A Shift From Competition To Facilitation With Abiotic Stress is Limited For Two Codominant Grass Species

It remains unclear how competitive exclusion is avoided between two ecologically, economically, and culturally important codominant grass species in the tallgrass prairie of the Great Plains, Andropogon gerardii and Sorghastrum nutans. These functionally similar C4 grasses appear to coexist despite considerable niche overlap, and asymmetric competition and drought tolerance in favor of A. gerardii. According to the stress gradient hypothesis, it may be that the sum of interactions between these species, which is typically negative (competitive) due to similar resource requirements, shifts to positive (facilitative) as abiotic stress increases. For instance, if the canopy cover of the stronger competitor reduces losses of subcanopy humidity or shallow soil moisture, recruitment of S. nutans tillers may be extended further into the drought event than would occur in the absence of A. gerardii. As later months of the growing season are drier on average where these species are codominant, such a mechanism may enable S. nutans to recover from early season asymmetric competition and stabilize their codominance. We tested this hypothesis in a greenhouse experiment in which we manipulated community composition and water availability in the latter half of the growing season. We found no evidence that a shift from a negative to positive interaction occurs, with each species performing similarly in mixed communities and monocultures. The similarities of the two species in their functional traits and responses to water limitations may limit such a shift in interaction net effects and suggests that other mechanisms are determining coexistence of these co-occuring C4 grasses.

Eating While the Eating’s Good: How Fire Creates a Magnet for Grazing Animals

Tallgrass prairie is disappearing because farming and development have replaced it. This ecosystem is home to a unique group of plants, animals, and microbial life. The processes of fire, grazing by animals, and drought are important to the tallgrass prairie. They can influence each other and prairie life. For example, pyric-herbivory is the interaction of fire and grazing on the landscape. Burned areas attract herbivores (plant eaters) like a magnet. After fires, plant growth is nutritious and easy to find. Herbivores prefer grazing recently burned areas, creating patches of different habitats that support many other wildlife species, too. You can see pyric-herbivory in action at the Tallgrass Prairie National Preserve, where fire, cattle, and bison are a part of the preserve’s management team! Healthy tallgrass prairie needs both fire and grazing.

PSIII-7 Seasonal effects on nutritional value of Oklahoma prairie grass when grazed in extensive or intensive systems

The objectives of this study were to determine the effects of sampling month and grazing system on tallgrass prairie forage nutritive value. Native tallgrass prairie pastures (n = 6) were assigned to either an extensive (EXT) or intensive (INT) grazing system for 5 consecutive years. Extensive system pasture annual stocking rate was 6.1 hectares per cow-calf pair with cows remaining in their respective pastures year-round. Intensive system pasture annual stocking rate was 3.0 hectares per cow-calf pair with cows present in the pastures 160 days per year [mid-May through mid-July (58 ± 6 d) and mid-September through mid-December (102 ± 4 d)]. Forage samples were collected monthly from two to four locations within each pasture. Samples were dried, separated into subsamples of leaf material and whole plant matter, ground, and analyzed using near-infrared spectroscopy. Data were analyzed using a mixed model (SAS 9.4) with grazing system, month, sample type (leaf or whole), and all two-way interactions as fixed effects and year as a random effect. Forage crude protein (CP) was not different between leaf or whole plant samples (P = 0.97) and increased (P < 0.001) in May (10.8% CP), declined in the fall, and stayed low (2 – 4% CP) in both systems. Neutral detergent fiber (NDF) was greater during May to July (P = 0.040) in the EXT than the INT pastures. Acid detergent fiber (ADF) differed by month (P < 0.001) and was lowest between April and May. In vitro true dry matter digestibility (IVTDMD) was greater in INT (63.7%) than EXT (62.3%) pastures regardless of month (P < 0.001), and both systems were greater during May through August (65 – 75%; P < 0.001) than the winter months (56 – 60%). These data indicate that protein supplementation is likely required outside of May and June, especially for lactating cows.