Variation in substrate chemistry along microtopographical and water-chemistry gradients in peatlands

A broad range of water chemistry (pH 3.5–8.2; Ca, 2–120 mg L−1) and substrate chemistry (pH 3.3–7.8; Ca, 4–138 mequiv. 100 g−1) exists among peatlands present in central Alberta. The six peatlands comprising the main study sites included strongly minerotrophic, moderately minerotrophic, and weakly minerotrophic systems. Variation in substrate chemistry along the hollow to hummock gradient in strongly and moderately minerotrophic peatlands was high (ranging from highly minerotrophic peats to ombrotrophic peats), while variation in substrate chemistry in weakly minerotrophic peatlands was slight (weakly minerotrophic peats to ombrotrophic peats) along the same gradient. Substrate chemistry and plant community composition of hollow and low-hummock communities varied considerably along the strongly minerotrophic to weakly minerotrophic peatland gradient. In contrast, the chemistry of the upper peat layers of the hummocks and hummock plant community composition were similar in all of the peatlands studied. Distributional patterns of plant species in weakly minerotrophic peatlands are not primarily in response to gradients in substrate chemistry but arise from gradients in substrate moisture and biotic interactions. This is not the case for strongly and moderately minerotrophic systems, where gradients in substrate chemistry may also strongly influence plant species distribution. The increase in complexity of the substrate environment in peatlands along the weakly minerotrophic to strongly minerotrophic gradient is reflected by a parallel increase in plant species diversity.