Combined effects of acid mist and frost drought on the water status of young spruce trees (Picea abies)

This investigation was focused on the effect of frost drought following acid mist exposure on the water status and CO2 assimilation of young spruce trees (Picea abies). Trees cultivated in special containers were exposed to acid mist with pH 3 and a control mist with pH 5 for 10 weeks with three mist events per week during autumn. Frost drought: indicating frozen soil with atmospheric conditions allowing transpiration, was simulated in spring for 6 weeks. The controls for frost drought treatments were treatments with humid non-frozen soil. Acid mist application resulted in degradation of the epicuticular needle waxes with melted wax threads and larger lesions in the wax layer. Control needles showed a well-developed epicuticular wax layer. Frost drought inhibited transpiration and CO2 uptake in both mist treatments almost completely due to stomatal closure. In contrast, trees showed considerable transpiration and CO2 uptake rates in the treatments with non-frozen soil. Needle water potential of trees (control soil) was normal (-0.60 MPa), whereas water potential in trees of frozen soil was significantly lower, with -1.49 MPa in the control and -2.26 MPa in the acid mist treatment indicating water stress. We suppose from this finding that the deteriorated wax layer impaired the water retention capacity of needles and allowed higher cuticular transpiration compared with the control treatment. Frost drought was associated with a large decrease in starch concentration of needles paralleled by an increase of glucose and fructose concentration, which is suggested to have increased the osmotic potential. This rise in osmotic potential obviously enabled plants of the frost drought treatment + control mist to maintain their turgor which was not different from the turgor of needles from the non-frozen soil treatments. Needles of the frost drought + acid mist treatment, however, had a weak turgor, indicating that in this treatment a strong water stress prevailed. It is suggested that also under natural forest conditions frost drought preceded by acid mist exposure may cause severe forest damage and decline. (C) 1998 Elsevier Science B.V. All rights reserved.