 . w 200 0 ~ 150 0 z 100 ~ 50 . ~ ........~~...... ........ ....... ........................... \ .. .... , , , I. II .1 II ii IY I II ~ II " 9 II DEe JAN FEB MAR APR /'ylAY JUNE JULY MISSIONARY RIDGE - 3200m MISSIONARY RIDGE-3500m ~ DATE OF CAMBIAL ACTIVATION (BLAUE,19[73) 1972 1973.......... 400 E 350 ~ 300 X ~ 250 . w a 200 ~ 150 0 Z 100 ~ 50 ##...... # ...... I ### ~~~ . ................ ..# ~~~ .... ..........0# .... '. " " \'1- , , , , , JULY DEe JAN FEB MAR APR WOLF CREEK PASS - 3100m 400 E 350 ~ 300 x ~ 250 . w 0 200 ~ 150 0 Z 100 ~ 50 #........ i ## ....' ### ~........ ..................... ........ '. " ............ " ", DEe JAN FEB MAR APR WOLF CREEK PA1S-3300m intensive sites, 1972-1913 Figure 4 - Snow depth and duration at 23