From King County's website: "Located in the center of Vashon Island, Island Center Forest is a 363-acre working forest and nature preserve that is managed to demonstrate sustainable forest management while protecting and restoring the health of the site's habitat. The property is owned and managed by King County Parks in collaboration with community partners who make up the Friends of Island Center Forest."
The only official access point listed on the King County website to have horse trailer parking space is at SW 188th St and 107th Ave SW (end of SW 188th St). Other access points for local riders are noted on the first map below.
From the north, follow Vashon Hwy and turn right onto SW 188th St (approximately 5.5 miles from ferry terminal). From the south, follow Vashon Hwy northwards and turn left onto SW 188th (a little more than 8 miles from ferry terminal). Follow 188th about 1/2 mile to its end.
Length: 9+ miles of trails
Surface: gravel, dirt
Share with: hikers, joggers, bicycles
King County Parks has added a downloadable "brochure" (including a map) for this park to their "Backcountry Trails Map Brochure Series."