Moisture dynamics of moss-dominated surface fuel in relation to the structure of Picea abies and Pinus sylvestris stands

We examined moss-dominated surface fuel moisture in a southern boreal fire environment by carrying out destructive moisture sampling on 47 days in Pinus sylvestris L. stands aged 0, 15, 30, 45, and in Picea abies (L.) Karst stands aged 0, 40, and 60 years since regeneration cutting. The moss layer was found to be consistently moister under dense canopy cover, formed by either of the studied tree species, than in the respective open stands. In the most exposed stand type (clear-felled Pinus), the frequency distribution of daily surface fuel moisture was strongly skewed towards the low end, with 37% of the observations falling below 50% MC. For 45-year-old Pinus, the corresponding figure was only 7%. Moisture difference between open and closed stands increased during the latter half of fire season, probably due to increased canopy-interception of radiation with lowering sun-angle. Average surface fuel moisture correlated with effective leaf area index and canopy openness with ρ ranging from −0.68 to −0.84 (p < 0.001). There was on average a two-fold difference in drying efficiency between the closed and cut-over stands. The Finnish fire risk index (a weather-based fuel moisture index that uses the data input of precipitation, solar radiation, temperature, relative humidity, and wind) predicted the average daily moisture content of surface fuels in most stand types with reasonable accuracy (R2: 0.56–0.98). Surface fuels reached a postulated ignition threshold moisture content of 30% and below in Pinus clear-cuts at an FFI value of 3.6, and in closed Pinus sylvestris stands at FFI 5.6, but never went below 40% in the closed P. abies stands. The results show that stand structure has a remarkable impact on surface fuel moisture regimes. Integrating spatially explicit stand data such as LAI with weather-derived fire risk indices would greatly improve moisture content predictions to the benefit of various fire management operations.