An investigation of fire scars in Pseudotsuga macrocarpa by Scanning X-Ray Fluorescence Microscopy

Abstract

Fire scars in dated sequences of tree-rings are regularly used for the reconstruction of histories of forest fire frequency and investigations of various exogenous factors (climate in particular) which may control such events. The potential of the tree-ring archive in this regard is such that in circumstances where no scarring occurs following a particular fire, or where sampling is limited to increment cores which may miss the zone of scarring, alternate means of detecting tree-ring evidence for fire impact must be sought. One possible alternative may be detection of changes in tree-ring chemistry associated with growth years following forest fires. If it was possible to characterize such a change in chemistry an independent proxy for forest fires in tree-ring series might be established. The behavior of various elements within the xylem is however extremely complex so for a dendrochemical approach to fire history to be established, new techniques are required to enhance existing knowledge of elemental behavior in trees affected by fire. In this study, elemental intensities were mapped across scarred and un-scarred vectors of Pseudotsuga macrocarpa (Vasey) Mayr (bigcone Douglas-fir) tree-rings from a site in the Los Padres National Forest, Southern California, using Scanning X-Ray Fluorescence Microscopy (SXFM). The aims were: to assess the potential of this technique for understanding elemental change in relation to fire scars and undamaged contemporary growth; to provide new information on specific elemental behavior in this species; and to contribute to wider research for dendrochemically establishing fire histories. The results highlight the potential of SXFM for mapping elemental changes associated with compartmentalization, callus and woundwood as well as providing some evidence for depletion of certain elements in contemporary un-scarred rings. They also provide a first step towards future work to use cores for dendrochemical construction of fire histories.