Ecological succession and the rehabilitation of disturbed terrestrial ecosystems

Abstract

Soil and vegetation development on surface-mined coal sites in a mixed grass prairie region were studied as (a) naturally revegetated chronosequences of 1, 7, 17, 30 and 45-year old sites, and (b) on sites that were regraded, received topsoil, and were fertilized and seeded. In both cases, vegetation and soils of adjacent, relatively undisturbed areas were also studied for comparison. Species richness was lowest (26 species) at the 1-year old site and highest (114 species) at unmined sites; species richness increased youngest to the oldest sites. Stand ordinations revealed that the site age was the most important factor influencing species richness and abundance. On the younger sites, the environment was characterized by unleached parent materials high in electrical conductivity, SO4, Mg, Ca, and Na and plants growing there were non-native, annual pioneers. At about 30 years, the edaphic conditions had moderated somewhat, and the species complex had a greater number of native species. However, even after 45 years, there were clear differences among mined and unmined sites both in species composition and edaphic conditions. Rates of nutrient accumulations over the 45-year time gradient were as follows: organic C 131 kg ha−1 yr−1, N 25 kg ha−1 yr−1, P 0.1 kg ha−1 yr−1, and K 4.9 kg ha−1 yr−1. C/N ratios showed the widest range for 1-year old sites (5–40), but 70% of the 45-year old sites showed values below 15, comparable to unmined sites. Distributional diagrams of plant species revealed their topographic and habitat preferences which may be helpful in formulating species prescriptions for revegetation. Stand-environmental complex ordinations with 53 variables in the matrix showed topographic variables to be the most important, followed by site age. This study attempted to evaluate simultaneously two major environmental factors—time and topography, as these influence the development of soils and vegetation concomitantly. The use of canonical correspondence analysis facilitated the resolution of these two major environmental factors in illustrating the relationships between the species and the environments in which they grow. Simulations with the CENTURY model of C and N dynamics on north- and south-facing slopes provided useful insights into the recovery time of mined systems. All carbon pools (except passive) were larger on the north-facing slopes than on the south, as was the accumulation of organic N. Rehabilitation of mined areas is now required by law and regulations applicable in the region under discussion mandate the following steps: regrading of ridges to blend in with the surrounding topography, spreading topsoil, fertilizing and seeding. The course of vegetation and soil development of such sites was studied intensively for four years and revealed that the first year vegetation, irrespective of desirable species seeded, was dominated by the pioneer forb, Kochia scoparia. Its replacement by seeded grasses was quick. The rapid replacement of Kochia by grasses appears to be related to the production of allelochemics by Kochia and its acting as a nurse crop in the first year. Although allelochemics may be produced by Kochia as a defense mechanism for herbivory, the compounds seem to produce ionic imbalances (P, Mn, Zn) which hasten its own demise through autotoxicity. Like the abandoned mined sites, dispersal of seeds in space (immigration) seemed to be more important than from seeds in the topsoil (seed banks- dispersal in time). But in contrast to abandoned mined sites, the managed sites showed (a) a very rapid replacement of pioneers, (b) three to five times faster rates of leaching of ions capable of diminishing plant growth, (c) 5–8 times faster rates of mineralization of ions that promote growth, and (d) successional trends followed the facilitation and auto-inhibition pathways. A comparison of abandoned and managed systems clearly shows that by following the legal mandates, the rehabilitation process can be achieved many decades sooner with human assistance.