Assessment of two spectral reflectance techniques for the estimation of fuel moisture content, equivalent water thickness, and specific leaf weight in Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) needles




Visintini, Fabio

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In the wildfire community fuel moisture content (FMC) is the quantity of choice when it comes to assess vegetation water status in relation to fire risk and fire behaviour. Field measurements of FMC are both expensive and time consuming and, in addition, sampling is often spatially inadequate. Remote sensing could represent an almost ideal solution both in terms of spatial and temporal coverage, if a consistent relationship between FMC and spectral reflectance could be established. A review of the literature suggests that it is difficult to retrieve FMC for dense forest canopies with remote sensing platforms. This study took a step back and explored the relationship between spectral reflectance and vegetation water content at the leaf level, where several confounding factors present at the canopy level are eliminated or controlled for. It also considered a conifer species, because relatively little research has been produced on this topic for this type of vegetation. The main goal was to establish if FMC can be derived directly from spectral reflectance in the solar spectrum using two well known approaches, such as spectral indices and continuum removal. It is also aimed at exploring if an alternative, indirect way to measure FMC as ratio of Equivalent Water Thickness (EWT) and Specific Leaf Weight (SLW) is feasible and accurate. The results derived from Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) needles used in this study suggested that FMC was not directly retrievable from spectral reflectance but vegetation water content could be assessed with sufficient accuracy in terms of EWT. Also the retrieval of SLW from reflectance of fresh foliage proved to be challenging. Finally, the study also highlighted several aspects in the relationships among foliar water content, dry matter content and reflectance that require additional research.



Douglas fir, moisture, forests and forestry