Physical top soil properties in pure stands of Norway spruce (Picea abies) and mixed species stands in Austria

This study was done to evaluate the impact of pure Norway spruce stands on physical soil properties of top soil in comparison to mixed species stands on comparable sites. It was hypothesized that the flat root system of spruce causes soil compaction, which would have a negative impact on the soil aeration and hydraulic properties and consequently on seedbed quality, as well as early tree growth and seedling establishment. Hence, this topic is important for forest restoration, especially converting secondary pure spruce stands to mixed species stands. Forty-eight sites (24 pairs pure spruce stand/mixed species stand) of different stand development stages (mature stage, pole stage) were selected on two different bedrock materials (Molasse, Flysch). Undisturbed soil cores were taken from 0-4 and 4-8 cm soil depth and the following soil parameters were determined: (total) bulk density, remaining fine soil bulk density, dry masses of coarse fragments, roots and forest floor, organic carbon content, total soil pore volume as well as macropore volumes (after free drainage for 24 h and after water desorption at 10 kPa). The remaining fine soil bulk density was a useful parameter for characterization of the state of compactness. Pure spruce stands caused a lower bulk density of the upper mineral soil due to lifting and loosening of the soil above the root system. Results of calculated macropore volumes after water desorption at 10 kPa were exactly conform with those obtained for the remaining fine soil bulk density, indicating significant differences for the grouping variables bedrock material and species composition. It is concluded from this study that changes of soil physical properties of the upper mineral soil (0-8 cm soil depth) by Norway spruce will not reduce germination and growth of mixed species trees. However, chemical and nutritional changes were not subject of this study, which are expected to limit the success of forest restoration. (C) 2000 Elsevier Science B.V. All rights reserved.