Biomass and density responses in tallgrass prairie legumes to annual fire and topographic position

Annually burned tallgrass prairie is purported to be a nitrogen-limited system, especially when compared to unburned prairie. To test the hypothesis that legumes, potential nitrogen-fixers, would increase in relative abundance in annually burned sites, we assessed their density and biomass for two seasons on upland and lowland soils in annually burned and unburned watersheds. Total legume density was significantly higher in burned (8.0 ± 1.0 [SE] stems/m²) than in unburned watersheds (3.0 ± 0.3 stems/m²). Species with higher (P < 0.05) densities in burned than in unburned prairie included Amorpha canescens, Dalea candida, Dalea purpurea, Lespedeza violacea, Psoralea tenuiflora, and Schrankia nuttallii. Desmodium illinoense was the only legume that responded negatively to annual fire. Total legume biomass did not differ between burned (11.3 ± 1.3 g/m²) and unburned prairie (10.5 ± 0.9 g/m²). Biomass productions of Dalea candida and Psoralea tenuiflora were higher (P < 0.05) in burned than in unburned sites, but biomasses of other legumes were similar between burn treatments. Average individual stem masses of Amorpha canescens and Baptisia bracteata were significantly greater in unburned than in burned prairie. Legumes were affected differentially by topographic location. Total legume density was higher (P < 0.05) on lowland soils (6.6 ± 1.0 stems/m²) than on upland soils (4.3 ± 0.5 stems/m²). However, total legume biomass was not different between lowland soils (12.0 ± 1.2 g/m²) and upland soils (9.9 ± 1.0 g/m²). Densities and biomasses of Amorpha canescens, Desmodium illinoense, and Lespedeza capitata were higher on lowland sites than on upland sites, whereas densities and biomasses of Baptisia bracteata and Dalea purpurea were higher on upland than on lowland soils. Most legume species are either fire tolerant or exhibit a positive response to fire and their persistence in annually burned prairie suggests that they may play an important role in the nitrogen budget of this ecosystem.     

Below-ground carbon and nitrogen accumulation in perennial grasses: A comparison of caespitose and rhizomatous growth forms

An experiment was conducted to compare below-ground soil organic carbon and total nitrogen accumulation between caespitose and rhizomatous perennial grasses in long-term (<25 yrs) grazed and ungrazed sites in semi-arid and mesic communities in the North American Great Plains. Development of greater nutrient pools beneath than between clones occurred at minimal clone basal areas (<60 cm²) for both caespitose species. Caespitose grasses accumulated substantially greater pools of carbon (20–200 fold) and nitrogen (50–500 fold) in soils to a depth of 10 cm beneath clones than rhizomatous grasses accumulated in rhizomes in both communities. Carbon and nitrogen pools in soils beneath caespitose clones exceeded combined (soil + rhizome) pools for rhizomatous grasses for a majority of the clone basal areas (>90 cm²) in the mesic community. In contrast, both pool sizes were smaller beneath the caespitose grass at all clone basal areas than the combined pools for the rhizomatous grass in the semi-arid community. The occurrence of larger soil nutrient pools beneath the rhizomatous species in the semi-arid community was largely a consequence of niche separation for microsites characterized by soils with higher nutrient concentrations, rather than plant-induced increases in nutrient concentrations. Although nutrient islands do not occur beneath rhizomatous grasses, their distribution in the semi-arid community was restricted to microsites characterized by soils with higher SOC and N concentrations. A greater efficiency of nutrient accumulation per unit rhizome mass and the maintenance of rhizome nutrient pools of similar magnitude to those of the rhizomatous grass in the mesic community may also contribute to the distribution of rhizomatous grasses in semi-arid communities. The existence of nutrient islands beneath a wide range of clone sizes in both mesic and semi-arid communities provides circumstantial evidence to suggest that nutrient islands beneath caespitose grasses may contribute to clone fitness in this growth form.     

Mitigating impacts of weeds and kangaroo grazing following prescribed fire in a Banksia woodland

Banksia woodlands are renowned for their flammability and prescribed fire is increasingly employed to reduce the risk of wildfire and to protect life and property, particularly where these woodlands occur on the urban interface. Prescribed fire is also employed as a tool for protecting biodiversity assets but can have adverse impacts on native plant communities. We investigated changes in species richness and cover in native and introduced flora following autumn prescribed fire in a 700‐hectare Banksia/Tuart (Eucalyptus gomphocephala) woodland that had not burnt for more than 30 years. Effectiveness of management techniques at reducing weed cover and the impacts of grazing by Western Grey Kangaroo (Macropus fuliginosus) postfire were also investigated. Thirty plots were established across a designated burn boundary immediately before a prescribed fire in May 2011, and species richness and cover were measured 3 years after the fire, in spring of 2013. Fencing treatments were established immediately following the fire, and weed management treatments were applied annually in winter over the subsequent 3 years. Our results indicate that autumn prescribed fire can facilitate increases in weed cover, but management techniques can limit the establishment of targeted weeds postfire. Postfire grazing was found to have significant adverse impacts on native species cover and vegetation structure, but it also limited establishment of some serious weeds including Pigface (Carpobrotus edulis). Manipulating herbivores in time and space following prescribed fire could be an important and cost‐effective way of maintaining biodiversity values.     

Assessing the Rate, Mechanisms, and Consequences of the Conversion of Tallgrass Prairie to Juniperus virginiana Forest

We assessed the determinants and consequences of the expansion of Juniperus virginiana L. (red cedar) populations into central US grasslands using historical aerial photos and field measurements of forest extent, tree growth, fire-induced mortality, and responses in herbaceous species diversity and productivity. Photos from northeast Kansas dating back to 1956 indicate that native tallgrass prairie can be converted to closed-canopy red cedar forest in as little as 40 years (a 2.3% increase in forest cover per year). Mean tree density in 21 forested sites ranged from 130 to 3500 trees/ha, with most sites at more than 800 trees/ha. In younger stands, maximum growth rates of individual red cedar trees exceeded 20 cm/y in height. Land management practices were critical to the establishment and growth of red cedar forest. Grazing reduced the fuel loads by more than 30% in tallgrass prairie. Based on measurements of mortality for more than 1800 red cedar trees, fire-induced mortality in grazed areas averaged 31.6% versus more than 90% at ungrazed sites. When tallgrass prairie was converted to red cedar forest, herbaceous species diversity and productivity were drastically reduced, and most grassland species were virtually eliminated. Consequently, community structure shifted from dominance by herbaceous C₄ species to evergreen woody C₃ species; this shift is likely to be accompanied by alterations in carbon storage and other ecosystem processes in a relatively short time period. Here we present a conceptual model that integrates the ecological and socioeconomic factors that underlie the conversion of grassland to red cedar forest. Received 29 August 2001; accepted 5 February 2002.