Laboratory measurements of plant drying: Implications to estimate moisture content from radiative transfer models in two temperate species

The estimation of live fuel moisture content (LFMC) is necessary for fire danger assessment. Several studies have successfully used satellite imagery to estimate LFMC, both using empirical and simulation approaches (Yebra et al., 2013). The latter are based on Radiative Transfer Models (RTM). They are generally more robust and easier to generalize, but they rely heavily on the proper parameterization. Since some of the input parameters are associated with different physiological processes, a better understanding of how those parameters co-vary is necessary for constraining the simulation scenarios, thus avoiding combinations of parameters that are unlikely to occur (for instance, in temperate ecosystems, it is unlikely to find simultaneously high values of leaf chlorophyll and low values of leaf moisture).To improve parameterization of RTM models for LFMC estimation, we conducted a laboratory experiment to measure trends in leaf and canopy variables of two tree species broadly distributed in Eurosiberian climates: Beech (Fagus sylvatica L.) and pedunculate Oak (Quercus robur L.). Measurements of LFMC, equivalent water thickness (EWT), dry matter content (DMC), chlorophyll (C_a+b), leaf area index (LAI), leaf angle distribution (LIDF), crown height to width ratio (CHW) and plant reflectance were performed. Significant positive correlations were found between LFMC and EWT (Rs >0.5), and negative ones were found between both parameters and C_a+b (Rs <-0.3). LFMC and EWT were positively related to DMC and LAI, with lower correlation coefficients for the latter. The effect of moisture variation in spectral reflectance was also analyzed using two indices: the spectral angle (SA) and the root mean square error (RMSE).The former contributed the most to the estimation of LFMC variations. Spearman correlation coefficients (Rs) between SA and LFMC were 0.656 and 0.554 for F. sylvatica and Q. robur, respectively; while for RMSE and LFMC they were 0.366 and 0.430, respectively.