Strong vertical gradient of foliar moisture content within tall conifers

Abstract Background Foliar moisture content (FMC) is a central determinant of plant flammability and strongly influences fire behavior and effects. In this study, we examined whether FMC varied with vertical crown position in three of the tallest conifer species of northwestern California: coast redwood ( Sequoia sempervirens ), Douglas-fir ( Pseudotsuga menziesii ), and Sitka spruce ( Picea sitchensis ). Specifically, we tested whether (1) FMC decreases with increasing crown position height, (2) the vertical FMC gradient persists across both new and old foliage, (3) the strength of the vertical FMC gradient varies by species, and (4) the vertical FMC gradient is associated with changes in leaf and shoot morphology. Results We found that FMC declined significantly with increasing relative crown position height for both new and old foliage in all three tree species. FMC around mid-crown positions was approximately 20% higher than at treetops. New foliage had approximately 14% higher FMC than old foliage but had consistent relationships with crown position height. The differences in FMC among species were weak and smaller than expected, with mean values differing by less than 10% among species. FMC was negatively associated with both leaf and shoot morphology, but shoot mass area was more strongly related to FMC than leaf mass area. Conclusions Our results provide evidence of strong vertical gradients in FMC within tall conifer crowns, likely driven by height-related hydraulic constraints and associated morphological changes. These findings suggest that FMC estimates derived from measurements of the upper canopy, including many remote sensing approaches, may underestimate whole-crown moisture and thus contribute to overestimation of crown fire initiation and spread. Incorporating vertical FMC gradients into fire behavior models could improve predictions of crown fire behavior, particularly in tall forest ecosystems.