Light ecology and regeneration on clearings of sessile oak-Turkey oak forests in the Visegrád Mountains, Hungary

摘要

Oak woods are the most important forest types growing on 570,700 hectares in Hungary that is about 32% of total forested lands. This paper reports results about the regeneration succession on clearings of sessile oak-Turkey oak forests, following successive clear-cut harvesting practice. Phytosociological relevés were taken, according to the space for time substitution model, covering stand ages from 2 to 28 years old. Major steps of the forest regeneration were analysed by multivariate methods and four stages were determined: I = 1-3 years, II = 4-11, III = 12-21(-25) and IV = (22-)26-28 years. Light climate of the four stages were characterised by measuring relative irradiation under clear sky conditions (RI) at four elevations (0, 20, 40 and 80 cm above ground). Herb layer phytomass was studied by the harvesting method in the same stands where RI was determined.Considering statistically significant differences in the studied variables among stages, two main stress periods were distinguished. The most drastic stress event appears during the transition from the mature forest stage to stage I. It is associated with a great and sudden increase in RI at the herb layer level (at 80 cm above ground RI was 95.5%). Also the amount of total herb layer phytomass of stage I increases considerably reaching more than three times higher values than that found under cutting age mature stands.The second stress period occurs in stage III. Significant decline of light intensity occurs during the transition from stage II to III, resulting an RI of 2-2.5%. Herb layer phytomass also becomes significantly reduced in this stage, amounting only 4 g DW/sqm. During this rather unfavourable period the majority of typical sessile oak-Turkey oak species disappear from the stands. A strong correlation between log(RI) and the herb layer aboveground phytomass was also established.The described two stress phases may lead to forest degradation, since the species able to survive the first stress phase with high RI probably cannot tolerate the very low illumination level in stage III. To prevent losses from the flora and vegetation, or at least to mitigate the damage, more frequent thinning in the thin pole phase (stage III) is recommended. Another solution would be the cultivation of uneven-aged forests, with selection cutting or single-tree selection.