SEEDLING ESTABLISHMENT IN RELATION TO MICROHABITAT VARIATION IN A WINDTHROW GAP IN A BOREAL PINUS-SYLVESTRIS FOREST

The characteristics of microhabitats of established Pinus sylvestris a nd Betula seedlings were studied in a small windthrow gap in a mature P. sylvestris-dominated forest in the Petkeljarvi National Park in eas tern Finland. Seedlings were strongly clustered in disturbed microhabi tats, particularly uprooting pits and mounds, formed by tree falls. Th ey covered 3 % of the 0.3.ha study area consisting of the gap and some of the forest edge. Although Betula occurred only as scattered indivi duals in the dominant canopy layer of the forest, it accounted for 30 % of the seedlings found in the study area. Betula regeneration was al most completely restricted to pits and mounds, where 91 % of the seedl ings were found. Uprooting spots were also the most important regenera tion microhabitats for Pinus, where 60 % of the seedlings grew, even t hough the seedlings were found in other substrates as well, particular ly on sufficiently decomposed coarse wood. Undisturbed field- and bott om-layer vegetation had effectively hindered tree seedling establishme nt, which emphasises the role of soil disturbance for regeneration. Wh ile the establishment of seedlings was found to be clearly determined by the availability of favourable regeneration microhabitats, the earl y growth of seedlings was affected by a complex interaction of environ mental variables, including the type of microhabitat, radiation enviro nment and interferences caused by competing seedlings and adjacent tre es. In the most important regeneration microhabitats, i.e. in uprootin g pits and on mounds, the distributions of the local elevations of Pin us and Betula seedlings were different. Pinus seedlings occurred close r to ground level, i.e. on the fringes of pits and lower on mounds, wh ile Betula seedlings grew deeper in pits and higher on mounds. The pos ition of the Betula seedlings indicate hat they may have a competitive advantage over Pinus seedlings in the dense seedling groups occurring in uprooting spots. We suggest that this initial difference in Pinus and Betula establishment may affect the subsequent within-gap tree spe cies succession and can, in part, explain the general occurrence of Be tula in conifer-dominated boreal forests.