Native Forest Restoration Alters Soil Characteristics for Improved Plant-Water Availability.

Field experiments have demonstrated significant alteration of soil hydraulic characteristics over fifteen years since initial revegetation with native species at the Auwahi subtropical dryland forest in Hawaii. At four 2.5-m-square plots within the reforested area and four within the adjacent unrestored grassland, we drip-irrigated and measured soil moisture and temperature down to one meter depth. Additional experiments assessed infiltration capacity, characteristic degree of preferential flow, and hydrophobicity (which tends to enhance preferential flow). During infiltration, at grassland plots water movement slowed with depth, whereas at reforested plots water moved rapidly through the entire uppermost meter of soil. Reforested plots also underwent greater changes in water content. With 75 small-scale (20-cm-diameter) infiltrometer measurements at the surface and at 25 cm depth, we found a generally greater degree of preferentiality of flow in the restored forest, as well as significantly greater field-saturated hydraulic conductivity (K_fs) and hydrophobicity measured on an 8-point scale. At 25-cm depth, below most of the grass root biomass, forest-vs.-grassland differences in both K_fs and hydrophobicity were even more pronounced than at the land surface.

All of these observed changes—heterogeneity of flowpaths, increased soil hydraulic conductivity and hydrophobicity, and larger changes in water content—act to distribute infiltrated water faster and deeper, as appropriate to meet needs of the deeper-rooted native plants. The deeper flow of water also suggests possible influence on aquifer recharge, though our study to date is insufficiently comprehensive for a conclusive statement on this topic. The results do show that, by processes associated with root growth and decay, enhanced production of hydrophobic materials, and probably numerous other root-zone effects, restored native plants in place for little more than a decade have altered soil structure and hydraulic function in ecohydrologically important ways.