Plant colonization of harvested peat surfaces

Abstract

Revegetation of initially bare peat surfaces, created by industrial peat harvesting, was studied over the years 1984-92 at several sites in Finland. Both a field survey and a field experiment approach were used to examine influences of both biotic and abiotic factors on the rate and pattern of revegetation. In general, the harvested peat surfaces were colonized by a low number of plant species. The dominating species were all perennials and native of nutrient-poor habitats. Substantial differences were found between sites in the rate at which the harvested surfaces were colonized by plants. Likewise, species composition of the early successional plant communities was found to vary a lot. Significant differences were found in physical and chemical characteristics of the peat substrate between sites. These differences were demonstrated to account for much of the variation in the rate of revegetation and structure of the plant communities. Sites with a thick peat layer, and surface peat with high content of soluble phosphorus and ammonium nitrogen were likely to become densely occupied by Eriophorum vaginatum alone. Different grass species were likely to dominate at sites with an excess of nitrate nitrogen over ammonium nitrogen and a thin peat layer. Woody species seemed to favour sites with intermediate values of these environmental variables. Large differences were observed in the growth in size of populations during the first five years of colonization at one study site. The rate at which different species colonized the bare peat surface was determined largely by their dispersal ability, growth rate and mode of reproduction. During the five year period, no associations were found between the three most abundant species, Eriophorum vaginatuni and Pinus sylvestris with homogeneous spatial patterns and Carex rostrata with a clumped pattern. During the prolonged stage of colonization, the importance of competition or other interspesific interactions in directing the early succession was found to be low. However, artificial plant cover was found to facilitate establishment of three abundant species and inhibit the establishment of one. Numbers of seeds of different species dispersed onto two newly abandoned peat harvesting sites differed substantially from those of established plant individuals of these species. The discrepancy between number of seeds and seedlings was most striking in Betula spp. and Calamagrostis spp. The relative importance of seed availability was found to be greater than that of soil quality to the rate of colonization. However, the failure of many species in establishing on the harvested peat surface, despite an abundant supply of their seeds, emphasizes the importance of substrate quality to colonization. This was also demonstrated by two separate field experiments, both with application of NPK fertilizer. In these experiments, plots with a presumably similar supply of propagules but dissimilar supply of nutrients differed significantly in terms of amount and composition of established vegetation.