Influence of heavy-metal contamination on plant response to water availability in white birch, Betula papyrifera

Elevated concentrations of heavy metals in growth substrate are known to reduce root growth more than shoot growth. We hypothesized an increased sensitivity to drought in plants exposed to heavy metals. The hypothesis was tested using birch seedlings grown on a substrate with three levels of Cu–Ni containing slag (0%, 0.5% and 2.5%) mixed with sand, and were either well watered or exposed to drought. The experiment was conducted outdoors for 4 months. Both the slag addition and low substrate moisture reduced plant dry mass. There was a significant slag × moisture interaction. The effect of moisture was most pronounced on slag-free substrate, whereas at the highest slag addition level there was no growth response to moisture. Stem diameter, length of radial file and cell size showed similar responses, but error variation was high and the individual effects were not always significant. The general picture is, however, clear, the effect of moisture on all growth parameters increasing with decreasing slag concentration in the substrate. We conclude that metal-contaminated substrate leads to an inability of the plants to respond to improved soil moisture, an effect which can be seen even at metal levels which do not show any large growth reduction. In addition, reduced plant size, caused by heavy metals, results in feedbacks that increase the relative availability of water and mineral nutrients. At the highest slag addition level, substrate moisture was slightly higher than in controls, probably due to a reduced transpiration, and the senescence was slower, probably due to lower nutrient requirements of the smaller plants.