DIGITALNA ARHIVA ŠUMARSKOG LISTA
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correlation of air temperature and soil volumetric water content was significant, negative and weak, i.e., r = -0.36 for the large gap and r = -0.22 for the small gap (Table 6 and 7).
The microclimate in the resulting forest gaps depends on the macroclimate of the area in which they are located, gap size or surface area, gap shape, gap direction, condition of natural regeneration in the gap and on the ratio between gap width and average height of the surrounding trees. In the present study, no significant differences were observed in the mean air temperature between the forest gaps and control forest stands, or between the large and small forest gaps. These results are in agreement with Muscolo et al. (2007), who did not find significant differences in air temperatures between gaps of different size, or between gaps and control plots. In this study, increasing forest gap area had no significant effect on air temperature, though it did result in a significant increase of soil temperature, while the air humidity and volumetric soil water content decreased. Muscolo et al. (2007) found that large gaps had higher values of soil temperature and photosynthetic active radiation, and lower soil moisture than small gaps. These results are comparable to the results presented here for soil temperature and soil moisture. Solar radiation is filtered by the forest cover through the tree layers to the forest soil, allowing the forest soil to absorb only a small percentage of energy. With the formation of forest gaps, the protective layer of trees is lost and higher quantities of solar energy reach the soil surface, changing the soil temperature regime in the newly formed gap (Ashton 1991). This alters the forest soil microclimate. Forest succession and undergrowth can also affect temperature. Soil temperature also depends on the number of plants from natural regeneration. If regeneration is good, i.e. if the density of juvenile plants is high, differences in soil temperature are lower as the soil is protected by the crowns of the young plants. Although regeneration was more pronounced in the large forest gap edge area than within the large gap, the number of plants was very small and insufficient from the silviculture standpoint, and hence had no major effect on equating the values of soil temperature among positions in the forest gap. Regeneration was also more pronounced in the small forest gap edge area than within the small gap, and the D/H ratio likely contributed to moderate variation in soil temperature in the small gap edge area in comparison with the control forest stand.
In artificially created gaps, the soil moisture in the gap was found to be higher than soil moisture in the forest stand, with a significant difference between the small gap and forest stand (Albanesi et al. 2008). This was also confirmed by Cutini et al. (2004) in the gaps of silver fir stands in the Central Apennines (Italy). In the present study, soil moisture differed significantly in all positions of the large and small forest gaps. Furthermore, soil moisture also differed significantly between the large and small gaps. The results for the values of soil water content in the forest gaps in the present study corroborate those of Muscolo et al. (2007) and Albenesi et al. (2008) for artificial forest gaps. The reduced soil moisture levels in the large gap compared to the small gap can be explained by the extremely hot and dry years, size of the gap and by large shadows of edge trees in the small forest gap. A significantly lower value of soil moisture in the small gap edge area compared to the large gap edge area can be explained by the large surface of the horizontal projection of edge tree crowns in the small gap and the consequential high amount of interception. High air