PURPOSE:
To estimate seed reproduction and population densities of three dominant prairie grasses: big bluestem, little bluestem, and indiangrass in each of the Konza Prairie LTER watersheds.
LOCATION OF SAMPLING STATIONS
In the grazed watersheds N01A and N04A, sampling is done in the paired 5 x 5 m exclosures (eight exclosures per watershed). In ungrazed watersheds 001C, 001D, 004A, 004B, 020B, and 020D the sampling is done close to the transects for plant species composition and above-ground biomass measurement on Tully and Florence soils.
FREQUENCY OF SAMPLING:
Once per year at the time of seed maturation (late Sept. or Oct.).
VARIABLES MEASURED (see sample data sheets, Figs. 19 and 20)
1) Flowering stem height in centimeters
2) Density of flowering stems (No. per m2)
3) Weight of flowering stems (g per m2)
4) Weight of seeds or inflorescences (g per m2)
METHODS:
Ungrazed Watersheds:
Sampling sites:
Because these measurements involve destructive sampling, no permanently marked plots were set up. All samples are taken near the permanently marked species composition plots at each LTER site.
Individual flowering stem heights and seed weights:
A quasi-random walk is initiated adjacent to the permanent LTER transects during which 25 sampling points per transect (100/LTER treatment) are located at intervals of about 2 m. At each sampling point, the stem height for the nearest flowering individual of each of the three species is measured to the nearest cm. Mean flowering stem height is calculated for each species at each site from the 100 values. At the first 10 points (first 10 plants sampled), the inflorescence of each individual is clipped and placed in a separate bag. The seed heads of each species are oven dried at 60oC and weighed. From this data (n = 40 plants per species), mean seed weight per plant is calculated for each species at each LTER site.
Weight and density of flowering stems:
Along a transect adjacent to the permanent LTER transects, six 50 x 50 cm plots per transect (x 4 transects = 24 per site) are sampled at every 10 paces. Within each of these plots, the number of flowering stems of each species is counted and the stems are clipped at ground level, bagged by species, oven dried, and weighed. The density of flowering stems (No. per m2) and the mean biomass of flowering stem (g per m2) are calculated for each species. Finally, total seed production (g per m2) for each species is calculated as the product of the flowering stem density and mean seed wight per individual stem.
Following drought years (when reproduction by the three species may be very low) flowering stem density is estimated during the fall by counting flowering stems in the 10 m2 circular plots. Flowering stem height and seed weights are measured on those species flowering adjacent to the transects. Using this technique, sample size is variable for flowering stem height and seed weight measurements. Low sample sizes may necessitate pooling replicate LTER treatments.
Grazed Watersheds (exclosures):
Height and Density of Flowering Stems:
In each grazed LTER watershed there are eight permanent 5 x 5 m grazing exclosures, four located on Tully soil sites and four located on Florence soil sites. Adjacent to each permanent exclosure, another 5 x 5 temporary exclosure is erected and remains in place only during the year during which stem density and biomass data are collected and only from May 1 until the vegetation reaches peak biomass. This provides exclosures in pairs, the permanent exclosure half providing an ungrazed treatment and the temporary exclosure half providing a grazed treatment (grazing is only temporarily prevented to allow regrowth biomass and flowering stem measurements).
Watershed Site Exclosure Nos. Year started
N01A Florence 1 - 4 1988
N01A Tully 5 - 8 1988
N01B Florence 1 - 4 1992
N01B Tully 5 - 8 1992
N04A Florence 9 - 12 1988
N04A Tully 13 - 16 1988
N04D Florence 9 - 12 1992
N04D Tully 13 - 16 1992
Within both the grazed half and ungrazed half of the paired exclosures, four 50 x 50 cm quadrats are randomly distributed (excluding the edge within 1 m of the exclosure fence) and the number of flowering stems of each of the three grass species are counted and recorded. In addition, four of the flowering stems of each species are randomly selected and their heights are measured to the nearest cm. The density of flowering stems (No. per m2) and mean flowering stem height are calculated for each species at each LTER site.