THE IRON AND MANGANESE STATUS OF 7 UPPER MONTANE TREE SPECIES IN COLORADO, USA, FOLLOWING LONG-TERM WATERLOGGING

1 Seven tree species were investigated for possible Fe and Mn toxicity following prolonged exposure to enriched groundwater in Rocky Mountain National Park, Colorado, USA. The groundwater contained abnormally high levels of Fe and Mn following a catastrophic flood in July 1982. The tree species were Pinus ponderosa, Pinus contorta, Pinus flexilis, Picea engelmannii, Abies lasiocarpa, Populus tremuloides, and Populus angustifolia. A shrub, Salix monticola, was also studied. The Fe-enriched conifers and Populus tremuloides died, while Populus angustifolia and Salix monticola remained healthy. 2 Foliar Fe and Mn concentrations were determined for normal and flood-affected trees. All seven tree species accumulated abnormally large amounts of Fe, with mean Fe concentration ratios (enriched:normal plants) ranging from 2.0 to 7.7. 3 A possible threshold of Fe toxicity was observed in Pinus ponderosa, Pinus contorta, Picea engelmannii, Abies lasiocarpa, and Populus tremuloides. In these species, we found a small range of foliar Fe concentrations separating the highest Fe concentration of normal, healthy trees and the lowest concentration in dead trees. 4 The mean Fe:Mn ratios in the adversely affected species exhibited large changes from normal values. The ratios increased between 1.4 and 3.9 times, indicating that these trees experienced a shift towards proportionately higher foliar Fe before death. 5 In the case of Pinus contorta, we compared the dead flood-affected trees with the results of a dose-response experiment and found that both groups died after about a doubling of the mean Fe:Mn ratio. Also, an increase in the Fe:Mn ratio above about 1.0 might represent a threshold of Fe toxicity. 6 The healthy, flood-affected Populus angustifolia accumulated large amounts of Fe and Mn, but the Fe:Mn ratio exhibited relatively little variation over a 5-year period of observation. The ability to maintain a stable Fe:Mn ratio might be an important mechanism that permits the success of the species in riparian habitats.