Just a few minutes uphill from the last visible farm structures (there is a large shed and some water tanks off to the left), at about 3,800 feet, you come upon another fenceline with a gate in it. This is the Haleakala National Park Boundary. The difference in vegetation between the 'farm' side of the fence and the park side of the fence is striking. On the park side, everything is much lusher and untouched.
Beautiful natural landscape
Above the park boundary, the combination of low trail use and thick untouched grasses makes the Kaupo Trail very faint through this section. There are a decent number of fairly obvious park service trail posts to help, though.
This section of trail is very beautiful as it winds through large koa tree forest.
At the 4,000-foot mark you encounter a sign indicating that it is a further 3.9 miles to Paliku camp (which is just beyond the terminus of the Kaupo Trail).
Above the 4,000-foot mark, the terrain alternates between grassy spots underneath arching trees and short stretches of open meadow. The tread underfoot gradually becomes less loamy and more dry and rocky as you ascend. The density of the forest, too, starts to decrease. As you approach 5,000-feet, the transition to open alpine scrubland is well underway.
Pleasantly winding upwards
courtesy JInnes
The trend to scrubbier terrain, a rocky path and cooler temperatures continue as you ascended towards 6,000 feet. The face of the eastern wall of Kaupo Gap, under which you have been hiking for some time now, gets higher and rockier. Deep side-canyons are cut into its slope.
The last thousand feet of elevation on the Kaupo Trail have a slightly more 'constructed' character: Instead of just meandering over the land surface, as it had done a bit lower down, up here the trail is more purposefully engineered, carefully graded and switchbacked, cutting through rock ridges, and built up when crossing over little ravines.
courtesy BConnell
Ramparts of upper Kaupo Gap
Eastern side of Kaupo Gap
At approximately 6400 feet, 11km (6.5 miles) from the Kaupo Trailhead, the trail flattens out, and you arrive at the junction with the Halemau'u trail. The Halemau'u trail is one of the main trails in the park that leads down from the main park road. From here, you are only a few flat-walking minutes away from the Paliku campsite.