Shifts in grassland invasibility: effects of soil resources, disturbance, composition, and invader size

• Published in: Ecology (Durham) Vol. 87; no. 9; pp. 2264 - 2277
• Main Authors: Renne, Ian J., Tracy, Benjamin F., Colonna, Ignacio A.
• Format: Journal Article
• Language: English
• Published: Washington, DC Ecological Society of America 01-09-2006
• Subjects: Amaranthus - growth & development; Amaranthus - physiology; Amaranthus hybridus; Analysis of Variance; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Biodiversity; Biological and medical sciences; Biomass; botanical composition; C3 grassland composition; C3 plants; C4 plants; Cattle; Demecology; disturbed soils; diversity; Ecological invasion; Ecology; Ecosystem; Environmental conditions; environmental factors; forbs; Fundamental and applied biological sciences. Psychology; Grasses; grassland soils; Grasslands; grazing intensity; invasion resistance; invasive species; Legumes; Lighting; Magnoliopsida - physiology; nitrate nitrogen; nutrient-induced invasibility reversal; Oxazines - analysis; pasture plants; plant available water; plant communities; plant growth; Plants; Plants and fungi; Population Dynamics; precipitation; Random Allocation; resource pulses; seedling emergence; Seedlings; shifting mosaic structure; smooth pigweed; Soil - analysis; soil disturbance; soil nutrients; Soil resources; Species; Species diversity; Synecology; Time Factors; Urine; Water; water availability; Weeds; Illinois; United States; North America; America; Availability; Grassland; soil disturbance; Composition; Amaranthus hybridus; C3 grassland composition; resource pulses; Diversity; Environmental factor; invasion resistance; Perturbation; Invasion; water availability; Soils; Invasive species; Sensitivity resistance; smooth pigweed; Nutrient; nutrient- induced invasibility reversal; Population dynamics; shifting mosaic structure
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/0012-9658(2006)87[2264:SIGIEO]2.0.CO;2

There is an emerging recognition that invasibility is not an intrinsic community trait, but is a condition that fluctuates from interactions between environmental forces and residential characters. Elucidating the spatiotemporal complexities of invasion requires inclusion of multiple, ecologically variable factors within communities of differing structure. Water and nutrient amendments, disturbance, and local composition affect grassland invasibility but no study has simultaneously integrated these, despite evidence that they frequently interact. Using a split-plot factorial design, we tested the effects of these factors on the invasibility of C₃ pasture communities by smooth pigweed Amaranthus hybridus L., a problematic C₄ forb. We sowed seeds and transplanted 3-week old seedlings of A. hybridus into plots containing monocultures and mixtures of varying composition, subjected plots to water, soil disturbance, and synthetic bovine urine (SBU) treatments, and measured A. hybridus emergence, recruitment, and growth rate. Following SBU addition, transplanted seedling growth increased in all plots but differed among legume and nonlegume monocultures and mixtures of these plant types. However, SBU decreased the number and recruitment rate of emerged seedlings because high residential growth reduced light availability. Nutrient pulses can therefore have strong but opposing effects on invasibility, depending on when they coincide with particular life history stages of an invader. Indeed, in SBU-treated plots, small differences in height of transplanted seedlings early on produced large differences in their final biomass. All facilitative effects of small-scale disturbance on invasion success diminished when productivity-promoting factors were present, suggesting that disturbance patch size is important. Precipitation-induced invasion resistance of C₃ pastures by a C₄ invader was partly supported. In grazed grasslands, these biotic and environmental factors vary across scales and interact in complex ways to affect invasibility, thus a dynamic patch mosaic of differential invasion resistance likely occurs in single fields. We propose that disturbance patch size, grazing intensity, soil resource availability, and resident composition are inextricably linked to grassland invasions and comment on the utility of community attributes as reliable predictors of invasibility. Lastly, we suggest temporal as well as spatial coincidences of multiple invasion facilitators dictate the window of opportunity for invasion.