Effects of native forest restoration on soil hydraulic properties, Auwahi, Maui

Background/Question/Methods

Over historic time Hawai‘i's dryland forests have been largely replaced by grasslands for grazing livestock; less than 10% of original dryland forest habitat remains. Invasive grasses prevented native plant regeneration and changed soil hydraulic properties to favor invasive species. On-going efforts have been undertaken to restore dryland forests to bring back native species and reduce erosion. The reestablishment of native ecosystems on land severely degraded by long-term alternative use requires reversal of the impacts of erosion, organic-matter loss, and soil structural damage on soil hydraulic properties. This issue is perhaps especially critical in dryland forests where the soil must facilitate native plants’ optimal use of limited water. These reforestation efforts depend on restoring soil ecological function, including soil hydraulic properties.  We hypothesized that reforestation can measurably change soil hydraulic properties over restoration timescales. At a site on the island of Maui (Hawai‘i, USA), we measured infiltration rate (K_fs), hydrophobicity, and abundance of preferential flow channels in a deforested grassland and in an adjacent exclosure where active reforestation has been going on for fourteen years. Improved characterization of how these properties change is crucial for understanding long-term ecological impacts of restoration on in this rapidly changing environment.

Results/Conclusions

Results from 55 tests support the hypothesis that reforestation at the Auwahi site has significantly altered plant-relevant soil hydraulic properties. Mean values of K_fs, hydrophobicity, and preferentiality are systematically higher for the restored forest than for the grassland.  Mean K_fs in the restored forest measured by infiltrometer tests was greater by a factor of 2.0. Hydrophobicity on an 8-point scale increased from average category 6.0 to average category 6.9. A 4-point empirical categorization of preferentiality in subsurface wetting patterns increased from an average 1.3 to 2.6.  Between measurements inside and outside the exclosure, differences in K_fs, hydrophobicity, and preferentiality of flow are significant at the 99% confidence level. K_fs values are also significantly different between the two dominate native species within the exclosure (olopua and ‘a‘ali‘i) at the 95% confidence level. Differences in preferentiality of flow between the olopua and ‘a‘ali‘i are also significant at the 95% confidence level with no significant difference in hydrophobicity. These results illustrate the important influence of vegetation on subsurface hydrologic processes. Results indicate that plants have the ability to alter soil properties on decadal time scales to facilitate optimal growth by promoting efficient water use.