Impact of Goats to native koa tree reproduction?

Goats came to Hawaii nearly 200 years ago and are considered wild and free to explore native Hawaii ecosystems. The climates the goats live in are dry and lowlands to the mountains. Studies done showed that goats would eat the native koa tree roots or suckers which help reproduction process and inhibit the growth of new koa trees on Mauna Loa. Because of the suckering the koa trees are endangered to grow maturely. Normally koa trees grow 10cm in the unfenced and free forest areas but at .5 meters to 2 meters in height they are missing their suckers. Researchers found that there are hundreds of koa trees growing inside the goat exclosure in which most koa trees are dead or about to die. This shows that the goats are being a nuisance in native forest areas and are truly affecting the reproduction cycle of the koa tree. What should be done to help stop the goats in hindering the reproduction cycle of the koa tree?