Spatial variability of soil hydrophobicity in fire-prone eucalyptus and pine forests, Portugal

Because of its implications for slope hydrology and soil erosion in the region and the lack of previous work on (i) spatial variability of hydrophobicity and (ii) hydrophobicity in a wet Mediterranean environment, this paper assesses the in situ severity and spatial variability of hydrophobicity of surface soils in dry summer conditions in burnt and unburnt Pinus pinaster and Eucalyptus globulus forests in north-central Portugal. Results of experiments to explore the origin of hydrophobicity are also reported. The molarity of ethanol droplet (MED) technique was employed. The average severity of hydrophobicity (MED > 24%) in both long-unburnt and recently burnt forests is among the highest recorded. In contrast to other studies, spatial variability of hydrophobicity is generally low for all land types. This is thought to be caused by a comparatively high release rate and thorough distribution of hydrophobic substances aided by the relatively wet climate combined with the fairly uniform character of the commercial forest stands investigated. Although forest fires are usually thought either to increase (for low ground temperatures) or to destroy (for high ground temperatures) surface soil hydrophobicity, burning in the study area had little impact on surface hydrophobicity. This is attributed to (i) preburn hydrophobicity already so severe that the organic compounds released from the litter during burning contribute no detectable additional hydrophobic effects and (ii) fire temperatures insufficient to destroy surface hydrophobicity. The results suggest that the relative spatial uniformity 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 hydrophobic substances, Extreme hydrophobicity in E. globulus stands is found to develop within 2 years of planting on previously hydrophilic plowed terrain.