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Understanding how plant communities of the past have responded to disturbance events can provide valuable insights when managing our natural resources and assessing human impacts on ecosystems. The geologic record has the potential to reflect these responses through the analysis of functional traits, which relate directly to plant function and ecosystem strategy. There is currently little evidence of how functional traits measurable in fossil leaves vary across succession in different forest types. Because of this, there is a limited ability to identify disturbance as the primary driver of vegetation change within the fossil record. To improve this ability, this study analyzes the carbon stable isotopic composition (δ 13C) of bulk organic matter sampled at the community-scale across successional gradients in a temperate deciduous forest (North Carolina, USA) and compares them against values from a previous study across succession in a tropical evergreen forest (Malaysian Borneo). Leaf δ13C is representative of a plant's water use efficiency (WUE), an important axis of ecological strategy representing the carbon assimilated per water lost in a plant during photosynthesis. Leaf δ13C as a functional trait has the advantage that it is often preserved during leaf fossilization and, integrated across a plant community, can be informative about prevalent ecological strategies, functional diversity , and community assembly dynamics. In Borneo, the community-weighted mean of leaf δ13C to be highest in early-succession plots, indicative of a higher WUE in plant communities closely following a disturbance event. Old growth plots were found to have a lower δ13C, and thus a more conservative WUE. This study will further investigate if this trend is followed within temperate forests, which is important as many mid-late Cenozoic plant assemblages come from what would have been temperate regions. Developing a method of identifying disturbances within the geologic record, will improve the ability to discern drivers of plant community change in the past. This improved knowledge will help guide management decisions across a range of ecosystems. 

Paleobotanical records provide opportunity to deepen an understanding of plant community ecology by reconstructing the outcome of large-scale ecological ‘experiments’ in Earth’s past. However, limited ability to describe ancient communities via plant functional traits and ecological strategies, rather than (para)taxonomic composition, can hinder the relevance of constructed datasets. Many functional traits are not measurable on fossil leaves and the link between leaf morphology and ecological strategy are currently unresolved. To help fill this gap, we analyze leaf traits applicable to fossil leaves (i.e., morphology, vein density, leaf mass per area) sampled at the community-scale from modern plots spanning successional gradients, where plant function and ecological strategies are expected to vary, in three different forest types: temperate deciduous forest (North Carolina, USA), tropical rainforest (Malaysian Borneo), and a tropical dry forest (Minas Gerais, Brazil). Preliminary results will be presented to draw empirical links between morphological leaf traits and ecological strategy. 

The U.S. Pacific Northwest (PNW) hosts an extensive suite of Miocene-aged fossil plants sites, with the potential to document changes in plant community ecology in response to regional climatic change during the Miocene Climatic Optimum (MCO; 17-14 Ma) and the ensuing Middle Miocene Climatic Transition (MMCT; ~14 Ma). The MCO was the most recent period of sustained global warming and thus provides some analogy to anthropogenic climate change. An important component of characterizing plant community ecology is the diversity and prevalence of ecological strategies present within a community. Many previous paleoecology studies rely on a nearest living relative approach to infer components of ecological strategy (e.g., plant functional types) from fossil plant assemblages. In contrast, much work in neo-ecology stresses the importance of functional traits in elucidating prevalent ecological strategies and functional diversity within plant communities. Here we take advantage of exquisitely preserved leaf compression fossils from Clarkia, northern Idaho (~16.9 Ma), representing the height of the MCO, to measure leaf functional traits and elucidate ecological strategies of dominant species in this ancient temperate mixed conifer-deciduous-evergreen forest. We focus on 13 species, representing the most abundant angiosperm taxa in the assemblage, including Betula, Castanea, and Quercus. We reconstruct assimilation rates using gas exchange modeling, address leaf hydraulic efficiency by measuring leaf vein density, and reconstruct water use efficiency by accounting for the ratio of carbon assimilation to transpirational water loss. As these species are prevalent in many other Miocene floras of the PNW, this study provides a benchmark by which to interpret changes in the dominance or presence of these species through time and, by inference, how Miocene climatic changes impact the functional composition and diversity of this forest type. We are also providing an example of how present-day mixed deciduous forests may respond to current anthropogenic changes in CO 2. 

Degree of canopy cover is linked to transpiration, carbon cycling and primary productivity of an ecosystem. In modern ecology, canopy structure is often quantified as Leaf Area Index (LAI), which is the amount of overstory leaf coverage relative to ground area. Although a key aspect of vegetation, the degree of canopy cover has proven difficult to reconstruct in deep time. One method, Reconstructed Leaf Area Index (rLAI), was developed to infer canopy structure using the relationship between non-grass leaf epidermal phytolith (plant biosilica) morphology, and leaf coverage in modern forests. This method leverages the observed correlation between epidermal phytolith size, shape (margin undulation), and light availability. When more light is available in a canopy, epidermal phytoliths tend to be smaller and less undulate, whereas less light availability is linked to larger and more undulate epidermal phytoliths. However, the calibration set used to develop this method was compiled from field sites and samples from localities in Costa Rica and it remains unclear how applicable it is to temperate North American fossil sites due to lack of data from relevant vegetation types and taxonomic differences between plant communities in the Neotropics vs. mid-latitude North America. For example, preliminary results measuring rLAI in phytolith assemblages from the Miocene of the North American Great Plains have yielded surprisingly high degrees of canopy density despite containing high relative abundances of open-habitat grasses. To test whether vegetational and taxonomic differences impact the calibration set, we constructed a new North American calibration using 24 quadrats from six sites, representing reasonable modern analogs for Miocene vegetation in eastern North America. Specifically, we sampled in Bennett Springs State Park in Lebanon, MO; Mark Twain National Forest in Rolla, MO; Tellico in Franklin, NC and Congaree National Park in Hopkins, SC. All sites include a range of canopy covers and vegetation types, from oak savannas and oak woodlands to mixed hardwood forests, pine savannas, and old growth bottomland forests. From each quadrat, we collected a soil sample and took hemispherical photos of the local canopy. From modern soil samples, biosilica was extracted in the lab, yielding phytolith assemblages which were scanned for epidermal phytoliths using a compound microscope. Recovered epidermal phytoliths size and margin undulation were measured and assemblage averages were used to predict measured LAI at each quadrat. Hemispherical photographs were processed using the software Gap Light Analyzer to obtain LAI values. We hypothesize there will be a linear relationship between actual LAI and LAI calculated from epidermal phytolith morphology, but its relationship will differ from that found in South America. Results will be used to reevaluate canopy coverage in sites within the Great Plains Miocene as well as applied to Pacific Northwest Miocene sites, both to understand changes to vegetation during global climatic events in their respective regions. 

The US Pacific Northwest (PNW), including Washington, Oregon, and Idaho, hosts an extensive suite of Oligocene–Miocene fossil plant sites that have the potential to showcase terrestrial vegetation and climate response to several pronounced environmental perturbations. These include the Mid-Miocene Climatic Optimum (MMCO; ca. 17-14 Ma), the Middle Miocene Climatic Transition (MMCT; ca. 14-13 Ma), and the eruption of the Columbia River Basalts (~95% of its volume 16.7 to 15.9 Ma). This collaborative study focuses on 18 PNW fossil plant sites spanning ca. 32 to 10 Ma, many of which have extensive pre-existing macrofossil collections. First, we radiometrically date interbedded tuffs at these sites to establish a high-resolution temporal framework, using U-Pb/CA-ID-TIMS. We present new dates for the Clarkia/Emerald Creek, Alvord Creek, Juliaetta, Pickett Creek, Whitebird, and Trout Creek fossil sites. Within this temporal framework, we are: 1) documenting regional climate change in the PNW during the MMCO and MMCT using paleobotany-based paleoclimate proxies, and 2) providing an integrated perspective on the response of plant communities to these mid-Miocene environmental changes by combining macrofossil, palynomorph, and phytolith evidence. Taken together, these data will provide a regionally-comprehensive perspective on the sensitivity of terrestrial vegetation and climate to global climatic events known more extensively from marine records.