No abstract

Models of plant-plant interactions underpin our understanding of species coexistence, invasive plant impacts, and plant community responses to climate change. In recent studies, models of competitive interactions failed predictive tests, thereby casting doubt on results of many past studies. We believe these model failures owe at least partly to heterogeneity in unmodeled factors (e.g., nutrients, soil pathogens) that affect both target plants and neighboring competitors. Such heterogeneity is ubiquitous, and models that do not account for it will suffer omitted variable bias. We used instrumental variables analysis to test for and correct omitted variable bias in studies that followed common protocols for measuring plant competition. In an observational study, omitted variables caused competition to seem like mutualism. In a quasi-experiment that partially controlled competitor abundances with seeding, omitted variables caused competition to seem about 35% weaker than it really was, even though the experiment occurred in an abandoned agricultural field where environmental heterogeneity was expected to be relatively low. Despite decades of research, consistently accurate estimates of competitive interactions remain elusive. The most foolproof way around this pr)oblem is true experiments that avoid omitted variable bias by completely controlling competitor abundances, but such experiments are rare.

Aristida purpurea (purple threeawn) is a competitive native perennial grass with monoculturistic tendencies and poor palatability. We examined effects of fire, defoliation, and interspecific/intraspecific planting for 1) threeawn responses in the presence of threeawn, Bouteloua gracilis, or Pascopyrum smithii, and 2) B. gracilis and P. smithii response with threeawn. Biomass, aboveground production, tillers, and axillary buds were analyzed following two fire and four clipping treatments applied to three species-pair combinations in a completely randomized factorial design with nine replications. Fire killed 36% of threeawn. Fire reduced surviving threeawn biomass 61% and reduced production 27%. Threeawn production was greatest when neither plant was clipped and least when competing species were moderately clipped, or when both plants were severely clipped. Tiller counts of burned threeawn were similar among clipping treatments, and less than non-clipped or moderately clipped plants not burned. Fire decreased threeawn axillary buds on average by 25%. Moderately clipped plants had greater production than those from other clipping treatments across species. Average threeawn percentage of pot biomass was greater with B. gracilis (46 6 3% SE) than P. smithii (38 6 3% SE). Fire reduced threeawn from 60 6 3% to 23 6 3% of pot biomass, indicating good potential for rapid reductions in threeawn dominance and restoration of plant diversity with fire.

No abstract

Fire behavior associated with wild and prescribed fires is variable, but plays a vital role in how a plant responds to fire. Understanding the relationship between fire behavior and rangeland plant community response will help to improve the use of prescribed fire to achieve management objectives. Fire is an important ecological process in many rangeland ecosystems and can be used as a tool to maintain grassland plant communities and shift community composition. Purple threeawn (Aristida purpurea Nutt.) is a grass native to North America that has poor forage quality and the ability to form near monocultures. Therefore, the identification of tools to reduce purple threeawn abundance is desirable. We assessed the effects of summer and fall prescribed fire on purple threeawn plant basal area, tiller production, and plant mortality one growing season post fire in the northern Great Plains. Thermocouples and portable data loggers were used to measure the maximum temperature, heat duration, and heat dosage that individual purple threeawn plants experienced. Fire reduced basal area and tiller production 59 % and 57 %, respectively. Heat dosage (C-statistic = 0.69) and heat duration (C-statistic = 0.65) were good predictors of purple threeawn mortality. A restrospective analysis showed maximum temperatures were similar for fall and summer fires but heat duration and dosage were 44 % and 21 % greater for summer fires, respectively. Our results indicate that purple threeawn is a fire sensitive species. The ability to predict purple threeawn mortality could enhance the efficacy of prescribed fire as tool to restore purple threeawn and other Aristida-dominated plant communities.