SPATIAL VARIABILITY OF SOIL HYDROPHOBICITY IN FIRE-PRONE EUCALYPTUS AND PINE FORESTS, PORTUGAL

Because of its implications for slope hydrology and soil erosion in th e region and the lack of previous work on (i) spatial variability of h ydrophobicity and (ii) hydrophobicity in a wet Mediterranean environme nt, this paper assesses the in situ severity and spatial variability o f hydrophobicity of surface soils in dry summer conditions in burnt an d unburnt Pinus pinaster and Eucalyptus globulus forests in north-cent ral Portugal. Results of experiments to explore the origin of hydropho bicity are also reported. The molarity of ethanol droplet (MED) techni que was employed. The average severity of hydrophobicity (MED > 24%) i n both long-unburnt and recently burnt forests is among the highest re corded. In contrast to other studies, spatial variability of hydrophob icity is generally low for all land types. This is thought to be cause d by a comparatively high release rate and thorough distribution of hy drophobic substances aided by the relatively wet climate combined with the fairly uniform character of the commercial forest stands investig ated. Although forest fires are usually thought either to increase (fo r low ground temperatures) or to destroy (for high ground temperatures ) surface soil hydrophobicity, burning in the study area had little im pact on surface hydrophobicity. This is attributed to (i) preburn hydr ophobicity already so severe that the organic compounds released from the litter during burning contribute no detectable additional hydropho bic effects and (ii) fire temperatures insufficient to destroy surface hydrophobicity. The results suggest that the relative spatial uniform ity of hydrophobicity in the study area is induced by the planting of E. globulus or P. pinaster. The litter layers of both species, and the root zone in the case of E. globulus, are identified as sources of hy drophobic substances, Extreme hydrophobicity in E. globulus stands is found to develop within 2 years of planting on previously hydrophilic plowed terrain.