Grasses are a diverse group of plants that play a significant role in many terrestrial ecosystems globally. Despite their importance, very little is known about where and when grasses originated. In particular, it remains unclear in which environment early grasses evolved. Current hypotheses, based primarily on phylogenetic work, suggest that early grasses emerged in closed habitats, such as forests, or in more open habitats along forest margins. However, there is little direct paleobotanical evidence to support either option. To understand the environmental context grasses evolved in, I will be reconstructing canopy openness using phytoliths from Argentinian fossil sites. Phytoliths are microscopic silica bodies deposited in or around plant cells, which can be preserved in the fossil record and used to reconstruct past vegetation. The phytoliths that I am analyzing are from the Las Violetas Formation (57.9-50.6 Ma) of Argentine Patagonia, a rock formation within an area known to hold the oldest records of grass phytoliths in South America. As a proxy for vegetation structure, I am using reconstructed Leaf Area Index (rLAI), which takes the area to perimeter ratio of non-grass phytoliths originated in the leaf epidermis and determines a value that corresponds to how much light passes through a canopy. A canopy with a high leaf coverage correlates to a high rLAI value and vice versa. Based on preliminary rLAI results, I expect to find that ancestral grasses lived in habitats with semi-open canopies, similar to modern shrublands. Investigating the ancestral environment of grasses can help us better understand their evolutionary history and potential environmental drivers that led to their success in terrestrial ecosystems. This information can help us gain insight on the vulnerability of grasses and grass-dominated habitats to environmental changes in the past, present and future.

Trait-based plant ecology can serve as a means to better understand shifts in ecological strategies within plant communities and how that affects greater ecosystem processes, such as productivity, across a period of major climate change. The most recent major global warming event prior to modern anthropogenic influences was the Miocene Climatic Optimum (MCO) ca. 17-14 million years ago. This event was a short aberration to a long-term cooling trend of the last 53 million years, with global temperature averages up to 8°C warmer than preindustrial averages. Changing climate conditions during the MCO may have led to plant community reshuffling, with many ecosystems possibly restabilizing with notably different optimal trait distributions. Functional traits such as leaf shape, size, and toothedness can be indicative of a plant’s ecological strategy. These leaf morphology variables have been shown to closely correlate with climate, highlighting their role in plant function and strategy, and can thus be used to statistically analyze community diversity across the MCO. I hypothesize that the MCO caused an overall increase in functional trait diversity through an increase in favorable environments, allowing plant reshuffling or migration of plants with new ecological strategies into existing communities. We expect to see this through trait distributions in a community diversifying as the MCO progresses. This study uses leaf functional trait data measured digitally from a range of Miocene fossil sites to assess trends and variances from before, during, and after the MCO. Statistical analysis will make use of a previously developed R package to assess functional diversity. These results will be crucial information in understanding the ecological response to today’s far more rapid climate change, as well as humanity’s response to the possible need for human-assisted plant community reshuffling by providing an example of how global warming affected vegetation in Earth’s past.

The geologic record has become an increasingly important source of information for scientists to observe how plant communities of the past have responded to disturbance. Currently, there is a limited ability to recognize disturbance as a primary driver of plant community change, as there is limited evidence of how functional traits – plant traits that relate directly with plant function and ecological strategy – that can be measured in fossil leaves vary across succession. In this study I will measure a functional trait to help better identify disturbance in the fossil record, the carbon stable isotopic composition (δ13C) of bulk organic matter in leaves sampled across a successional gradient following a disturbance. This trait is often preserved during leaf fossilization and is representative of a plant's water use efficiency (WUE), or the amount of carbon dioxide used by the plant during photosynthesis for a given amount of water that is lost during transpiration. It is currently not known the extent to which carbon isotopes measured at the community-scale reflect the successional stage of a plant community. In an effort to develop this tool, I hypothesize that the WUE of plant species within a community will become more conservative in later successional stages. In support of this hypothesis, I predict that the abundance-weighted community average of leaf δ13Cwill increase through succession. In addition, I hypothesize that δ13C as a proxy for WUE will be most confounded in early succession, before a tree canopy forms, due to seedling utilizing water resources more rapidly without having established root systems and thus predict a higher variance of δ13Cvalues in this earliest stage of succession (Cernusak 2020). This research will help develop a method of identifying disturbances within geologic records which can give guidance on management decisions regarding modern ecosystems.

In this study I investigate potential changes in plant community ecology in response to Earth’s most recent major global warming event, the Miocene Climatic Optimum (MCO). During the MCO (from 17-14 million years ago) global temperatures increased by approximately 8° C and CO2 levels increased by 300-400ppm. In assessing these ecological changes, I use minor leaf vein density (mLVD), a leaf functional trait correlated with photosynthetic rate, as a proxy for understanding plant community strategies. This trait corresponds with the spectrum of “fast” versus “slow” growing strategies described in plant physiology, with high mLVD in fast-growing plants facilitating higher photosynthetic rates, and low mLVD reflecting slow-growing persistence strategies with lower rates of photosynthesis. I hypothesize that global warming led to long growing seasons that enabled the dominance of ecological strategies that prioritize persistence over productivity (i.e., slow growing strategies), and more favorable climates increased the diversity of ecological strategies present within the community. Currently, I am measuring fossil leaf mLVD from specimens collected in the Pacific Northwest from sites representing before, during and after MCO. I examine the community-level distribution of this trait (mean, variance, kurtosis) and compare these values between sites, and thus across the MCO. I predict that plant community ecological diversity would increase during this global warming event; I also expect to see higher variance in distribution of mLVD values as warming temperatures opened new ecological niches, while mean mLVD would decrease due to an increase in persistence strategies correspondent with low mLVD. This work will help us not only to understand how plant communities responded to rising temperatures in the past but also how plant communities could potentially respond to changing climates in the future.

The Miocene Climatic Optimum was a period of rapid warming that occurred from 17 to 14 million years ago where temperatures rose 2-4°C above pre-warming estimates and CO2 concentrations increased to ~400-600 ppm. This event was coeval with the eruption of the Columbia River Basalts (16.6-15.9 Ma), a series of large flood basalts covering much of the Pacific Northwest. The combined forces of these events led to this period being characterized by tumultuous changes to Pacific Northwest plant communities. To quantify these changes, I am reconstructing canopy openness. Ranging from open deserts to closed rainforests, degree of canopy openness describes the amount of sunlight reaching the understory of a plant community. These differences in sunlight exposure affect the size and shape of leaf epidermal cells. Leaves grown in shaded conditions tend to have larger, more undulated epidermal cells when compared to those grown in full sunlight. In the fossil record, silica casts of those cells called phytoliths can be measured to reconstruct the canopy openness of ancient ecosystems. I am using samples from four fossil sites in Central Oregon: Hawk Rim (16.4-16.2 Ma), Mascall (15.1 Ma), Haystack Valley (23-18 Ma), and Picture Gorge Basalts (17.23-16.06 Ma). These sites range from immediately before the Miocene Climatic Optimum (MCO) through the first two million years of warming. They also include samples from sedimentary layers interbedded with basalts. Therefore, they will provide insight on changes that occurred within the plant community both as warming began and because of volcanic eruption. I hypothesize that increased temperature and CO2 concentrations resulted in longer growing seasons and a CO2 fertilization effect. These conditions promoted high vegetation productivity and therefore closed canopies. Additionally, I expect that areas impacted by eruption will exhibit open canopies due to repeated disturbance preventing the re-establishment of forests.