Trace Metal Uptake and Accumulation in Trees as Affected by Environmental Pollution

Abstract

Regional-scale increases in combustion of fossil fuels, trace metal deposition profiles in lake sediments, and metal accumulation patterns in forest soils suggest that forests have been exposed to increasing levels of trace metals in recent decades. Such increases are of concern because metals such as Al, Cd, Cu, Fe, Mn, Ni and Zn typically have long residence times in soil-plant systems and can adversely affect plant physiological and litter decomposition processes at sufficiently high concentrations. Chronic effects of these metals at present regional concentrations are largely unknown, and their increased availability to trees can occur through multiple pathways, most of which are poorly characterized. Our surveys of changes in tree-ring chemistry of conifers show that increased tissue concentrations of some metals are quite similar to temporal increases in local and regional fossil fuel combustion, suggesting a relationship between the two. Highest metal concentrations in tree rings were found near a power plant and at high-elevation sites remote from point sources. Levels of many trace metals, particularly Al, Fe, and Mn, in tree phloem tissues are at or exceed levels reported as toxic thresholds in other plant species. In reviewing the literature, it is clear that toxic thresholds are strongly influenced by soil characteristics, plant species, and plant part. Thus, until more information is obtained on toxic thresholds for individual and combined levels of trace metals on plant physiological and forest ecosystem-level processes, their role, if any, in forest declines must be considered conjectural.