Moorland fire history from microscopic charcoal in soils and lake sediments

Abstract

Microscopic charcoal analyses of sediment cores are used widely by palaeoecologists for reconstructing proxy records of past fire activity. Few studies, however, have reconstructed fire histories from UK and Irish moorland environments, a rather surprising situation considering the fact that much of the heather-dominated moorland in the UK and Ireland has been managed and maintained using fire for many centuries, and in some instances millennia. This thesis addresses the main issues regarding the use and applicability of microscopic charcoal analyses in moorland contexts. The literature pertaining to the theory and practical application of microscopic charcoal analyses is comprehensively reviewed, and all aspects of microscopic charcoal analysis, from charcoal production through to the interpretation of sediment charcoal profiles, are discussed and wherever possible related to their applicability in moorland contexts. An investigation of the taphonomy of microscopic charcoal around small moorland fires was conducted in order to provide an appreciation of the processes of charcoal production and the extent of charcoal particle dispersal. The results suggest that small moorland fires produce differential quantities of charcoal particles of different size ranges, smaller particles are produced in significantly greater quantities than progressively larger ones. The majority of charcoal particles produced by small muirburns are deposited locally, within approximately 70-100 m of parent fires, and the wind direction at the time of the fire may be a key factor determining the dispersal of microscopic charcoal particles, the majority being deposited down-wind of fires, few are dispersed laterally or into the wind. A microscopic charcoal quantification technique was developed to reconstruct extended (>50 year) fire histories from moorland soil profiles. A number of fire histories from mor humus-rich moorland soil cores of approximately several centuries duration were reconstructed. Fossil charcoal assemblages produced by in situ fires were distinguished from those produced by nearby ex situ fires on the basis of differences in gross charcoal abundance and charcoal size class distributions. Charcoal assemblages produced by in situ fires are determinable from those produced by ex situ fires because they generally contain a greater total abundance of charcoal particles and higher proportions of medium to large particles. Microscopic charcoal analyses of lake sediment cores from seven UK and Irish moorland catchments were used to reconstruct long-term (>100 year) fire histories. The reconstructed fire histories were used to assess whether changes in fire activity in the catchments may have been responsible for initiating past -episodes of peat erosion, inferred from loss on ignition measurements, and declines in Calluna cover, inferred from pollen analyses, evident at all of the sites. The results suggest that moorland burning may have been an influential factor contributing to the initiation of peat erosion at only one of the seven sites studied. Similarly, fire activity was only significantly related to the loss of Calluna at two of the seven sites studied.