REVERSIBILITY OF DAMAGES TO FOREST FLOOR PLANTS BY EPISODES OF ELEVATED HYDROGEN-ION AND ALUMINUM-ION CONCENTRATIONS IN THE SOIL SOLUTION

Herbs and grasses may episodically be exposed to elevated concentratio ns of potentially toxic elements as H+ and Al3+ due to considerable te mporal variability in the chemical composition of soil solutions in th e uppermost layer of the soil profile. Greenhouse experiments were per formed to test the effect of episodes of different length (1 and 2 wee ks) and concentrations of H+ and Al3+ (pH 3.8, 4.0, 4.2 and 4.5; 0, 20 and 70 mu M Al) on root and shoot growth, designed to elucidiate the reversibility of growth inhibition. Three forest floor species were st udied - Galium odoratum and Lamium galeobdolon, having similar pH dist ribution ranges in the field and Poa nemoralis which also occurs at sl ightly lower pH. The plants were grown for 5 weeks (episode and recove ring time) in a synthetic soil solution in a flowing solution system w ithout recirculation. The species reacted in three different ways. Gal ium odoratum was the most sensitive species and seemed to be irreversi bly damaged (ceased growth) by 2 week episodes of pH less than or equa l to 4.0 and Al greater than or equal to 20 mu M at pH less than or eq ual to 4.2. Lamium galeobdolon was about equally sensitive during the episodes but it had a much greater ability to recover. Poa nemoralis w as rather insensitive to the episode treatments tested. It is conclude d that episodes of elevated H+ and Al3+ concentrations may be decisive for plant performance. It is therefore important to consider the extr eme environmental conditions which plants may be exposed to in the fie ld, in addition to long-term averages of e.g. soil solution concentrat ion of potentially toxic elements, when studying species distribution and performance in relation to soil chemical properties.