2004. By 2007 the uplift had slowed, though the area was still considered potentially active. A study published in 2010 described the magma intrusion as having a volume of about . Scientists determined in 2013 that the uplift had slowed to a rate of about per year, compared to up to per year in the early 2000s. Because of the uplift at South Sister, the USGS planned to increase monitoring of the Three Sisters and their vicinity by installing a global positioning system (GPS) receiver, sampling airborne and ground-based gases, and adding seismometers. The agency completed a GPS survey campaign, including planning, documentation, and data processing and archiving, at South Sister in 2001, as well as annual InSAR radar observations from 1992 to 2001, and it followed up with campaign GPS and tilt-leveling surveys in August 2004. As of 2009, semi-permanent GPS networks have been deployed every year at the Three Sisters, continually showing that inflation persists at South Sister. Ecology The ecology of the Three Sisters reflect their location in central Oregon's Cascade Range. The westside slopes from lie in the Western Cascade Montane Highlands ecoregion, where precipitation is abundant. Forests here consist predominantly of Douglas-fir and western hemlock, with minor components of mountain hemlock, noble fir, subalpine fir, grand fir, Pacific silver fir, red alder, and Pacific yew. Vine maples, rhododendron, Oregon grape, huckleberry, and thimbleberry grow beneath the trees. Douglas-fir is dominant below , while western hemlock dominates above. Timberline at the Three Sisters occurs at , where the forest canopy opens and the subalpine zone begins. These forests lie in the Cascade Crest montane forest ecoregion. Mountain hemlock trees dominate the forest in this area, while meadows sustain sedges, dwarf willows, tufted hairgrass, lupine, red paintbrush, and Newberry knotweed. Tree line occurs at . The vegetation in this harsh alpine zone consists of herbaceous and shrubby subalpine meadows. This zone has a large winter snowpack, with low temperatures for much of the year. There are patches of mountain hemlock, subalpine fir, and whitebark pine near the treeline, as well as wet meadows supporting Brewer's sedge, Holm's sedge, black alpine sedge, tufted hairgrass, and alpine aster. Near the peaks of the Three Sisters, there are extensive areas of bare rock. The lower eastern slopes of the Three Sisters below lie in the Ponderosa Pine/Bitterbrush Woodland ecoregion. This ecoregion has less precipitation than the western slopes and has