Towards a new paradigm in fire severity research using dose–response experiments

Abstract

Most landscape-scale fire severity research relies on correlations between field measures of fire effects and relatively simple spectral reflectance indices that are not direct measures of heat output or changes in plant physiology. Although many authors have highlighted limitations of this approach and called for improved assessments of severity, others have suggested that the operational utility of such a simple approach makes it acceptable. An alternative pathway to evaluate fire severity that bridges fire combustion dynamics and ecophysiology via dose–response experiments is presented. We provide an illustrative example from a controlled nursery combustion laboratory experiment. In this example, severity is defined through changes in the ability of the plant to assimilate carbon at the leaf level. We also explore changes in the Differenced Normalised Differenced Vegetation Index (dNDVI) and the Differenced Normalised Burn Ratio (dNBR) as intermediate spectral indices. We demonstrate the potential of this methodology and propose dose–response metrics for quantifying severity in terms of carbon cycle processes.

Spectral Indices Accurately Quantify Changes in Seedling Physiology Following Fire: Towards Mechanistic Assessments of Post-Fire Carbon Cycling

“…The strong relationship between dNDVI and PN observed in these fire dose-response experiments supports the methodology of [8] and further demonstrates that spectral indices have the potential to improve burn severity quantification through the monitoring of plant physiological metrics. While many studies have tested the utility of spectral indices for quantifying broad burn severity metrics (i.e., canopy cover, soil color, etc.)…”

“…The results of this and the prior studies [8] demonstrate that at one month post-fire a clear dose-response relationship between FRED and ecophysiology metrics is apparent. This study demonstrated that at one year post-fire the surviving seedlings of the 0.4 MJ¨m´2 and 0.8 MJ¨m´2 treatments were not significantly different from the control.…”

“…[8] to include a comparison with Larix); (c) P N predicted from dNDVI four weeks post-fire (adapted from [8] to include a comparison with Larix); and (d) chlorophyll fluorescence predicted from dPRI one week post-fire.…”

“…Details of seedling culture and growing conditions are reported in more detail in a prior study [8]. In total, 36 Larix occidentalis (western larch) and 36 Pinus contorta (lodgepole pine) seedlings were grown in an open-sided greenhouse at the University of Idaho Pitkin Forest Nursery in Moscow, ID, USA, using 3.8 L pots through two and a half growing seasons under natural light conditions.…”

“…From the fire science literature [31,32], it is well-accepted that the consumption of pure fuel beds of known type, loads, and moisture contents will release predictable amounts of FRED. Using data presented in [33] and following the methods described in [8], we determined the loads (kg¨m´2) of dry (~0% fuel moisture content) Pinus monticola (western white pine) needles to produce total FRED "doses" of 0.4 MJ¨m´2 (low group), 0.8 MJ¨m´2 (moderate group), and 1.2 MJ¨m´2 (high group). A control group containing the same number of replicates was included that was not exposed to fire.…”