Studies of plant populations are critical for linking organismic to ecosystem-level phenomena and for understanding mechanisms driving responses to global change and patterns of biodiversity. I take a population approach to address questions of variation in productivity, plant responses to resource availability, and plant responses to ecosystem drivers in the context of global change such as species invasions and altered climate. I am broadly interested in the determinants of plant species coexistence and diversity, plant population dynamics, persistence mechanisms of rare species and demographic modeling, particularly matrix models. I endeavor to bring well-developed population theory and models to bear on new challenges presented by global climate change.
A central theme in my research is to understand linkages between individual interactions and population, community and ecosystem processes. For example, my dissertation research addressed questions about the community and ecosystem consequences of bud bank populations and meristem limitation across the central Great Plains grasslands. I found that the effects of key factors such as fire, grazing, and climatic variability on plant communities and aboveground net primary productivity may be mediated principally through effects on bud bank population dynamics. An important outcome of this research is that systems with a large reserve bud bank may be the most responsive to future climatic change or nutrient enrichment and may be most resistant exotic species invasions.
Regulation of spatial and temporal variability in aboveground net primary productivity
Knapp and Smith compared patterns of inter-annual variability (range and CV) in ANPP across 11 LTER sites. Contrary to predictions that biomes with the most variable precipitation (deserts) would also have the greatest inter-annual variability in ANPP, they found that ANPP was most variable in grassland biomes that were intermediate in mean annual precipitation and productivity. They hypothesized that low ANPP variability in desert
and arid grassland biomes is explained by meristem limitation, which constrains the ability of plants to respond to pulses of high resource availability. My work supports the meristem limitation hypothesis that meristem limitation along this regional gradient constrains responses to inter-annual changes in resource availability and demonstrates that bud bank densities decrease along a decreasing precipitation and productivity gradient among grasslands in North America (Dalgleish and Hartnett 2007). Grass bud banks show similar seasonal patterns in density across the precipitation gradient as well.
Demographic responses to limiting nutrients (N and water)
A clonal plant can be studied as a population of ramets with the size, shape, and growth of the genet determined by the demography of its parts. Growth of tallgrass prairie plants, many of which are clonal and maintain substantial bud banks, can be limited by water, nitrogen (N), and light. I hypothesized that tallgrass prairie plants may respond to increases in a limiting resource (N) through demographic effects on the bud bank. The objective was to test the effect of a pulse of N on (1) bud bank demography (2) plant reproductive allocation, and (3) ramet (tiller) size. I parameterized matrix models for genets of Sporobolus heterolepis and Koeleria macrantha considering each genet as a population of plant parts (buds and tillers). This study strongly supports that demographic mechanisms of the bud bank are important for driving grass responses to resource availability (Dalgleish et al. in review). Understanding bud bank dynamics has important implications far beyond the individual plant. Because of the importance of bud banks in grasslands, enhanced understanding of bud bank dynamics may lead to lead to improved predictive models of productivity and potential grassland responses to environmental change.
Fire and grazing ecology
I used long-term experiments at Konza Prairie Biological Station to explore the underlying demographic mechanisms responsible for tallgrass prairie responses to two key ecological drivers: fire and grazing. The data indicate that belowground bud banks mediate tallgrass prairie plant response (Dalgleish and Hartnett, in review). Fire and grazing altered rates of belowground bud natality, tiller emergence from the bud bank, and both short-term (fire cycle) and long-term (>15 year) changes in bud bank density. Lastly, the size of the reserve grass bud bank is an excellent predictor of long-term ANPP in tallgrass prairie and also of short-term interannual variation in ANPP associated with fire cycles, supporting our hypothesis that ANPP is strongly regulated by belowground demographic processes. Meristem limitation due to management practices such as different fire frequencies or grazing regimes may constrain tallgrass prairie responses to inter-annual changes in resource availability and to future climatic change. In addition, grasslands with depleted bud banks may have lower resistance to exotic species invasions.
Comparative research in Botswana savannas
My research builds the case that in North American grasslands, the effects of key factors such as fire, grazing, and climatic variability on plant communities and ANPP may be mediated principally through their effects on soil bud bank dynamics. In conjunction with colleagues at the University of Botswana, I am testing the generality of these ecological patterns. Compared with species of North American grasslands, African grasses in a preliminary study produced fewer belowground buds but showed a much higher percentage of tillers producing seed (Hartnett et al. 2006). A more extensive study was conducted in the spring of 2007 to compare inter-species allocation patterns to buds and seeds, as well as to compare bud bank size along a similar precipitation gradient to that found in North America (Dalgleish et al. in prep).
Impact of climate variability on population dynamics and species coexistence
I am currently learning new matrix modeling techniques (Caswell 2007) to explore the transient vs. long-term effects of climate variability on the demography and population dynamics of grassland and shrubland plants. The ability to apply classical population biology theory and models to climate change questions is actualized by remarkable, recently-digitized datasets from Idaho, Kansas, and Montana. This work represents an exciting opportunity to explore the relative importance of climate and biotic interactions for driving changes in population dynamics for individual species that will give insight into the potential effects of climate change.
My study of the bud bank has piqued my interest in the diversity of reproductive strategies employed by plants, specifically the population and community consequences of that reproductive diversity. Plants display an impressive array of reproductive patterns, strategies and requirements that can have consequences for genetic diversity, population dynamics, species coexistence and community diversity. I am currently developing a project using seed and bud bank studies in conjunction with modeling to explore the impact of reproductive allocation and on population dynamics and species coexistence.