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(A) Absorptive root surface area (ARSA) for untreated spruce trees (control) and trees fertilized with potassium (K+) growing close to Zürich, Switzerland. (B) Fine root surface density for living fine roots of the corresponding spruce trees classified for diameter cut-offs 1 mm (black columns) and 2 mm (grey columns). Data were obtained by the earth impedance method (A) and by soil coring (B), respectively and are given as means ±SE.
