RESPONSES OF PICEA, PINUS AND PSEUDOTSUGA ROOTS TO HETEROGENEOUS NUTRIENT DISTRIBUTION IN SOIL

The spatial distribution of plant-available mineral nutrients in fores t soils is often highly heterogeneous. To test the hypothesis that loc al nutrient enrichment of soil leads to increased root proliferation i n the nutrient-rich soil zone, we studied the effects of nutrient enri chment on the growth and nutrient concentrations of Douglas-fir (Pseud otsuga menziesii (Mirb.) France), Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) roots. Three-year-old seedlin gs were grown for 9 months in split-root containers filled with nutrie nt-poor forest mineral soil, with one side supplemented with additiona l mineral nutrients. Root dry weight and root length in Scots pine and Norway spruce were increased in the nutrient-supplemented soil compar ed with the nonsupplemented side, whereas root growth in Douglas-fir w as unaffected by nutrient enrichment. Of the three species examined, N orway spruce exhibited the highest root and shoot growth and the highe st nutrient demand. Specific root length (m g(-1)) and the number of r oot tips per unit root length were not affected by local nutrient addi tion in any of the species. Despite increased root growth in Norway sp ruce and Scots pine in nutrient-supplemented soil, their root systems contained similar nutrient concentrations on both sides of the split-r oot container. Thus, coniferous trees may respond to local nutrient su pply by increased root proliferation, but the response varies dependin g on the species, and may only occur when trees are nutrient deficient . As a response to local nutrient enrichment, increases in root dry ma tter or root length may be better indicators of pre-existing nutrient deficiencies in conifers than increases in root nutrient concentration s.