Eco-hydrological controls on microclimate and surface fuel evaporation in complex terrain

The micrometeorological factors driving variation in litter moisture (GWClit) at the landscape scale are poorly understood, particularly in areas with heterogeneous vegetation and complex terrain. In this research we seek to quantify how climate and eco-hydrology contribute to variation in litter moisture and potential evaporation at the forest floor. Research sites were established at 12 locations in southeast Australia with variable precipitation, solar exposure, and drainage position. We measured solar radiation, air temperature, relative humidity, litter moisture, and litter temperature. Spatial patterns of GWClit, examined during two drying phases, show that moisture in the litter and its rate of evaporation are closely linked to vegetation density, which is largely a function of aridity. This creates a pattern whereby aridity-related differences in vegetation structure controls the spatial variation in GWClit, with regional effects being driven by precipitation while local effects are caused by variation in solar exposure and drainage position. By parametrising a model of daily potential evaporation (Ep) at the forest floor we explore how vegetation and topography influence evaporative demand above the litter layer. The model shows that Ep is driven primarily by net radiation and that the role of vapour pressure deficit is almost negligible due to high moisture content within the sub-canopy air mass and high aerodynamic resistance. In dry forests the net radiation is directly related to shortwave radiation and Ep remains high despite low temperatures. In the tall wet forests, commonly found at high elevations, along drainage lines and on slopes with polar-facing aspects, the long-wave radiation (i.e. temperature) was just as important for Ep as the shortwave radiation. Low energy inputs to the forest floor in these tall forests means that a significant rainfall event results in surface fuels that remain wet for much longer than fuels in the dry open forest. This leads to a spatial pattern of flammability whereby surface fuels in densely vegetated areas are more often less likely to burn than those in the open forests. These wet compartments limit landscape-scale fire activity by creating dis-connectedness in the available fuel. Heatwaves and the duration of dry spells determine the degree with which these wet compartments persist as barriers to fuel connectivity through a fire season. Elsewhere in the landscape, however, the large inputs of shortwave radiation to the forest floor means that surface fuels reach flammable conditions within several hours or days after rainfall and are therefore flammable for much of the fire season.