Alex W. Ireland

Hydroclimatic variability drives episodic expansion of a floating peat mat in a North American kettlehole basin

The coming century is predicted to feature enhanced climatic variability, including increased frequency, intensity, and duration of extreme climatic events. Ecologists are faced with the critical challenge of anticipating potentially nonlinear ecosystem responses to these changes. High-resolution paleoecological data sets that capture past ecosystem responses to climate variability provide valuable long-term perspectives on the sensitivity of ecosystems to climate-forced state shifts. We used a suite of paleoecological analyses at Titus Bog in northwestern Pennsylvania, USA, to test the hypothesis that the development and expansion of floating peatlands in kettlehole basins represents a threshold response to hydroclimate variability. In contrast with expectations of gradual autogenic peat mat expansion, our results indicate that peat mat expansion at Titus Bog was highly episodic and occurred in three distinct pulses centered on 800, 650, and 400 cal yr BP. Each of these expansion events coincided with or immediately followed decadal-to-mutlidecadal droughts recorded in regional paleoclimate reconstructions. These patterns indicate that peatland development in kettlehole basins can follow nonlinear trajectories, with episodes of rapid advancement triggered by climatic forcing. Future climate changes may increase the likelihood of peatland expansion in kettlehole basins, potentially leading to abrupt changes in adjacent lake ecosystems.

Drought as a Trigger for Rapid State Shifts in Kettle Ecosystems: Implications for Ecosystem Responses to Climate Change

Global climate change has raised important questions about ecosystem resilience and the likelihood of unexpected and potentially irreversible ecosystem state shifts. Conceptual models provide a framework for generating hypotheses about long-term ecosystem processes and their responses to external perturbations. In this article, we review the classic model of autogenic peatland encroachment into closed-basin kettle lakes (terrestrialization) as well as studies that document patterns of terrestrialization that are inconsistent with this hypothesis. We then present a new conceptual model of episodic, drought-triggered terrestrialization, which is consistent with existing data and provides a mechanism by which climatic variability could cause non-linear patterns of peatland development in these ecosystems. Next, we review data from comparative studies of kettle lakes along a peatland-development gradient to explore potential ecological and biogeochemical consequences of non-linear patterns of terrestrialization. Finally, we identify research approaches that could be used to test conceptual models of terrestrialization, investigate the ecological implications of non-linear patterns of peatland development, and improve our ability to predict responses of kettle systems to climate changes of the coming decades and century.

Widespread dust deposition on North American peatlands coincident with European land-clearance

Ecosystems around the world are being subjected to numerous human disturbances. Climate change and land degradation are the most obvious of these disturbances and have received much attention. However, easily overlooked, indirect disturbances can also alter ecosystem structure and function. Dust deposition is a prime example of an easily overlooked disturbance process. We hypothesized that historic European settlement and land-clearance in eastern North America led to widespread wind erosion of upland soils and subsequent dust deposition onto otherwise undisturbed peatlands, potentially fertilizing these naturally nutrient-poor ecosystems and causing shifts in plant communities. We tested these hypotheses by analyzing 11 peat profiles collected across a broad region of eastern North America. We documented a strong correlation between the concentrations of Ambrosia pollen grains and microscopic mineral particles, interpreting this as a signal of dust deposition coincident with European settlement and land-clearance. Analysis of Sphagnum macrofossils revealed substantial site-to-site variability in both the degree and the direction of ecological response to dust deposition, but suggested that increasing magnitude of dust deposition increased the likelihood of a decline in the relative abundance of Sphagnum. Results also suggested that raised bogs were more sensitive to dust deposition than kettle peatlands. We conclude that European settlement and land-clearance resulted in widespread dust deposition on peatlands, leading to ecological changes in some of these ecosystems, and leaving behind a coherent dust horizon in the late-Holocene peatland stratigraphy of eastern North America. This easily overlooked indirect disturbance process could be ongoing today in areas of widespread soil disturbance and could potentially further alter dust-receiving ecosystems.

Bob Marshall's forest reconstruction study: three centuries of ecological resilience to disturbance1

Abstract In 1924, Bob Marshall and his Harvard University advisor Richard Fisher developed an integrated historical approach for forest reconstruction to address ecological questions and provide insights to forest management. Their approach utilized complementary methods and data: dendrochronology, diverse historical records (e.g., deeds, town and oral histories, and census and lumber mill records), and intensive field mapping and sampling of the site including stumps and uproot mounds, and forest composition and structure. Marshall applied this approach broadly to a 61-ha Tsuga canadensis (eastern hemlock)-Pinus strobus (eastern white pine) forest on sandy dry soils and intensively to a 0.15-ha sub-plot. He sought to test his hypothesis that pine and hemlock displayed compositional resilience to disturbance and to understand the life-history and growth characteristics of the two species that enabled co-dominance. Marshall concluded that, in contrast with the successional tendencies of white pine across New Englands mesic uplands, pine and hemlock had dominated on these dry, sandy soils since at least the early 1700s and through multiple episodes of logging. Based on his interpretation of the contrasting growth rates and shade tolerance of these species, he developed a simple model of forest development and silviculture to guide management on dry sites for pine and hemlock timber production. Fisher, Marshall, and Harvard Forest colleagues used these historical insights to plan a suite of harvesting experiments on the site in 1924–25 to perpetuate hemlock and pine. Unfortunately, the hurricane of 1938 and subsequent salvage logging terminated the experiment. In 2007, we tested Marshalls interpretations and prediction of resilience in this forest by examining its long-term response to the series of intense disturbances. We synthesized decades of observations, photographs, and data, and relocated Marshalls plot to measure forest age and size structure, composition, and site features including decaying stumps, pits, mounds, and bent and sprouting individuals. The current hemlock and pine forest is strikingly similar in structure and composition to that of 1924. These results reinforce Marshalls conclusion that hemlock and white pine forests on well-drained sandy soils can be remarkably resilient in composition to intense disturbance. The work highlights the development and application of an integrated approach to forest reconstruction by Marshall and Fisher and underscores the contribution of historical insights to addressing basic ecological questions, designing large long-term field experiments, and guiding forest conservation and management.

Provisional, Forested Ecological Sites in the Northern Appalachians and Their State-and-Transition Models ☆

On the Ground The identification of unique areas of vegetative potential across the Northern Appalachians is complicated by a long land-use history of vegetation management. We introduce provisional ecological sites and associated state-and-transition models for the region, which can be differentiated by latitudinal drivers of: precipitation and temperature; local parent material and resulting soil differences; and landscape position, slope, or aspect. Identification of ecological sites and associated States or Phases in the Northern Appalachians provides land managers with quantifiable benchmarks for assessing forest compositional shifts due to natural or anthropogenic disturbance.