Above-ground biomass accumulation patterns in moorlands after prescribed burning and low-intensity grazing

Abstract

Shrub-dominated ecosystems such as moorlands are recognized internationally as cultural landscapes with high biodiversity conservation value. These ecosystems are commonly managed using prescribed burning to reduce the impact of wildfires, increase biodiversity and ecosystem productivity for grazing. Given that ecosystem responses are sensitive to the above-ground balance within the vegetation, knowledge of the above-ground biomass accumulation patterns on moorlands is an important issue for planning management action. Here, we used the replicated long-term manipulative grazing and burning experiment at Moor House (UK) to explore the cumulative effects of multiple fires and low-grazing. The study comprised a comparison between no-burn reference plots (no-burn since ca. 1923) and an experiment where all plots were burned in 1954/1955. Within the experiment, the effects of low sheep grazing vs. no grazing and three burning rotations were tested (no-burn since 1954/1955, repeat-burning at 10- and 20-year intervals). We hypothesized that prescribed burning and grazing will interact, affecting both the above-ground biomass and vegetation height. The results reveal that although the main above-ground biomass was constrained in three fractions (litter, Calluna and bryophytes) there was no significant effect of sheep-grazing or its interaction with prescribed burning (graze×burn) on any biomass variables or vegetation height. Significant reductions in above-ground biomass and vegetation height were only produced by repeated burning. There were no significant differences in biomass or vegetation height between the no-burn since 1954/1955 treatment and reference plots. Moreover, Calluna biomass and vegetation height showed a positive significant asymptotic association with time since the last burn with an asymptote at 20 and 15 years after fire, respectively. This work demonstrates that burning rotations lower than 20 years reduced the above-ground biomass and vegetation height on this moorland compared to stands unburned for more than 50 years. In order to maximize the C fixation, fire return-intervals should be around the Calluna biomass accumulation asymptote 20 years since last fire. Furthermore, the vegetation height asymptote of 36cm, indicating when the vegetation is at its maximum stage, could be a useful tool for guiding when to implement prescribed burning for carbon accumulation purposes.