Above- and belowground growth response of Picea abies seedlings exposed to varying levels of soil relative bulk density

Abstract

During mechanized forest operations, machines with high wheel loads are operated directly on the soil and can pose serious concerns for the integrity and quality of soil properties. Simply relying on absolute soil density (g cm−3 or t m−3) as a measure of qualitative assessment remains site specific. Instead of absolute soil density, the aim of this study was to assess and quantify the above- and belowground biomass growth response of Norway spruce (Picea abies [L.] Karst) seedlings exposed to varying relative bulk densities (RBD), expressed as a ratio between field bulk density and maximum bulk density achieved through the standard Proctor test. In the laboratory study, silt loam collected from a forest stand located near Munich, Germany, was placed inside 24 chambers (30.6 cm in length, 8.2 cm in width, and 42.0 cm in height) and compacted layerwise with a hydraulic press until the target RBD levels (0.67, 0.72, 0.77, and 0.82) were reached. After compaction, 120 Norway spruce seeds were sown and growth was monitored for 156 days. Morphologic measurements of 102 seedlings were performed after the growing period where root, shoot, and needle growth characteristics and mass were recorded. Detailed root analysis was performed with the WinRHIZO software, and nutrient analysis was performed on the soil as well as the root, shoot, and needle components of seedlings. Results indicate statistically lower average root (48%), shoot (43%), and needle (43%) mass per seedling for seedlings growing on the RBD 0.82 as compared to those growing on the RBD 0.67. Most analyses revealed no statistical differences between the two lower RBD levels, while statistical differences were detected as the RBD increased to 0.77 and 0.82. The use of RBD as a means of qualitative assessment of soil properties showed promise and should be investigated further for other economically important European tree species, while taking into consideration a longer monitoring period.