Photosynthesis of damaged and undamaged forest trees - A study with spruce in the Fichtelgebirge : Waldschadensforschung - Wissensstand und Perspektiven. Internationaler Kongress

1990

S. 527-550

11 Abb., 32 Lit. Ang.

Unselbständiges Werk

200068794

Symptoms of the decline in Norway spruce in the Fichtelgebirge are photosensitivity and loss of pigments of the photosynthetic apparatus of the needles, as well as their reduced photosynthetic capacity and carboxylation efficiency. Those symptoms do not occur, or even disappear, when measures are taken to prevent Mg2+ deficiency in the needles. With damaged trees, magnesium was found to be withdrawn from the older needles by ghe competition of the new flush (mainly from the 1-year-old-needles). This withdrawal of nutrients resulted in the depression of photosynthetic activity. Soils of the forests at our experimental site are poor in Mg2+, Ca2+, and Zn2+. Figure 6 has shown that healthy at the sites were able to mobilize much more of these minerals than declining trees. Obviously, there is a close relationship between the viability of the trees and their ability to mobilize essential nutients. Mineral shortage of forest soils is a sommon phenomenon. It is accentuated by acid deposition, which causes protonation of cation binding sites in the soil, exposing nutrients to leaching and resulting in the liberation of toxic cations such as Al3+ (Ulrich 1980). Indeed, whereas needles of the damaged trees contained less Mg2+, Ca2+, and Mn2+ than healthy trees, their aluminium contents were increased (Fig. 6). The changes in soil chemistry affect root growth and mycorrhiza development and, in turn, exacerbate the difficulties of the trees in nutrient uptake (Schulze et al. 1989 c). Sulfur contents of the needles were high both in healthy and declining trees. By far, the major part of the sulfur must be sulfate, which had entered the needles in the form of SO2. In 4-year old needles, equivalent rations of the cations Mg2+, Ca2+ and K+ to sulfate were higher by a factor of two in healthy trees than in declining trees. Clearly, the cation/anion balance was disturbed in the declining trees. Effects of acid deposition on the soil are cumulative, but the changes in soil chemistry brought about by acidification are slow processes. However, tree decline in the Fichtelgebirge has not been slow. It has initially progressed rapidly since about 1981, causing significant tree death since about 1984 (Uhlmann et al. 1989). Tree growth on mineral deficient soils may be retarded, but deficiency alone is not likely to cause immediate death of the trees. We could demonstrate recovery after sufficient magnesium was made available to initially damaged trees. It has been proposed that the increased nitrogen availability, which is caused by the deposition of airborne nitrate and ammonium at the forest sites, results in stimulating growth of trees and thus exhausts even more the stored magnesium reserves (Schulze 1989). .......