Roots are stressed quire often under natural conditions, e.g. when considering sloping terrain, layers of fluvial deposits, huge layers of melting snow, load of heavy forest machinery during logging and hauling operations, recreational activities of people, high density of deer or cattle, etc. We focused our experiments on Norway spruce (Picea abies [L.] Karst.) seedlings grown in containers with glass walls under the permanent load of 5.1 kPa during the whole growing season. The applied pressure affected roots both directly and indirectly due to the occurrence of hypoxia. Root growth ceased under such conditions. Growth dynamics and capability to occupy available soil also changed. For example, the total root area of experimental plants decreased to 52% but the root area index (RAI) was higher by 33% when compared to the control. It indicates that the pressure applied to the soil surface caused the development of only smaller root systems but more compacted into smaller volumes of soil. Mean longitudinal growth of stressed roots decreased by about 50% when compared to the control. Growth of experimental roots was also delayed, which is a typical general response to stress. However, a tendency to create dense and small root systems is in contradiction with the typical strategy of tree root systems.

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ABSTRACT:In summit parts of the Orlické hory Mts., massive dieback of forests occurred during an air pollution/ ecological disaster in the 70s to the 80s of the 20 th century. Destroyed stands were cut down and deforested areas were planted by more resistant species including mountain pine (Pinus mugo Turra). The aim of the paper was to assess effects of mountain pine on the prosperity of other tree species. Four localities with mountain pine stands were selected at altitudes from 1,000 to 1,115 m. In 2003-2005, growth analyses of shrubs were carried out there and basic parameters of stands were determined such as: height; cover of mountain pine, other tree species and herbs; radiation transmittance and temperature gradients. Results show the importance of the height of mountain pine stands in mitigating climatic changes in the ground layer of air, identification of the light compensation point for the mountain pine prosperity, the dependence of a growth type on outer and inner conditions of mountain pine including partly its age. Under given conditions, the present stands of mountain pine on top parts of the Orlické hory Mts. show their justification because they create a good protection for young seedlings, self-seeding and newly planted tree species. If forest spreads there, the markedly heliophilous mountain pine will recede similarly as in case of more than hundred-year-old mountain pine in the Sowie gory Mts. in Poland.

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