Christopher L. Fastie

Spatial and temporal variability in the growth and climate response of treeline trees in Alaska

In this study, we investigated the response of trees growing at the cold margins of the boreal forest to climate variation in the 20th century. Working at eight sites at and near alpine and arctic treeline in three regions in Alaska, we compared tree growth (from measured tree ring-widths) to historical climate data to document how growth has responded to climate variation in the 20th century. We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest.

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.

Recent changes in treeline forest distribution and structure in interior Alaska

Abstract Although the forest-tundra boundary is likely to be sensitive to future climate warming, the degree to which treeline response may lag climate change and the extent to which sensitivity to climate may vary among sites remain largely unknown. We used tree-ring analysis to reconstruct white spruce (Picea glauca) density from 1800 to present at and beyond the current forest limit at seven altitudinal treeline sites in two regions of interior Alaska. Treeline advance was ubiquitous: cone-bearing spruce are present beyond the current forest limit at all but one site, and tree density has increased at and beyond the forest limit in recent decades at all sites. Increases in stand density were positively correlated with summer temperature at most, but not all, sites. The timing of inferred advances in treeline differed significantly between regions, beginning in the mid- to late 1800s in the White Mountains and in the mid-1900s in the Alaska Range. These differences in the timing of treeline advance may be caused by differences in the rate of forest response to climate or by differences in regional climate history, which remains poorly known. Despite the variation in timing of an advance of treeline, the similarities among sites in the pattern (if not the timing) of change at treeline suggest that recent shifts in the location of the forest-tundra border are a widespread response to recent warming in Alaska.

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 ...

Spruce succession, disturbance, and geomorphology on the Tanana River floodplain, Alaska

A long-standing paradigm in the ecology of the Alaskan taiga states that black spruce (Picea mariana [Mill.] BSP) replaces white spruce (Picea glauca [Moench] Voss) after several centuries of prima...