Average percent coverage of each plant species per 5-meter segment along a 30-meter line intercept perpendicular to each of 91 stations on each of the three LTER-I permanent transects. Data consists of week number, transect, station number, species, carbon reduction pathway, habit, form, and average percent cover. It is sorted by transect, station number, then plant species. Measurements are made on 30-meter line intercept transects located perpendicular to each of 91 stations on each of the 2.7 km long Control, Treatment, and Alternate Control Transects established at the beginning of LTER-I. Following January 1, 1987, spring line- intercepts will only record cover of annual species. All perennial cover data will be limited to fall samples. Both annuals and perennials are measured during the fall. Measurements were made biannually from 1982 - 1988. After this they are measured every 5 years. Annuals are measured through 1998; however, after this only perennials are measured and only in the fall. Each plant line transect is divided into 6 5-meter segments. Annuals are measured only in the first meter of segments 1, 2, and 3 and in the last meter of segments 4, 5, and 6. Perennials are measured for the full length of all segments. BACKGROUND. In the spring of 1982, as part of the establishment of the Jornada Long-Term Ecological Research site in southern New Mexico, a 135 ha portion of a 1500 ha, internally drained, watershed was exclosed from grazing by domestic livestock. Prior to exclosure the watershed, as well as the rest of the Jornada basin, had been moderately to heavily grazed for the past 100 years. Concurrent with grazing, the vegetation had undergone a dramatic change from desert grassland, with an almost continuous cover of C4 perennial grasses, to isolated patches of the original grassland in a mosaic with desert shrub dominated plant communities (Buffington and Herbel, 1965). The exclosure lies along a northeast facing piedmont slope at the base of a steep isolated mountain peak, and covers a variety of component landforms from the foot of the mountain to the basin floor. This provided the opportunity to investigate the response of vegetation with respect to landscape characteristics as well as release from grazing.
Standing biomass is sampled three times a year: in winter (February - March), before shrubs begin spring growth; in spring (May), when shrubs and spring annuals have reached peak biomass; in fall (late summer; October), when summer annuals have reached peak biomass but before killing frosts. At each sample date, each site is visited (order of sampling may vary, according to phenological stage of sites) and the dimensions of each plant on each quadrat are measured and recorded. Recorded for each observation are: date, zone, plot, quadrat #, species (4 letter acronym), observation # (one for each measurement of that species in that quadrat), cover (percentage of quadrat covered by canopy of that individual or species), height (vertical extent of that individual or species), count (if multiple individuals with the same dimensions are present), and phenological stage (Flowering/fruiting or Vegetative). Attention: These data are not appropriate for estimates of percentage cover. NPP-associated percent cover measurements were developed for and are used solely to provide the best estimate of biomass production. Becuase the methodology results in measurements of overlapping subcanopy systems and canopies of adjacent individuals, NPP percent cover measurements are not an appropriate measure of actual aerial plant cover. Doing so will result in inflated numbers for the "actual" vegetative cover.