Peak detection in sediment - charcoal records: impacts of alternative data analysis methods on fire-history interpretations

Abstract

Over the past several decades, high-resolution sediment–charcoal records have been increasingly used to reconstruct local fire history. Data analysis methods usually involve a decomposition that detrends a charcoal series and then applies a threshold value to isolate individual peaks, which are interpreted as fire episodes. Despite the proliferation of these studies, methods have evolved largely in the absence of a thorough statistical framework. We describe eight alternative decomposition models (four detrending methods used with two threshold-determination methods) and evaluate their sensitivity to a set of known parameters integrated into simulated charcoal records. Results indicate that the combination of a globally defined threshold with specific detrending methods can produce strongly biased results, depending on whether or not variance in a charcoal record is stationary through time. These biases are largely eliminated by using a locally defined threshold, which adapts to changes in variability throughout a charcoal record. Applying the alternative decomposition methods on three previously published charcoal records largely supports our conclusions from simulated records. We also present a minimum-count test for empirical records, which reduces the likelihood of false positives when charcoal counts are low. We conclude by discussing how to evaluate when peak detection methods are warranted with a given sediment–charcoal record.

Highly episodic fire and erosion regime over the past 2,000 y in the Siskiyou Mountains, Oregon

“…The charcoal stratigraphy, developed at a 2-y mean resolution (1-cm intervals), displays a set of 71 statistically significant peaks (28), which correspond closely with nearby mapped historical fires and tree-ring-dated fire scars (Fig. 2).…”

“…Particles sieved with a 250-μm mesh sieve showed similar trends similar to the 125-μm fraction, and the two sievings were counted together. Concentrations (particles/cm 3 ) were converted to charcoal accumulation rates (CHAR, particles cm −2 y −1 ) in 2-y intervals (the median sample deposition time) using CharAnalysis software (28). A slowly varying component of the CHAR, representing secondary charcoal deposition and long-term change in charcoal production (30), was identified by smoothing the influx data with a locally weighted regression robust to outliers (200-y window).…”

Wildfires in boreal ecosystems: past, present and some emerging trends

“…These forests therefore appear, at some level, to be resilient to changes in fire activity . Nonetheless, even though fire is a long-term intrinsic property of the boreal forest (Bremond et al 2010;Carcaillet et al 2010;Higuera et al 2010), its occurrence is increasingly linked with human activities (Marlon et al 2008;Bradshaw et al 2010;Niklasson et al 2010;Wang and Anderson 2010). This influence has up until now been direct, via ignition, but the indirect influence through human-induced climate change is becoming worrisome (Flannigan et al 2009;Bergeron et al 2010;Metsaranta 2010).…”

“…These advances include reconstructions from fire-scarred trees, stand establishment records, tree rings, charcoal particles preserved in lake sediments, peat-bog and soil deposits, and ammonium concentration [NH 4þ ] in ice cores (see Whitlock et al 2010b for an overview of paleofire research). The first four papers of this issue (Bradshaw et al 2010;Bremond et al 2010;Carcaillet et al 2010;Higuera et al 2010) report on analyses of sedimentary charcoals preserved in lake sediments, whereas the last paper on this topic (Niklasson et al 2010) is based on dendrochronological analysis.…”

“…Nevertheless, this science is leading the way towards rapid development of new techniques for improved and more robust fire reconstructions. The work by Higuera et al (2010) notably provides a historical and critical overview of the statistical techniques used for filtering out fire signals from the background noise in sedimentary charcoal records, and concludes with sound recommendations for follow-up studies. The study by Bremond et al (2010) addresses the question of carbon emissions from fires and proposes an original method for the estimation of such quantity based on sedimentary charcoal records, with an application to eastern boreal Canada.…”