Shortgrass Steppe LTER

Widely varying Modeling results underscore the value of long-term data

The shortgrass steppe is a semi-arid ecosystem, receiving approximately 320 mm of precipitation a year. Precipitation is the primary influence on livestock forage production in the shortgrass steppe, so it is important for researchers to understand the nature of this relationship in order to make predictions about how the ecosystem may respond to climate change. Lauenroth and Sala (1992) analyzed a 52-year data set of annual forage production and precipitation collected at the Shortgrass Steppe LTER site. They compared their model of vegetation response to precipitation over a long time period in one ecosystem to another model that had been constructed using precipitation and forage production data from many ecosystems across a precipitation gradient in the Great Plains at one point in time. The model based on data collected from one point in time offered significantly different predictions about the response of vegetation to precipitation in the shortgrass steppe as compared to the long-term model developed by Lauenroth and Sala. The one-point-in-time model overestimated forage production during wet years and underestimated forage production during dry years (Fig. 1). This is a significant source of error should the one time model be used to predict responses of a single ecosystem to climate change that would not have been discovered without the use of long-term data.

Figure 1. Relationships between aboveground net primary production and annual precipitation for a regional model (one-time model) for the Central Grassland Region of the U.S. (Sala et al. 1988) and for the long-term model developed in Lauenroth and Sala 1992. Shaded areas represent 95% confidence intervals.

Forage production and precipitation on the shortgrass Steppe vary greatly, which makes management and planning a constant challenge. Furthermore dry periods are common but droughts are not. Severe droughts are very infrequent (only three appeared in our 52 year dataset). Years with low forage production are also frequent and don't always correspond to low total precipitation. Forage production is related to total precipitation but precipitation explains less than half of the variability in forage production. Finally, there do not appear to be any cycles in either precipitation or forage production.

24-Year Dataset reveals that grazing less important than precipitation for predicting total forage

The shortgrass steppe is extensively grazed by domestic cattle. Researchers at the Shortgrass Steppe LTER investigated the relative effects of variation in annual precipitation and different levels of livestock grazing on annual forage production by analyzing a 24 year dataset. They determined that annual forage production was more sensitive to variations in precipitation than to long-term differences in grazing regimes (Milchunas et al. 1994), an important consideration when making management decisions about livestock grazing in shortgrass steppe.

Studying exotic plant invasion in the shortgrass steppe

In the early 1970s, LTER researchers added water, nitrogen, and the combination of the two to experimental plots on the shortgrass steppe, in amounts far exceeding what this ecosystem normally experiences. These treatments were continued for five years, and data on plant community composition were collected for each year of treatment. At the end of this experiment, the plots which had received excess water and water plus nitrogen had significantly higher biomass than the nitrogen or control plots, but at that time no large differences in plant community composition between the treatments were apparent (Lauenroth et al. 1978). The researchers predicted a gradual reversion back to normal vegetation with the cessation of enrichment. Seven years after the cessation of the treatments, researchers returned to these plots and sampled the vegetation for an additional 10 years. They observed large changes in community composition as a result of the past enrichments, most notably the water and nitrogen enriched plots had been invaded by exotic weeds which were dominating the community (Milchunas and Lauenroth 1995), a condition that persists into today (Lowe 2000). Conclusions and predictions about how the shortgrass steppe responds to resource enrichment based on the initial short-term experiment were vastly different from conclusions based on long-term monitoring of these plots, stressing the important of long-term research in our understanding of community response to disturbance.

Tracking Global Change on the Great Plains: 23-Year record illustrates climate change and ecosystem response

Bouteloua gracilis (blue grama) is the dominant plant species on the shortgrass steppe, and the dominant forage plant for domestic cattle in this system. Alterations in the abundance of this species could significantly impact livestock production and community composition. Researchers at the Shortgrass Steppe LTER analyzed a 23 year data set investigating the effect of temperature on the aboveground production of several important shortgrass steppe species, including blue grama. The researchers found that average annual minimum temperatures were increasing and had a negative impact on the production of blue grama, and a positive effect on the production of exotic, annual forbs (Alward et al. 1999). The examination of this long-term dataset provided scientists with a basis for predictions about how global warming may affect the shortgrass steppe. These results would not have been possible if the study was limited to the traditional time frame for conducting ecological studies, which is 3-5 years in duration.

Long-term Studies on the Effects of Nitrogen Fertilization and the uptake of Greenhouse Gases

Long-term research is essential to our understanding of how ecosystems will respond to the diverse factors involved in global change, such as how the ecosystem will respond to increasing amounts of nutrients. Aerobic soils, which characterize much of the earth’s land mass, are a significant sink for methane, a greenhouse gas. Short-term experiments on the shortgrass steppe that explored the relationship between nitrogen fertilization and the soil uptake of the greenhouse gas methane found no reduction in the soil uptake of methane with nitrogen fertilization (Burke, unpublished data). However, data from a long-term fertilization experiment (1976 to 1989) on the same soil type with a similar rate of nitrogen application, found that annual application of nitrogen reduced the uptake of methane by 41% (Mosier et al. 1991).