A meta-analytical approach to understanding the charcoal source area problem

The sedimentary charcoal proxy is plagued by the source area problem: estimates of the spatial scales for which charcoal serves as a reliable recorder of fire history span several orders of magnitude, thereby precluding higher-order fire-climate and fire-vegetation analyses requiring spatially explicit reconstructions. This problem is tackled using a meta-analytical approach: (1) published estimates of charcoal source area are compiled to assess all available data and constraints, (2) the limitations and implications of these source area estimates are discussed, and (3) the parameter space of the relationship between charcoal size and dispersal distance is modelled using the published estimates as constraints and with the goal of estimating the functional charcoal source area (FCSA), or the globally applicable source area necessary to facilitate spatially-explicit fire history reconstructions and data-model comparisons. Source area estimates exist in numerous forms and flavors including direct assessments of source area (both relevant (RSAC) and potential charcoal source area (PCSA)) as well as indirect and/or opportunistic observations of dispersal distance. These estimates span several orders of magnitude and do not exhibit size dependent dispersal which may be due to differences in the flavor of source area constrained by each study, limitations of the inferences by the observed fire history at each site in time and space, the physical inability of size class differences examined in paleofire research (e.g. micron scale), to result in observable size-dependence of dispersal, or the effects of other unconstrained influences affecting dispersal and source area. The model results show the FCSA of all size fractions is limited to 50 km. This estimate of the FCSA enables spatially explicit paleofire reconstructions and climate model comparisons. However, the meta-analysis results and broad strokes approach of the model in this paper highlight the need for further research characterizing the parameters affecting charcoal dispersal as well as the limitations of fire history in delineating charcoal source area.