Search for: All records

Paleoclimate reconstructions for the western US show spatial variability in the timing, duration, and magnitude of climate changes within the Medieval Climate Anomaly (MCA, ca. 900–1350 CE) and Little Ice Age (LIA, ca. 1350–1850 CE), indicating that additional data are needed to more completely characterize late-Holocene climate change in the region. Here, we use dendrochronology to investigate how climate changes during the MCA and LIA affected a treeline, whitebark pine ( Pinus albicaulis Engelm.) ecosystem in the Greater Yellowstone Ecoregion (GYE). We present two new millennial-length tree-ring chronologies and multiple lines of tree-ring evidence from living and remnant whitebark pine and Engelmann spruce ( Picea engelmannii Parry ex. Engelm.) trees, including patterns of establishment and mortality; changes in tree growth; frost rings; and blue-intensity-based, reconstructed summer temperatures, to highlight the terminus of the LIA as one of the coldest periods of the last millennium for the GYE. Patterns of tree establishment and mortality indicate conditions favorable to recruitment during the latter half of the MCA and climate-induced mortality of trees during the middle-to-late LIA. These patterns correspond with decreased growth, frost damage, and reconstructed cooler temperature anomalies for the 1800–1850 CE period. Results provide important insight into how past climate change affected important GYE ecosystems and highlight the value of using multiple lines of proxy evidence, along with climate reconstructions of high spatial resolution, to better describe spatial and temporal variability in MCA and LIA climate and the ecological influence of climate change. 

We present a lake-sediment record of pre-Columbian agriculture and fire history from the lowlands of southern Pacific Costa Rica that captures the arrival of maize agriculture at ca. 3360 cal yr BP in the Diquís subregion of the Gran Chiriquí archeological region. Our 4200-year record from Laguna Los Mangos begins 1000 to 2000 years earlier than other lake records from the region and provides the first microfossil and geochemical evidence of vegetation and fire prior to the establishment of maize agriculture. This early portion of the record shows evidence of fire events associated with land clearance or field preparation and maintenance for subsistence activities. Alternatively, these were wildfires ignited unintentionally by people or naturally by lightning or volcanism. Evidence of early maize by ca. 3200 cal yr BP was found at Laguna Zoncho in the southeastern section of the Diquís subregion. Our discovery of early maize agriculture at ca. 3360 cal yr BP in the Laguna Los Mangos watershed in the northwestern portion of the Diquís subregion indicates a rapid adoption of maize agriculture in the region after initial introduction. Pre-Columbian agriculture and fire activity at Los Mangos is nearly continual until historic times, but with a decline after ca. 1170 cal yr BP, coincident with the early Terminal Classic Drought (TCD). We infer a pronounced drying of the lowland environment at Laguna Los Mangos based on a depositional hiatus in the record at ca. 950 during late TCD. Agricultural proxies indicate reduced watershed activity during the ‘Little Ice Age’ following Spanish contact in southern Central America until the 20th century.