Ecological forecasts are critical to managing and mitigating the effects of invasive species and climate change. Oftentimes, such forecasts rely on data that describe ecological patterns, rather than the processes by which those patterns occur. While some estimation is more useful than none, process-based forecasts of species responses to environmental change provide the opportunity to understand underlying mechanisms. Garlic mustard (Alliaria petiolata), an invasive biennial herb in the United States, can serve as a model system for both methodological development and comparative forecasting. Here, we compare the phenology and demography of garlic mustard to a native ecological analog, tower mustard (Arabis glabra), and four forest understory woody shrubs and vines in the northeastern United States. We ask whether earlier springtime phenology is linked to demography, and whether demography can aid our forecasting of regional species distributions with climate change. The data are derived from experimental plots established throughout the region, and represent a unique “experimental biogeography” approach to studying species-level responses to environmental gradients. We forecast population growth rates for establishing populations across the region with climate change using novel demographically-driven species distribution models based on integral projection models.
Garlic mustard on average bolted a week later, but flowered 4 days earlier than the native tower mustard. Annual growth was positively correlated with early bolting and flowering, suggesting that earlier reproduction provides a demographic advantage. Bolting of both herbs occurred at least 20 days after and with less variation than leaf-out of woody plants. Integral projection models parameterized with current climate, light availability, and soil characteristics and projected across the region showed distribution patterns consistent with known populations for all species. The models uncovered compensatory responses of growth and survival with seed production for garlic mustard, allowing its persistence in both open and closed canopy environments. Climate change projections revealed an unexpected reduction in garlic mustard establishment by mid-century, due to the negative response of all vital rates to higher temperatures. The mechanistic distribution model explains why garlic mustard might struggle to keep up with climate change in the northeastern US, and provides testable hypotheses about biogeographic range limits more broadly. The projected response of garlic mustard to climate change highlights the potential for negative effects of climate change on invasive plants and underscores the need to incorporate process into future forecasts of both native and invasive plants.