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Wildfire is a ubiquitous disturbance agent in subalpine forests in western North America. Lodgepole pine ( Pinus contorta var. latifolia), a dominant tree species in these forests, is largely resilient to high-severity fires, but this resilience may be compromised under future scenarios of altered climate and fire activity. We investigated fire occurrence and post-fire vegetation change in a lodgepole pine forest over the past 2500 years to understand ecosystem responses to variability in wildfire and climate. We reconstructed vegetation composition from pollen preserved in a sediment core from Chickaree Lake, Colorado, USA (1.5-ha lake), in Rocky Mountain National Park, and compared vegetation change to an existing fire history record. Pollen samples ( n = 52) were analyzed to characterize millennial-scale and short-term (decadal-scale) changes in vegetation associated with multiple high-severity fire events. Pollen assemblages were dominated by Pinus throughout the record, reflecting the persistence of lodgepole pine. Wildfires resulted in significant declines in Pinus pollen percentages, but pollen assemblages returned to pre-fire conditions after 18 fire events, within c.75 years. The primary broad-scale change was an increase in Picea, Artemisia, Rosaceae, and Arceuthobium pollen types, around 1155 calibrated years before present. The timing of this change is coincident with changes in regional pollen records, and a shift toward wetter winter conditions identified from regional paleoclimate records. Our results indicate the overall stability of vegetation in Rocky Mountain lodgepole pine forests during climate changes and repeated high-severity fires. Contemporary deviations from this pattern of resilience could indicate future recovery challenges in these ecosystems.

Floodplain development, land-use, and flooding on the lower Ohio River are investigated with a 3100-year-long sediment archive from Avery Lake, a swale lake on the Black Bottom floodplain in southern Illinois, US. In all, 12 radiocarbon dates show that Avery Lake formed at 1130 BCE (3100 cal. yr BP), almost 3000 years later than previously thought, indicating that the Black Bottom floodplain is younger and more dynamic than previously estimated. Three subsequent periods of extensive land clearance were identified by changes in pollen composition, corresponding to Native American occupations before 1500 CE and the current Euro-American occupation beginning in the 18th century. Sedimentation rates prior to 1820 CE changed independently of land clearance events, suggesting natural as opposed to land-use controls. Comparison with high-resolution paleoclimate data from Martin Lake, IN, indicates that lower Ohio River flooding was frequent when cold-season precipitation originating from the Pacific/Arctic predominated when atmospheric circulation resembled positive Pacific North American (PNA) conditions and the Pacific Decadal Oscillation (PDO) was in a positive mean state (1130 BCE to 350 CE and 1150–1820 CE). Conversely, Ohio River flooding was less frequent when warm-season precipitation from the Gulf of Mexico prevailed during negative PDO- and PNA-like mean states (350 and 1150 CE). This flood dynamic appears to have been fundamentally altered after 1820 CE. We suggest that extensive land clearance in the Ohio River watershed increased runoff and landscape erosion by reducing interception, infiltration, and evapotranspiration, thereby increasing flooding despite a shift to negative PDO- and PNA-like mean states. Predicted increases in average precipitation and extreme rainfall events across the mid-continental US are likely to perpetuate current trends toward more frequent flood events, because anthropogenic modifications have made the landscape less resilient to changing hydroclimatic conditions.

Transfer of nutrients from terrestrial to aquatic ecosystems is a natural process with climatic, biotic, and geologic controls. Recently, increasing concern about human manipulation of global nutrient cycles has required a long-term approach to assessing the nutrient status of aquatic systems. Data available in palaeorecords can assess current trophic status, baseline conditions, and long-term processes controlling nutrient fluxes on decadal to millennial timescales. Here, we review three palaeolimnological methods used to reconstruct nutrient cycling: (1) chemical compounds preserved in lacustrine sediment, (2) aquatic biotic indicators (often using a quantitative transfer function), and (3) quantitative empirical sediment flux estimates. The millennial-scale regulation of nutrient cycling by climate and catchment geochemistry leads to a gradual trajectory of dystrophication over the Holocene in many temperate lakes. In many systems, the magnitude of recent anthropogenic changes to nutrient cycling is la...

The proportional cover of forest and grassland vegetation, known as landscape openness, has been particularly difficult to reconstruct because of differences in pollen productivity and transport between the two vegetation types. To begin to calibrate landscape openness in eastern North America, we collected 21 samples of surface sediments from small ponds (less than 60 ha) in the Upper Connecticut River Valley of New England, USA. Pollen assemblages from these surface samples were compared with vegetation composition around each pond assessed with three different remote sensing methods: land cover digitized from aerial photographs, the National Land Cover Database and a spectral mixture analysis of vegetation structure using LANDSAT data. A principal component analysis indicates that most of the variation in land cover among sites is captured by pollen data and each of the three vegetation methods. Because spectral mixture analysis contains at least as much vegetation information as the other two techniques, while avoiding the problem of classifying vegetation into categories, it is better for comparison with pollen data and detection of landscape openness. Even though there is a relationship between non-arboreal pollen percentages and a metric of landscape openness, it is inadvisable to use pollen percentages alone to infer landscape openness across regions either in modern or ancient times. Quantitative assessments and reconstructions of plant abundance at a higher taxonomic resolution will require detailed vegetation surveys to correlate with pollen data from surface samples and possibly simulations of modern and ancient landscapes. Open areas in a landscape that is over 75% forested can be detected, allowing for future quantitative reconstructions of landscape openness.

Biotic causes of changes in palaeorecords are discussed as a counterbalance to the (over) emphasis on climatic drivers in the interpretation of palaeoecological change. Biotic and climatic drivers are considered as competing hypotheses with particular reference to the importance of early-Holocene drought in the northern central grassland region of North America. It is concluded that the observed patterns are explained better by grazers than by the direct effects of climatic variation. The elevation of biotic factors as competing hypotheses with climate will require new conceptual and methodological approaches to palaeoecology in North America.

Archaeological records show that, as early as 2000–1500 bc, Native Americans in the eastern USA domesticated a group of native plant species with starchy or oily seeds that helped support extensive Woodland societies. Previously, the location and extent of cultivation of these ancient crop plants has been unknown, but was hypothesized to be small clearances on river floodplains. Here, pollen analyses of two 2000-year-old sediment records from Fort Ancient, an upland Hopewell archaeological site in southwestern Ohio, show exceptionally high amounts of pollen from cultigens- Iva annua, Polygonum erectum and Chenopodium – deposited during Hopewell occupation of the site (100 bc to ad 400). It is likely that this earthwork is a site of prehistoric cultivation, not exclusively a ceremonial structure. Pollen spectra indicate the area surrounding the ponds was largely deforested, and the sediment record indicates high rates of erosion during this time. Therefore, North American Woodland societies were likely modifying vegetation more extensively through agriculture and other land-clearing activities than has been assumed previously.