OCR Text |
Show 8 m winter, but assimilation starts again with thaw although the tree may still be covered with snow. Assimilation shows a sharp rise late in spring when daily average needle temperature exceeds 10° C, only to fall off again when sprouting starts late in July. A substantial difference between amount of carbon dioxide absorbed and the amount of mass added by growth has been observed but not yet explained. Investigation of temperatures and freezing point of Siberian pine needles shows annual variation of frost hardiness. Old trees are much more frost-resistant than young ones, and young trees from nurseries may be easily damaged if snowfall does not protect them from frost. The greatest damage to Siberian pines is caused by low- temperature parasitic fungi, mainly the snow- blight, which is most effective in zones of long-lasting snow cover. The maximum damage occurs in just those plant communities which offer the most favorable seed beds for natural pine reproduction, hence the maximum winter snow depth takes on a critical importance. This species of tree is not suitable for reforestation with present techniques in areas of persistent deep snow cover. Extensive wind tunnel and field tests with wind- deflection structures ( snow fences, kolktafeln, jet roofs, etc.) have developed techniques for extending areas susceptible to reforestation. From the standpoint of plant growth, these structures are used to equalize snow distribution, preventing deep accumulations and depositing snow on otherwise bare sites exposed to wind scour. From the standpoint of avalanche prevention they break up the mechanical continuity of slab formation. In essence they perform artifically these two functions of a forest cover and allow reforestation to proceed unhindered by avalanches |