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Jackson, Stephen T

Aim: A conspicuous climatic and biogeographical transition occurs at 40?45� N in western North America. This pivot point marks a north?south opposition of wet and dry conditions at interannual and decadal time-scales, as well as the northern and southern limits of many dominant western plant species. Palaeoecologists have yet to focus on past climatic and biotic shifts along this transition, in part because it requires comparisons across dissimilar records [i.e. pollen from lacustrine sediments to the north and plant macrofossils from woodrat (Neotoma) middens to the south]. To overcome these limitations, we are extending the woodrat-midden record northward into the lowlands of the central Rocky Mountains. Published...
ABSTRACT: Samples from 107 piñon pines (Pinns edulis) at four sites were used to develop a proxy record of annual (June to June) precipitation spanning the 1226 to 2001 AD interval for the Uinta Basin Watershed of northeastern Utah. The reconstruction reveals significant precipitation variability at interannual to decadal scales. Single-year dry events before the instrumental period tended to be more severe than those after 1900. In general, decadal scale dry events were longer and more severe prior to 1900. In particular, dry events in the late 13th, 16th, and 18th Centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900. The last four decades of the 20th Century also represent...
Evidence from woodrat middens and tree rings at Dutch John Mountain (DJM) in northeastern Utah reveal spatiotemporal patterns of pinyon pine (Pinus edulis Engelm.) colonization and expansion in the past millennium. The DJM population, a northern outpost of pinyon, was established by long-distance dispersal (approximately 40 km). Growth of this isolate was markedly episodic and tracked multidecadal variability in precipitation. Initial colonization occurred by AD 1246, but expansion was forestalled by catastrophic drought (1250-1288), which we speculate produced extensive mortality of Utah Juniper (Juniperus osteosperma (Torr.) Little), the dominant tree at DJM for the previous approximately 8700 years. Pinyon then...
Key risks associated with projected climate trends for the 21st century include the prospects of future climate states with no current analog and the disappearance of some extant climates. Because climate is a primary control on species distributions and ecosystem processes, novel 21st-century climates may promote formation of novel species associations and other ecological surprises, whereas the disappearance of some extant climates increases risk of extinction for species with narrow geographic or climatic distributions and disruption of existing communities. Here we analyze multimodel ensembles for the A2 and B1 emission scenarios produced for the fourth assessment report of the Intergovernmental Panel on Climate...
Modern pollen assemblages from 18 small ponds and wetlands on the spatially isolated and forested Kaibab Plateau were studied to determine how the pollen assemblages recorded vegetation patterns. Vegetation of the Plateau consists of an inner core of subalpine Picea/Abies forests, surrounded by mixed Abies/Picea/Pseudotsuga/Pinus ponderosa forests, which are in turn surrounded by extensive Pinus ponderosa forests. The flanks of the Plateau are vegetated by Pinus edulis/Juniperus woodlands, with scattered Quercus populations. Arboreal pollen assemblages were dominated by Pinus (70-98%), which was most abundant in the P. ponderosa forests. Picea, Abies, Pseudotsuga and Populus pollen were abundant only at sites in...
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