Grass-dominated ecosystems cover about one-third of Earth's land surface, influence global biogeochemical processes (e.g. water, carbon, and nutrient cycling), and serve as major food sources for humans and herbivores. These ecosystems are among the most sensitive to future changes in climate and atmospheric chemistry. This sensitivity can be partially attributed to the differential responses of two major functional groups, C3 and C4 plants. C4 plants gain competitive advantages under certain environmental conditions (e.g. fire, heat, drought, and low pCO2). C3 and C4 plants in turn have distinct influences on major biogeochemical processes; for example, the role of C4 plants in global net primary production is disproportionately large. Despite these well-known differences, it remains controversial what environmental factors directly control the relative abundances of C3 and C4 plants. Paleoecological studies can help decipher the responses of C3 and C4 plants to a wide range of environmental conditions, thereby offering information for anticipating future changes. This project will evaluate the effects of fire, climate, and atmospheric CO2 concentration on the variation of C4-grass abundance over the past 25,000 years in East Africa and Australia. The researchers will analyze sediment cores for a suite of ecological proxy indicators and develop a novel technique to estimate the abundance of C4 plants in geological records. This project provides an excellent context for educating the general public on climate change and grassland ecology. The researchers will organize summer courses targeting middle and high school teachers as well as land managers.
Intellectual Merit: Typical ecological studies are no more than 3-5 years in length and even the longest long-term ecological studies span only several decades. Yet, many ecological processes (particularly at the ecosystem level) vary at time scales that are longer than most observational studies. The paleoecological approach, which is employed in our study, is invaluable because it enables observation of ecological responses to a wide range of environmental conditions over long periods of time and across broad spatial scales. Our project tested a long-standing controversy about the environmental controls of grass-dominated landscapes. Understanding the relative importance of factors (e.g., fire, climate, CO2 concentrations) controlling grass-dominated ecosystems, and particularly C4-grass abundance, is important because C4-grass dominated grasslands and savannas are globally widespread, and uncertainty exists concerning their response to future environmental change. Our project made a significant technical advance to the field of paleoecology. A major impediment to previous paleo-grassland studies has been the inability to distinguish C4 and C3 grasses, because the most common proxy for grasses in paleorecords, grass pollen, is morphologically indistinct beyond the family level. The novel technique (Single Pollen Isotope Ratio AnaLysis = SPIRAL) that we have developed for analyzing the stable carbon isotopic composition of individual grass-pollen grains overcomes this challenge. As part of our project we improved this novel technique. We expanded an existing SPIRAL training dataset of pollen carbon isotope composition and validated it using field data collected from the surface sediments of lakes in Africa and Australia. We also developed and tested a statistical model to estimate the presence/absence and relative abundance of C3- and C4-grass pollen in unknown samples. We tested a hypothesis regarding the relative importance of pCO2 and climate conditions as controls of the evolution of C4 grasses; we found that C4 grasses originated in some parts of the world when atmospheric CO2 concentrations were high, challenging the paradigm that a precipitous decrease in atmospheric pCO2 led to the origin of C4 plants. Our charcoal data indicate that fire in the dry lowland savanna of southeastern Kenya was not fuel-limited during the Last Glacial Maximum (LGM) and Late Glacial, in contrast to the prevailing pattern in many other regions throughout the world. The relative importance of savanna burning in the region varied primarily in response to changes in rainfall and dry-season length, which were controlled by tropical monsoon dynamics. The fuel limitation that characterizes the region’s fire regime today appears to have begun around 5,000-6,000 years ago, when warmer interglacial conditions coincided with prolonged seasonal drought. These results showed that climatic impacts on dry-savanna burning are temporally heterogeneous, which has important implications for anticipating future shifts in fire-mediated ecosystem processes. At our high elevation sites in Africa and Australia C4-grass abundance was high during the glacial period, declined during the glacial-interglacial transition, and remained relatively low during the Holocene (last 11,500 years), suggesting that C4-grass abundance was driven by shifts in pCO2 and/or moisture. At low elevations, C4-grass abundance fluctuated with no pattern corresponding to the glacial to interglacial transition in environmental conditions. These fluctuations were likely driven by varying influences of rainfall and evaporation. These pronounced spatiotemporal variations in the environmental controls of C3 and C4 grass abundances indicate that future vegetation shifts are likely to be non-stationary and that unanticipated shifts in vegetation composition may occur. Broad Impacts: Our outreach activities centered on two summer workshops (in 2010 and 2012) for middle- and high-school teachers. These workshops provided 17 science educators with a framework for linking ecological concepts and cutting-edge research, with a focus on past, present, and future ecosystem change. The workshops helped the teachers build on core ecosystem concepts through current research, develop a framework for engaging students in critical thinking and active learning about ecology, and make professional connections among themselves, scientists, and students. Materials used or developed as part of the course were distributed to the teachers, published in a peer-review article, and presented at several national conferences. The materials are available to a broader audience at www.life.illinois.edu/hu/eew. We incorporated assessment of our summer short-courses in order to help us to better develop our outreach activities and also reveal important information about the skills and aptitude required for students to learn ecology. Our project also made significant contributions to human resource development by providing opportunities for research, teaching, and/or mentoring to ~6 undergraduate students, ~8 graduate students, and a postdoctoral researcher.
