Long-Term Recovery of the Mamane Forest from Feral Sheep Grazing, Mauna Kea, Hawaii.

摘要

Seven exclosure sites, established in the 1960s and 70s, were sampled to characterize long-term recovery of the mamane (Sophora chrysophylla) forest from feral sheep grazing, and to assess the impacts of exotic plant species and recurrent sheep presence on forest recovery. The forest is critical habitat for a native honeycreeper, the palila ( Loxoides bailleui). Vegetation was sampled inside of exclosures during 1972-1976, 1998, and 2009, and also outside of exclosures during 2009. After 33-46 years of protection from sheep grazing, patterns of recovery varied among exclosures, but overall mamane trees and native shrubs showed a trend of increased cover between the 1970s and 1998, and slowing or declining recovery between 1998 and 2009. Trends in cover of native herbaceous vegetation were variable between the 1970s and 1998, and then showed a general pattern of decline between 1998 and 2009. Mamane height class distributions indicated that tree recruitment was initially high but then declined as heights shifted toward larger size classes, presumably reflecting a shift to an older age distribution. We found some evidence of a negative effect of exotic species on forest recovery, but the effect was not consistent over time or among sites. Recurrent sheep presence had a negative effect on mamane canopy density and perhaps tree density across all sites, and on mamane condition at some sites. Our results indicate that the mamane forest has shown modest recovery at some sites, especially those protected for the longest amount of time. However, these protected sites represent a small portion of the mamane forest. Sheep grazing continues to impact mamane recovery in unprotected areas, and thus habitat quality for the palila.