Water uptake by planted Picea abies in relation to competing field vegetation and seedling rooting depth on two grass-dominated sites in southern Sweden

Norway spruce (Picea abies L. Karst.) seedlings were planted on 2-, 3- and 4-yr-old clear-cuttings. At the time of planting, the ground vegetation on the clear-cuttings was dominated by the grass Deschampsia flexuosa. Effects of various site preparation treatments (mounding and herbicide) were analy sed during the first 3 yrs after planting. After the third growing season, seedling dry mass was significantly higher in the herbicide and mound treat ments than in the control, but then was no significant difference between t he herbicide and mound treatments. During a drought period in August 1995, soil water potentials were lower in untreated plots and in mounds than in t he herbicide treatment, but seedling water potentials showed no evidence of water stress. The hypothesis that seedling growth was supported by water t aken up from beneath the rooting zone of the ground vegetation during droug hts was tested in two ways. By studying the relation between the root morph ology of planted seedlings and the rooting density of the ground vegetation , and by comparing the natural abundance of an isotope of oxygen (O-18) in seedling xylem water with its abundance at various depths in the soil profi le. More than 70% of the total root biomass of the ground vegetation was in the top 15 cm of the soil profile. Although the planted seedlings also had the major part of their root system in the top 15 cm, each seedling also h ad at least one sinker root extending down to greater depths, where the roo ting density of the ground vegetation was low. Soil water from the topsoil had a significantly higher delta(18)O than water from the subsoil, but its delta(18)O was about the same as or lower than that of xylem water. Therefo re, it was concluded that seedlings took up water mainly from the topsoil, irrespective of the treatment, and the above hypothesis was rejected.