Stephen T. Gray

Ecology and the ratchet of events: Climate variability, niche dimensions, and species distributions

Climate change in the coming centuries will be characterized by interannual, decadal, and multidecadal fluctuations superimposed on anthropogenic trends. Predicting ecological and biogeographic responses to these changes constitutes an immense challenge for ecologists. Perspectives from climatic and ecological history indicate that responses will be laden with contingencies, resulting from episodic climatic events interacting with demographic and colonization events. This effect is compounded by the dependency of environmental sensitivity upon life-stage for many species. Climate variables often used in empirical niche models may become decoupled from the proximal variables that directly influence individuals and populations. Greater predictive capacity, and more-fundamental ecological and biogeographic understanding, will come from integration of correlational niche modeling with mechanistic niche modeling, dynamic ecological modeling, targeted experiments, and systematic observations of past and present patterns and dynamics.

The unusual nature of recent snowpack declines in the North American cordillera.

The snowpack covering the mountains of western North America has decreased dramatically during the past 50 years. In western North America, snowpack has declined in recent decades, and further losses are projected through the 21st century. Here, we evaluate the uniqueness of recent declines using snowpack reconstructions from 66 tree-ring chronologies in key runoff-generating areas of the Colorado, Columbia, and Missouri River drainages. Over the past millennium, late 20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming that is due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western United States.

Patterns and sources of multidecadal oscillations in drought‐sensitive tree‐ring records from the central and southern Rocky Mountains

[1] Tree-ring records spanning the past seven centuries from the central and southern Rocky Mountains were studied using wavelet analysis to examine multidecadal (>30–70 yr) patterns of drought variation. Fifteen tree-ring series were grouped into five regional composite chronologies based on shared low-frequency behavior. Strong multidecadal phasing of moisture variation was present in all five regions during the late 16th century megadrought. Consistent oscillatory modes in the 30–70 yr domain persisted until the mid-19th century in two of the five regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought. The 16th/17th century pattern of severe multidecadal drought followed by decades of unusually wet conditions resembles the 1950s drought and post-1976 wet period. The 16th century megadrought, which may have resulted from coupling of a decadal (20–30 yr) Pacific cool phase with a multidecadal warm phase in the subtropical North Atlantic, marked a substantial reorganization of climate variation in the Rocky Mountain region. INDEX TERMS: 1812 Hydrology: Drought; 1833 Hydrology: Hydroclimatology; 1854 Hydrology: Precipitation (3354); 9350 Information Related to Geographic Region: North America; KEYWORDS: drought, treerings, multi-decadal variability, western U. S., Pacific Decadal Oscillation, Atlantic Multidecadal Oscillation. Citation: Gray, S. T., J. L. Betancourt, C. L. Fastie, and S. T. Jackson, Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the central and southern Rocky Mountains, Geophys. Res. Lett., 30(0), XXXX, doi:10.1029/2002GL016154, 2003.

ROLE OF MULTIDECADAL CLIMATE VARIABILITY IN A RANGE EXTENSION OF PINYON PINE

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 quickly replaced juniper across DJM during a few wet decades (1330-1339 and 1368-1377). Such alternating decadal-scale droughts and pluvial events play a key role in structuring plant communities at the landscape to regional level. These decadal-length precipitation anomalies tend to be regionally coherent and can synchronize physical and biological processes across large areas. Vegetation forecast models must incorporate these temporal and geographic aspects of climate variability to accurately predict the effects of future climate change.

INFLUENCE OF LANDSCAPE STRUCTURE AND CLIMATE VARIABILITY ON A LATE HOLOCENE PLANT MIGRATION

We analyzed and radiocarbon-dated 205 fossil woodrat middens from 14 sites in central and northern Wyoming and adjacent Utah and Montana to document spa- tiotemporal patterns of Holocene invasion by Utah juniper (Juniperus osteosperma). Ho- locene migration into central and northern Wyoming and southern Montana from the south proceeded by a series of long-distance dispersal events, which were paced by climate variability and structured by the geographic distribution and connectivity of suitable habitats on the landscape. The migration of Utah juniper into the region involved multiple long- distance dispersal events, ranging from 30 to 135 km. One of the earliest established populations, on East Pryor Mountain in south central Montana, is currently the northernmost population of the species. Establishment by long-distance dispersal of that population and another in the Bighorn Basin occurred during a period of relatively dry climate between 7500 and 5400 years ago. Further expansion of these initial colonizing populations and backfilling to occupy suitable sites to the south was delayed during a wet period from 5400 to 2800 years ago. Development of dry conditions 2800 years ago led to a rapid expansion in which Utah juniper colonized sites throughout its current range. Landscape structure and climate variability play important roles in governing the pattern and pace of natural in- vasions and deserve close attention in studying and modeling plant invasions, whether exotic or natural.

Climatic Controls on the Snowmelt Hydrology of the Northern Rocky Mountains

Abstract The northern Rocky Mountains (NRMs) are a critical headwaters region with the majority of water resources originating from mountain snowpack. Observations showing declines in western U.S. snowpack have implications for water resources and biophysical processes in high-mountain environments. This study investigates oceanic and atmospheric controls underlying changes in timing, variability, and trends documented across the entire hydroclimatic-monitoring system within critical NRM watersheds. Analyses were conducted using records from 25 snow telemetry (SNOTEL) stations, 148 1 April snow course records, stream gauge records from 14 relatively unimpaired rivers, and 37 valley meteorological stations. Over the past four decades, midelevation SNOTEL records show a tendency toward decreased snowpack with peak snow water equivalent (SWE) arriving and melting out earlier. Temperature records show significant seasonal and annual decreases in the number of frost days (days ≤0°C) and changes in spring minim...

Tree-Ring-Based Reconstruction of Precipitation in the Bighorn Basin, Wyoming, since 1260 a.d

Abstract Cores and cross sections from 79 Douglas fir (Pseudotsuga menziesii) and limber pine (Pinus flexilis) trees at four sites in the Bighorn Basin of north-central Wyoming and south-central Montana were used to develop a proxy for annual (June–June) precipitation spanning 1260–1998 a.d. The reconstruction exhibits considerable nonstationarity, and the instrumental era (post-1900) in particular fails to capture the full range of precipitation variability experienced in the past ∼750 years. Both single-year and decadal-scale dry events were more severe before 1900. Dry spells in the late thirteenth and sixteenth centuries surpass both magnitude and duration of any droughts in the Bighorn Basin after 1900. Precipitation variability appears to shift to a higher-frequency mode after 1750, with 15–20-yr droughts becoming rare. Comparisons between instrumental and reconstructed values of precipitation and indices of Pacific basin variability reveal that precipitation in the Bighorn Basin generally responds ...

Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana

Instrumental climate records suggest that summer precipita- tion and winter snowpack in Glacier National Park (Glacier NP), Montana, vary significantly over decadal to multidecadal time scales. Because instru- mental records for the region are limited to the twentieth century, knowledge

Upper Green River Basin (United States) Streamflow Reconstructions

The Upper Green River represents a vital water supply for southwestern Wyoming and Upper/Lower Colorado River Compact states. Rapid development in the southwestern United States combined with the recent drought has greatly stressed the water supply of the Colorado River system, and concurrently increased the interest in long-term variations in streamflow. The current research developed six new tree-ring chronologies in and adjacent to the Upper Green River Basin (UGRB). Nine proxy reconstructions (three main-stem streams and six headwater streams) of UGRB streamflow were created by combining these new tree-ring chronologies with existing tree-ring chronologies from sites adjacent to the UGRB. All UGRB streamflow reconstructions extended back to the year 1615 or earlier. The variance explained ( r2 ) by these reconstructions ranged from a low of 0.44 at one headwaters gauge to 0.65 for the lowest main-stem gauge in the drainage. An extended reconstruction of the main-stem Green River gauge near Greendale, ...

Climate Zone Delineation: Evaluating Approaches for Use in Natural Resource Management

Recent efforts by the United States Department of the Interior (DOI) have the potential to make climate zones the basic geographic units guiding monitoring and resource management programs in the western U.S. We evaluated a new National Park Service approach for delineating climate zones that will likely be a model for other DOI agencies. Using the test case of the Greater Yellowstone Area in Wyoming, Montana and Idaho, we conducted three separate analyses, each based on a different dataset. Cluster analysis of 1971–2000 temperature and precipitation normals grouped weather stations according to similarities in seasonal patterns. Principal Components Analysis (PCAs) of 1895–2008 monthly data grouped stations by similarities in long-term variability. Finally, an analysis of snow data further subdivided the zones defined by the other two analyses. The climate zones produced by the cluster analysis and the PCAs were roughly similar to each other, but the differences were significant. The two sets of zones may be useful for different applications. For example, studies that analyze links between climate patterns and the demography of threatened species should focus on the results of the PCAs. The broad similarity among results produced by the different approaches supported the application of these zones in climate-related monitoring and analysis. However, since choices in data and methodology can affect the details of maps depicting zone boundaries, there are practical limitations to their use.