Effects of mounding and soil clay content on postplanting success of Norway spruce

Effective soil preparation by mounding is a common practice that precedes the outplanting of Norway spruce (Picea abies (L.) Karst.) in Finland, because of the improved survival and early growth of seedlings. On fine-grained soils, however, the postplanting performance of seedlings with mounding has been poorer than on coarser soils. The ecology of seedling growth and the effects of mounding on it on sorted fine-grained soils are still poorly known.This study examined the effects of mounding and soil clay content on the postplanting performance of Norway spruce container seedlings on soils ranging from medium-grained tills to fine-grained sorted clay soils in reforested sites in south-western Finland. First, regeneration results were inventoried on five plantations of differing soil texture. These reforestation sites had been mounded and planted with Norway spruce five to six years earlier through practical forest management. Secondly, mounding treatments (normal ditch mounding with a mound height of 15–25cm, ditch mounding with a lowered mound height of 5–10cm, spot mounding and unprepared treatment) were applied on five clearcut sites with different soil textures. These sites were mounded in the autumn and planted in the following spring, and the postplanting performance was measured after three growing seasons.The results showed that mounding decreases early postplanting seedling mortality and increases growth, thus promoting plantation establishment on forest soils with varying soil textures. The mounding method or mound height showed no clear difference in seedling growth in the studied soils and years. Early postplanting seedling height growth and survival (three years from planting) were slightly and root-collar diameter a bit better related to the soil clay content; the seedling attributes tended to be poorest on silty soils with a clay content of 20–30% (or silt and fine sand content of over 60%), compared with coarser or finer (clayey) soils. The results suggest that even effective soil preparation cannot fully alleviate all of the inherent disadvantageous effects of sorted silty soils on early postplanting and later growth; but corroborative research is still needed.