Primary production in an ecosystem refers to the total amount of living plant matter produced in a unit area, such as per acre or per square meter. Annual primary production in a northern temperate climate refers to the total production of plant biomass by the end of the growing season. Plant growth stops with water stress or cold temperatures. A fall or winter season with freezing temperatures stops plant production until the following spring when plants renew their growth.
At the western edge of the tallgrass area, Konza Prairie has averaged 416 g/m2 annual aboveground net primary production over the last 20 years. Spring fires increased plant biomass on lowland sites in all but the driest years. In annually burned sites grasses contributed more than 80% of the biomass. Sites burned after several years of protection from fire had the greatest positive response. Forbs contributed more in unburned areas. Topographic position strongly influenced primary production in burned areas, but in unburned areas upland and lowland biomass weights/unit area were not significantly different. In intermittently burned areas, variability was higher than could be accounted for by meteorological conditions. Multiple limits to primary production might be light, nitrogen (N) availability or water.
The purpose of this activity is to assess the total aboveground biomass per unit area in plots of native prairie in late September after peak primary production has occurred. These observations will allow a long-term evaluation of changes in biomass production over time as well as from site to site across Kansas. Variability may be due to differences in soil type, slope, rainfall or other climatic factors.
The aboveground biomass is sampled in plots not subject to trampling or other disturbance. These plots are defined as on a hillside (slope), upland or lowland site. A large area for aboveground biomass samples is marked with permanent stakes for long-term recognition. Above ground biomass is clipped at ground level in 10 or more quadrats (0.1 sq. m.) per site. Every quadrat clipped is marked at its center with a plastic flag or more permanent marker to avoid subsequent re-clipping in following years. After 4 years the same area could be clipped again. A map or “gps” location of the clipped quadrats should be kept on file.
The SLTER plots are sampled toward the end of September after peak biomass has occurred. Total aboveground biomass/unit area (dry weight) will be assessed. Prairie vegetation is separated into live vs. dead and also could be sorted into grass/sedge vs. forb or shrub components. Material is separated before air or oven drying. Samples from areas not burned that spring must be sorted for old growth vs. this year’s growth before drying. Discard the old growth. Samples from areas burned that spring are not sorted in this way unless it is noted that some of the material did not burn and is from last year. Annual production graphs for burned and unburned prairie are plotted from averages of at least 10 samples.
In burned areas, all biomass is clipped at ground level and carefully bagged in the field. In unburned sites, it is best to sort the current year's growth from the previous years' dead biomass in the lab on top of a plastic bag or large piece of paper. The current year's growth is sorted into live grass/sedges, forbs and woody components. Woody plants are separated from forbs because of the difference in drying time. Each quadrat is sorted into these three components. All sorted samples are air- or sun-dried for 1-4 days or oven-dried at 60C for 24-48 hours. Samples should be acclimated to room conditions (air-conditioning) for about an hour before weighing.
A copy of the data sheet used to record weights is given in the Teacher’s Resources section.