Spatiotemporal variation and difference analysis of global leaf fuel moisture content based on MODIS data

Forest fires are major natural disasters and key to the loss of forest biodiversity and degradation of ecosystem functions. Fuel moisture content (FMC) is closely related to occurrence of forest fires and is a critical parameter in forest fire risk assessment and in calculating fire spread rates． In this study, we used Google Earth Engine (GEE) platform to generate spatiotemporal distribution maps of global FMC and analyzed spatiotemporal differences in FMC across various regions and vegetation types． First, ground-measured data and spectral indices are used to construct empirical estimation models for equivalent water thickness (EWT) and dry matter content (DMC) for forest and grassland vegetation types, respectively． Then, EWT and DMC are used as intermediate variables, to develop a stepwise estimation model for FMC, to ultimately produce global monthly FMC spatiotemporal distribution maps for the period from 2010 to 2022． To assess estimation accuracy, both direct and indirect validations are conducted by integrating site data (Global-LFMC) and standardized precipitation evapotranspiration index (SPEI) data． For forests, normalized difference infrared index (NDII) is used to estimate EWT (r = 0.89) and normalized difference water index (NDWI) is used to estimate DMC (r = 0.26)． For grasslands, simple ratio water index (SRWI) is used to estimate EWT (r = 0.85) and NDWI used to estimate DMC (r = 0.27), both yielding high accuracy． Thus, these methods are used to construct stepwise estimation model for FMC． Direct validations indicate that the correlation coefficient between estimated FMC and measured values is 0.79 (with a significance level of P < 0.000 1), demonstrating significant correlation． Indirect validations reveal that FMC exhibits a trend consistent with SPEI, albeit with a temporal lag． The spatiotemporal dynamics of global vegetation FMC is then further explored． FMC shows significant seasonal variations and marked differences between the Northern and Southern Hemispheres as well as among different vegetation types． In tropical regions, FMC values remain relatively high throughout the year with minimal fluctuations． In temperate regions, FMC values increase significantly during the spring and summer． In polar and arid regions, FMC values are relatively low year-round． In mid-to-high latitude areas, FMC is significantly influenced by extreme seasonal climate events． Areas with low FMC coincide with regions of high fire occurrence． Evergreen forests maintain high moisture content throughout the year, whereas FMC in deciduous forests and grasslands fluctuates markedly with seasons． Spatial distribution characteristics of FMC outliers are confirmed． Analysis of these outliers reflects the significant impact of factors such as climate and sunlight on vegetation water status, further elucidating influence of extreme climate on FMC． MODIS-derived global monthly FMC spatiotemporal distribution maps generated for 2010–2022 reveal FMC distribution characteristics across different regions and vegetation types, providing ground for formulating scientific prevention and control strategies in high fire-risk areas, and they are of significant values for forest fire prevention and ecosystem management．