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Abstract "Heft 33"


Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich


Heft 33


Peter Trüby

Zum Schwermetallhaushalt von Waldbäumen


Freiburg im Breisgau 1994

ISSN 0344-2691


Summary:

Introduction

The investigation was focussed on pathways of heavy metal uptake and the internal transport in forest trees. The objective was to distinguish rules of heavy metal distribution by deter-mining detailed distribution patterns in the roots, stem and assimilation Organs. The experimental design is based on pedogenicly/geogenicly and atmosphericly contaminated trees respectively. The hypothesis was that there are characteristic distribution types, which depend on the source of contamination and enable the reconstruction of the pathways of uptake and internal translocation. Furthermore it was aimed to clear, whether the radial heavy metal distribution is feasible for the reconstruction of imission history of a site and the role heavy metals play in forest decline.

To obtain significant effects trees were mainly selected from contaminated sites. The atmo-spheric deposition and the total heavy metal contents of the soil Substrates were up to three Orders of magnitude higher than on normal forest sites.

In Order to exclude the uptake of airborne heavy metals via roots trees were selected from carbonatic soils, where the availability of heavy metals is low. Trees from acid silicatic soils were used as control.

The investigation concentrates on the heavy metals Mn, Cu, Zn, Cd, Pb, partly Ni, Cr, As, and Sb. In addition to these heavy metals the nutrient elements P, K, Ca, Mg were analysed too. The selected tree species were spruce (Picea abies), douglas fir (Pseudotsuga menziesii), silver fir (Abies alba), scots pine (Pinus silvestris), oak (Quercus robur) and beech (Fagus silvatica).

Material and Methods

Trees were selected on different sites in the Black Forest and in the heavy metal polluted area of Stolberg (West-Germany).

In the Black Forest pairs of trees were selected on heavily contaminated ore mine spoils and uncontaminated Gneiss-substrates. The ore mine spoils contained up to 3.520 µ g/g dm Zn, 12 itg/g dm Cd, 14.980 µ g/g dm Pb, 4.490 µ g/g dm As and 14.900 µ g/g dm Sb. On these sites the atmospheric deposition is the same and the absolute level is low.

In contrast to the Black Forest sites the Stolberg area is extremely strong polluted. The atmospheric deposition is 10-100 times higher than on "normal" forest sites in Germany. This also leads to increased heavy metal Contents in the upper soils. Depending on the local deposition rate the Contents vary between 1.000 and 10.000 ixglg dm. Geogenicly contaminated Substrates occur, where Galmei ores reach the surface. In this instance the maximum Contents increase up to 73.000 µg/g dm Zn, 201 µ g/g dm Cd and 20.700 µ g/g dm Pb.

Detailed element distribution patterns were determined for root, stem, branches, and needles/leaves. Wood samples were analysed after separating annual tree-rings. Bark fracti-ons were taken from the periderm tissue and the tissue of inner and outer bast.

For the preparation of wood and bark samples Special titanium or titanium-nitrid covered tools were used. This enabled to minimize the contamination in preparing samples with low heavy metal contents.

The precise analysis of heavy metals in different biological matrices required an optimization and an adaptation of the Standard methods. The precision of the analysis was essentiallyim proved by optimizing the digestion procedures. The Standard error of the heavy metal analy-sis was <10% often <5%.

For the characterization of plant available heavy metals an extraction method with ion ex-change resins was developed. The method determines water soluble, exchangeable and acid soluble heavy metals partly. The acid soluble amount can be varied by conditioning the resin with potassium. This allows acidity changes that occur in the rhizosphere to be simulated and to estimate mobil reserves.

Heavy metal distribution in tree compartments

The level of the heavy metal Contents primarily depends on the amount of available heavy metals in the soil. This holds even true for lead which is predominantly accumulated on the surface of needles/leaves, bark and roots. Substantial amounts of heavy metals in the tree were taken up from the soil. Therefore roots are the most important pathway of uptake. The-re was no indication for an essential heavy metal uptake via assimilation organs or the bark.

Needles /leaves

The distribution patterns are predominantly influenced by the supply of heavy metals in the soil and the (nutrient) element composition in total. The Contents depend on the age of the tissue and the position within the crown. With the exception of Cu heavy metal Contents in-crease with the age of the tissue. Three characteristic distribution types were identified.

Type I: The distribution is characterized by an accumulation which corresponds with the age of the tissue. These patterns were found for Ca, Zn, Cd, and Pb.

Type II: The element contents increase with decreasing age of the tissue. This is caused by retransloction and redistribution, which is controlled by the demand. These distribution patterns were found for P and Cu.

Type III: The element contents correspond with the age of tissue until the maximum content is reached in 3-4 year old needles. Due to leaching the contents in the older needles decrease. Typical patterns were found for K, Mn.

Branches

The principles of element distribution in the bark of the branches are similar to assimilation organs. Leaching, redistribution and accumulation corresponding with the age of the tissue are the most important processes. No continous vertical and axial gradients were found. Occasionally Pb- and Ca-contents increase slightly from the top to the bottom of the crown. A decrease is observed for those elements, which are redistributed or leached from the older tissue. In the branchwood two characteristic distribution types occur. With the exception of Pb the element Contents decrease from the tip of branch to the stem. However Pb-contents increase.

Stem

In general the element Contents were higher in the bark than in the stemwood. Periderm tissue had lower contents than inner and outer bast. This is probably caused by redistribution and/or leaching.

In most instances no clear axial gradients of element Contents were observed. However Pb and Ca contents decrease from the crown to the bottom of the tree. They correspond with the axial gradients of the tissue age. Sometimes high contents of P, K, Cu, Zn, and Cd were found in the top of the tree, but there were no continous gradients along the stem. The radial distribution in the stemwood is differentiated individually. The distribution pattern is influenced by the anatomic structure, but there are no specific patterns for a Single tree species. However, the distribution patterns of beech and oak are different from those of co-nifers. In total 4 characteristic types were identified.

Type I,1: The element contents in the heartwood and the older sapwood are relatively low and no radial gradients were found. In the cambial zone there is a strong increase up to the maximum value, which was always found in the youngest tree-ring. Characteristic type for P and Cu.

Type I,2: The distribution is similar to type 1,1. However the contents increase at the sap-wood-heartwood boundary to a constant level before the maximum value is reached in the cambial zone. Characteristic type for K.

Type II: The contents follow a maximum-curve. Maximum contents occur at the boundary between sapwood and heartwood. Characteristic type for Pb.

Type III: The distibution curve corresponds with a hyperbola and the contents decrease from the heartwood to the sapwood. Characteristic type for Ca.

Roots

The element distribution  in the roots is heterogeneous. There are no specific distribution patterns  for  Single  elements  or  tree species. The  most  important  factor  is  soil  acidity respectively  the  supply  of available  elements in the  soil.   High  pH-values  increase the accumulation  in the perderm-tissue.
In  general  the element Contents of the fine roots are higher than in the tissue  of bark and wood.   Heavy metals are accumulated very effectively in fine roots.   Mycorrhization,   which was  found  at  all  trees,   might play an important role in  the effectivity  of heavy   metal accumulation.
The element distribution in the roots is characterized by the following 3 types.
Type    I:   The  element Contents, especially the heavy metal Contents of the periderm  tissue and fine roots are higher than in the bast tissue. This is interpreted as an effective discrimination at the root surface.
Type  II:   The  element  Contents  of fine roots, bark- and periderm-tissue are  at the  same level.
Type III:   The element Contents of the periderm tissue are lower than in the bast tissue. This distribution   pattern - found primarily for nutrient elements - indicates a  selective uptake.



Conclusions

The internal distribution processes are not specific for Single elements or tree species. There-fore individual distribution patterns exist, which depend on the soil properties especially the element  availability and the element composition of the organic matter. Transpiration is  the most important distribution process for heavy metals. In contrast to the phloem mobile  elements the active transport of heavy metals takes place only on short distances.
Heavy metals are predominantly taken up by the roots. There was no indication of an uptake via bark or assimilation Organs.
The distribution patterns of heavy metals in the stemmwood are individually differenciated and depend on different accumulation processes. So there is no chance for the reconstruction of immission-history by determining the heavy metal distribution in tree-rings.

Heavy metal supply in Norway spruce

Heavy metal supplies were calculated by the Contents of the tree compartments and the corresponding biomass.   Biomass was measured and partly estimated by a model.  Wood totalled more than 80%, bark and needles each are nearly 10% of biomass. About 3/4 of total Pb and Cd are stored in the stemwood, 1/4 in the bark. The supplies in the needles are lower than 5 %. However the amounts of Cu and Zn in wood were  substanti-ally  lower. Depending on the supply of the soil there is a streng accumulation in  the  bark. Only  25% is calculated airborne Pb, the remainder of 75% must be taken up by  the roots. Zn and Cd is completely taken up from the soil. It is proposed that considerable amounts of Zn and Cd are leached, especially from the bark. An estimation of airborne Cu supplies was not possible.
The turnover by litterfall is a substantial component in the heavy metal budget of the trees. Nearly   1/3 of total Zn respectively 1/5 of total Cd amount is turned over by a 120 year old spruce. Pb from atmospheric deposition is mainly accumulated on the surface of the needles. Thereafter it is cycled passively by litterfall. The amount exceeds 30% of the total supply.

Mycorrhization

In spite of extremely high soil heavy metal Contents the roots of all trees were mycorrhized. However the number of species was relatively low. Vital mycorrhizas were even found at the roots of Scots pine on the Galmei-sites, containing 104 g/g dm Cd and 73.000 f*g/g dm Zn. Obviously no long term damage of mycorrhiza occurs by potentially toxic heavy  metals.

Toxic effects on forest trees

Even with high atmospheric or geogenic heavy metal pollution no specific damages were observed. Norway spruce, Douglas fir, Scots pine, Silver fir, oak and beech are able to grow normally without any Symptoms even under long term and extremely high pollution. The toxicity level was not exceeded on any of the experimental Sites. Due to the very low pollution level on normal forest sites" in West Germany it can be concluded that a participation of heavy metals on the new type forest decline can be excluded.




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