Tree Cover Changes in Mamane (Sophora chrysophylla) Forests Grazed by Sheep and Cattle

Abstract

Using aerial photographs taken in 1954, 1965, and 1975, percentage of tree cover was determined for three sections of the sheep- and cattlegrazed mdmane (Sophora chrysoph lla) forest of Mauna Kea , Hawaii. In one section , the Ka'ohe Game Management Area, where grazing by sheep was judged light, tree cover increased slightly during the 21-yr period , and tree cover did not change significantly along an elevation gradient. This condition was probably the result of the predominance of naio (Myoporum sandwicense) trees, which are not as palatable as mamane and, therefore, are less sensitive to browsing. In the Mauna Kea Forest Reserve, the most heavily sheep browsed of the three section s, a significant loss of tree cover was observed between 1965and 1975near tree line where feral sheep tended to concentrate their browsing. Of the three sections examined, Parker Ranch, which was grazed mainly by cattle, sustained the greatest loss of tree cover during the 21-yr period, reflecting the more destructive nature of cattle browsing as compared to sheep browsing. Increases of tree cover in areas relatively free of sheep within the Mauna Kea Forest Reserve indicated that habitat for the palila, an endangered bird that depends on the mamane forest , will improve slowly after feral sheep are removed.