SGS-LTER Ecosystem Stress Area - long-term density dataset following nutrient enrichment stress on the Central Plains Experimental Range in Nunn, Colorado, USA 1975-present

Water, nitrogen, and water-plus-nitrogen at levels beyond the range normally experience by shortgrass steppe communities were applied from 1971 through 1975, plant densities were sampled through 1977, and then sampling resumed in 1982, with sampling frequencies changing from annually to every other year. The initial sampling from 1970 to 1974 showed that the water and water plus nitrogen treatments had the strongest effect on plant community structure, both treatments increased biomass, and exotic weed species were noted on the water plus nitrogen treatment. Later sampling from 1982 to 1991 showed a ten-fold increase in exotic weed species on the water plus nitrogen plots as compared to the controls (Milchunas and Lauenroth 1995), a community change that has persisted on this site due to a chronic elevation of soil nitrogen caused by a plant tissue/soil organic matter feedback mechanism (Vinton and Burke 1995). In 1998, Six new treatments were superimposed on the historic study site. The six new treatments were: control, sugar, lignin, sawdust, lignin and sugar, and sawdust and sugar.In 2010, plots will be sampled every 5 years. Our objective in this study is to examine how plant communities change through time and explore implications of these changes for monitoring potentially stressed ecosystems.

1971年至1975年，研究人员向短草草原（shortgrass steppe）群落施加了超出其正常耐受范围的水分、氮肥及水氮复合处理；1971年至1977年持续开展植物密度采样，随后采样工作于1982年恢复，且采样频率从每年一次调整为每两年一次。1970年至1974年的初始采样结果显示，水分处理与水氮复合处理对植物群落结构的影响最为显著，两类处理均提升了群落生物量（biomass），且水氮复合处理样地中出现了外来杂草物种。1982年至1991年的后续采样表明，相较于对照组，水氮复合处理样地的外来杂草物种数量增长了10倍（Milchunas与Lauenroth，1995）；由于植物组织-土壤有机质（soil organic matter）的反馈机制引发土壤氮素长期富集（Vinton与Burke，1995），该样地的群落变化得以持续维持。1998年，研究团队在该历史研究样地增设了6组新处理措施，分别为：对照组、蔗糖组、木质素组、锯末组、木质素+蔗糖组以及锯末+蔗糖组。2010年起，样地采样频率调整为每5年一次。本研究旨在探究植物群落随时间的演变规律，并探讨这些变化对潜在受胁迫生态系统监测的启示意义。