Date of Award
Doctor of Philosophy
Tallgrass prairie is one of the most threatened grasslands in North America. Conservation of tallgrass prairie focuses on both effective management of remaining native prairie, and restoration of formerly cultivated fields to tallgrass prairie. This research focused on processes and properties relevant to restoration and conservation of tallgrass prairie. Community assembly theory attempts to explain the formation of communities, which can be governed by deterministic or stochastic processes, or some combination of both. Fire and grazing are widely used to manage grasslands for conservation purposes, but few studies have evaluated the effects of these drivers on the conservation value of plant communities measured by the floristic quality index (FQI). Pollination services play a vital role in the reproductive stability of the plant community in prairies, though this has not been well studied in restored prairie. The first chapter of this dissertation reports on a sequential restoration approach used to gain insights into the extent to which community assembly is deterministic and stochastic events change the trajectory of community development. The sequential restorations consisted of former agriculture fields restored to prairie, varying only in time since abandonment. Species composition and aboveground net primary production were quantified over time in sequentially restored communities to reveal the predictability of ecological restoration in producing desired communities and ecosystem functions. The sequential restoration plots were established in a block design. The same suite of species was seeded using the same seeding rates in each restoration sequence. Species composition was recorded each September in the year of seeding and each June and September in the two subsequent years for each block. Annual aboveground net primary productivity was collected from 2 randomly placed 0.1 m2 plots per subplot during peak biomass. There was a significant sequence by age interaction for sown, volunteer, and total species composition. Sown, volunteer, and total cover, diversity, and richness also were affected by a sequence by age interaction. Annual net primary production (ANPP) also was affected by a sequence by age interaction for sown and volunteer species. However, total ANPP was only affected by the variable age. Results show that interannual climate variability (specifically growing season precipitation) inhibits a priori determinations of community assembly, which suggests that stochastic processes play a significant role in the community assembly process in tallgrass prairie restoration. Variations in annual precipitation during the installation years likely drove the initial differences in species composition and ANPP. In general this study revealed that drought conditions at the time of restoration may be more deleterious than drought conditions occurring at other times post-establishment. The influence of fire and grazing on soil properties and functions is difficult for land managers and restoration practitioners to assess. Therefore, the objectives for the second study were to (i) to quantify the independent and interactive effects of grazing and fire frequency on floristic quality in native tallgrass prairie, and provide potential benchmarks for community assessment, and (ii) to explore whether floristic quality can serve as an indicator of soil structure and function for more holistic ecosystem assessments. A factorial combination of fire frequencies (1-2, 4, and 20 y return intervals) and grazing (by bison or ungrazed) treatments was sampled for plant species composition and several indicators of soil quality in lowland tallgrass prairie. Floristic quality, diversity, and richness were higher in grazed than ungrazed prairie over all fire frequencies. Available inorganic N, microbial biomass N, total soil N, and soil bulk density were also higher in grazed prairie soil over all fire frequencies. Microbial biomass C, total soil organic C, and total soil N were positively correlated with FQI. This study demonstrated that floristic quality and soil N pools are more strongly influenced by grazing than fire and that floristic quality can be an indicator of total soil C and N stocks in never cultivated lowland prairie. In tallgrass prairie, 85 – 90 % of angiosperms require an insect or other animal for pollen transfer. Restorations can play a vital role in the reestablishment of pollination services and simultaneously help maintain high levels of diversity in the tallgrass prairie ecosystem. Missed pollination, via temporal asynchronies, could have a number of biological disadvantages for a plant population. In the third study we addressed the effects of missed pollination on floral period, photosynthetic activity, leaf N content, and seed set in a common native tallgrass prairie forb, Penstemon digitalis. In each of 12 plots, 6 individual plants were either bagged to prevent pollination, or left unbagged, to allow for pollination. There was no difference in mean flower duration between netted and open plots. There was a treatment by time interaction for relative chlorophyll concentrations (P = 0.0005). Mean relative chlorophyll concentrations values at sampling times (prior to flowering; Pre, 10 days after FFD; Plus, 10 days after cessation of flowering; Post, and at the time of leaf collection; Coll) all differed from one another in bagged plots. Comparisons of relative chlorophyll concentrations values only showed a difference between bagged and open-pollinated plots at sampling time ‘Coll’. There was no difference in percent leaf N between bagged and open-pollinated plots and the amount of seed set in bagged plots was significantly lower than the amount of seed set in the open-pollinated plats. Any disruption to plant-pollinator interactions can create temporal asynchronies, however, the impact of those asynchronies is equivocal.
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