Appendix A. Descriptions of study sites and procedures for measuring productivity.
Jornada
Jornada is located in southern New Mexico (32o30' N, 106o48' W, elevation ca. 1300 m), at the northern terminus of the Chihuahuan Desert. Average annual precipitation is 263 mm (Knapp and Smith 2001). The permanent plots at Jornada include five community types: two grassland communities (black grama grassland [Bouteoloua eriopoda] and playa grasslands) and three shrubland communities (creosote shrubland [Larrea tridentata], mesquite scrub [Prosopis glandulosa], and tarbush shrubland [Flourensia cernua]). Three regions occured within each of the five community types and each of the 15 regions contained 49 1-m2 plots dispersed at 9-m intervals in a 7 × 7 grid (Huenneke et al. 2001). Therefore, the area sampled by plots in each region was 49 m2 and the mean distance among regions was 5.9 km. Individual plants were identified to species and aboveground biomass was estimated non-destructively by using species-specific regressions of volume and weight (Huenneke et al. 2001). Plant volume was calculated on the basis of plant cover and height. Data were collected three times per year, in the fall, spring, and winter.
The annual composition for each plot was determined by aggregating species composition across all seasons within a calendar year (i.e., if a species is present in at least one season, it is counted as present for the year). The annual aboveground net primary productivity (ANPP) for each plot in a particular year equaled the sum of individual species productivities for that year (g·m-2·yr-1). The annual productivity for individual species was calculated as the sum of the positive increments in biomass of that species between (1) winter and fall, (2) spring and winter, and (3) fall and spring (Huenneke et al. 2001). All plant species except Yucca elata were included in estimates of productivity and richness. The life history of Yucca results in large interseasonal pulses in aboveground biomass, attributable to the formation of large flowering stalks. These stalks likely draw on belowground resources stored over many years (Smith and Ludwig 1976, 1978) and may not provide a good estimate of annual production.
Konza
Konza Prairie Biological Station is a 3487 ha tallgrass prairie preserve located in the Flint Hills of northeastern Kansas (39°05'N, 96°35' W, elevation ca. 400 m). The climate is continental and variable (Knapp and Smith 2001) with an average annual precipitation of 835 mm (75% between April and September). Grassland communities at Konza Prairie are dominated by a few warm-season (C4) grass species, Andropogon gerardii and Sorghastrum nutans, and to a lesser extent Panicum virgatum, A. scoparius and Bouteloua curtipendula. A number of less common warm- and cool-season (C3) grass, forb, and woody species compose the bulk of the diversity (Freeman and Hulbert 1985).
Konza is divided into 60 watershed units (ca. 60 ha each) that are subjected to different fire frequency (1-, 2-, 4- and 20-year return interval) and seasonality (burn in spring, summer, fall, or winter) treatments. Each watershed contains a similar topographic gradient (ca. 100 m), with soil type and depth differing between upland and lowland areas. Five permanent circular 10-m2 plots are located along four randomly placed 50 m long transects in upland and lowland areas in each watershed. The five plots were arranged in a linear array within each transect and there was no dispersion between adjacent plots within a transect. For each circular plot, species composition (% cover of each species) was estimated twice during the growing season, once in late May and in mid- to late August. Hence, species composition at Konza is reflected only by those species that were present at Konza from May to August. All of the aboveground biomass that accumulated over a growing season was harvested from five 0.1-m2 plots located adjacent to each species composition transect at the end of the growing season when production is at its annual maximum (Briggs and Knapp 1995). ANPP was estimated as the sum of grass, forb, and current-year dead biomass after drying and expressed as g·m-2·yr-1. Data from the species composition plots within a transect were pooled together as it is not possible to associate any of the biomass plots with a specific species composition plot. Consequently, we refer to an entire species composition transect as a 50-m2 plot, with four of these plots (i.e., transects) within each topographic position within each watershed.. Biomass plots were averaged at the transect level. The mean distance among transects within a region was 154 m and the mean distance between regions (irregardless of whether they occured in the same watershed) was 1.1 km. The mean distance between upland and lowland regions within a watershed was 249 m.
We restricted analyses to data from eight watersheds (i.e., those burned annually in the spring, fall, or winter) to avoid potential confounding effects of succession and different rates of disturbance on estimates of species turnover. Watersheds burned in the summer were excluded because they were not burned consistently on an annual basis.
LITERATURE CITED
Briggs, J. M., and A. K. Knapp. 1995. Interannual variability in primary production in tallgrass prairie: climate, soil moisture, topographic position and fire as determinants of aboveground biomass. American Journal of Botany 82:1024–1030.
Freeman, C. C., and L. C. Hulbert. 1985. An annotated list of the vascular flora of Konza Prairie Research Natural Area, Kansas. Transactions of the Kansas Academy of Science 88:84–115.
Huenneke, L. F., D. Clason, and E. Muldavin. 2001. Spatial heterogeneity in Chihuahuan Desert vegeation: implications for sampling methods in semi-arid ecosystems. Journal of Arid Environments 47:257–270.
Knapp, A. K., and M. D. Smith. 2001. Variation among biomes in temporal dynamics of aboveground primary production. Science 291:481–484.
Smith, S. D., and J. A. Ludwig. 1976. Reproductive and vegetative growth patterns in Yucca elata Engelm. (Liliaceae). Southwestern Naturalist 21:177–184.
Smith, S. D., and J. A. Ludwig. 1978. Further studies on growth patterns in Yucca elata Engelm. (Lilliaceae). Southwestern Naturalist 23:145–150.