Century to Millennial Scale

The absence of direct meteorological observations for most time periods and geographic areas at the century to millennial scale forces investigators to use proxy evidence from which to infer information concerning the variability of climate. At this scale, therefore, the ecological response is being used to provide information concerning the climate. We thus admit, in this section, to engaging to a certain degree in circular argument while discussing the "inferred" climate variability (as a "cause") and ecosystem response (as a "result"). The fields for which we have the most information at this timescale are those related to tree-ring thickness variability (Graumlich and Brubaker 1986; Graumlich 1987; Holmes et al. 1986; Buckley et al. 1992; Wiles et al. 1995; Garfin and Hughes 1996) and tree-ring-based (Weisberg and Swanson 2003) and lake-charcoal-based forest fire histories (Long et al. 1998), as well as vegetation change noted from pollen analysis (Warona and Whitlock 1995; Sea et al.1995; Grigg and Whitlock 1998).

Warm periods from 1400 to about 1575 and from 1800 to about 1925 were associated with widespread forest fires at the Andrews site and in the western Cascades (Weisberg and Swanson 2003). During the cool period from 1700 to about 1775, there was a marked decrease in the extent of forest fires. Forest fire histories based on tree rings at the Andrews site and in other study areas in western Oregon indi cate widespread forest fires during the periods of 1475-1550 and 1850-1900. The most recent period of widespread fire is associated, among other factors, with a warmer, drier climate beginning about 1840, as noted in the tree-ring record (Weisberg and Swanson 2003). They point to anthropogenic factors acting synchronously with climate variability to produce the overall fire history.

The Weisberg and Swanson study suggests that as for crown fire-driven landscapes in general, the PNW may have exhibited high, spatiotemporal variability at any spatial scale. Climate variability at the century to millennial scale operating through the provision of periods for variable forest fire frequency leads to a highly dynamic ecosystem. Swanson has noted that forest establishment after fire may take place in periods of unusually stressful climate. He speculates that this may have affected succession and ultimately the development of present-day old growth forests in ways unlike the potential consequences of forests established by natural processes or management actions in areas with other climate conditions.

Forest fire histories based on lake charcoal for a site about 140 km west of the Andrews Forest complied by Long et al. (1998) extend our information on the interaction of climate and forest fire back even further. Climate models and known changes in the timing of the perihelion and the tilt of Earth's axis (Kutzbach et al. 1993) indicate that, between about 9000 and 6850 years before present (b.p.), the climate was warmer and drier than it is today. During this time fire intervals in the Oregon Coast Range averaged 110 ±20 years. From about 6850 to 2750 b.p., there was an onset of cool, humid conditions, and, although there was an increase in the abundance of fire-sensitive species, the fire interval lengthened to 160 ± 20 years. From 2750 b.p. to the present, cool, humid conditions resulted in mesophytic taxa, and the mean fire interval increased to 230 ± 30 years. Although the actual fire intervals may be different in the Cascades and near the Andrews site, the overall pattern of changing climate and the ecological response in terms of relative fire intervals might have been similar.

The same overall climate changes that affected fire regime led to pronounced vegetation changes in the PNW. The long-term record shows that the composition of the forests has not been static, but instead has changed continuously with climate changes. Records from Little Lake (central Oregon Coast Range), Indian Prairie (Oregon Western Cascades), and Gold Lake Bog (central Oregon Cascades), for example, show changes in forest composition in the past that were most likely a response to shifts in summer drought and winter precipitation (Worona and Whitlock 1995; Sea et al. 1995). These, in turn, were driven by changes in the seasonal amplitude of insolation, the position of winter storm tracks, and the strength of the northeast Pacific subtropical high-pressure area.

The paleoecological record also suggests that forest communities in this region can change fairly rapidly with climate change. One episode of rapid vegetation change occurred at Little Lake around 14,850 years ago (Grigg and Whitlock 1998) when the pollen record shows that spruce forest was replaced by forest dominated by Douglas-fir in less than a century. A douglas-fir forest then persisted for about 350 years, when it reverted back to spruce forest. The increase in Douglas-fir at Little Lake was preceded by a prominent charcoal peak, which suggests that one fire or several closely spaced fires helped trigger the vegetation change by killing spruce and creating soil conditions suitable for Douglas-fir establishment. Warmer conditions than before allowed Douglas-fir to remain competitive for several decades or even centuries before spruce returned. Additional records of comparable resolution are necessary to determine whether this event is of regional significance. Nonetheless, the Little Lake data suggest that vegetation changes can occur rapidly when climate alters disturbance and tree regeneration conditions.

The PNW is a topographically complex area. Whitlock (1992) has described the Holocene vegetation history for this area as a response by plants to a hierarchical set of environmental controls of which climate is but one. Vegetation changes at the millennial timescale appear to respond to warming and, in this part of the world, drying associated with the retreat of the main Laurentide ice sheet. From 20,000 to 16,000 b.p., there was an influx of xerothermic subalpine vegetation (Picea en-gelmanni and Artemisia). Mesophytic subalpine vegetation appeared (Tsuga mertensiana, Picea sitchensis, and Alnus sinuata) after 16,000 b.p. when the main storm tracks are believed to have shifted northward. The later establishment of warm-loving and drought-adapted species from 12,000 to 6000 b.p. is associated with greater solar radiation and an expansion of the subtropical high-pressure zone. Pseudotsuga and Alnus then dominated the forests. Prairies and grasslands also appeared. At shorter timescales, Whitlock notes that fires were probably more frequent in the early Holocene warm dry period, so early successional and forest-opening species would have been more abundant. Also, at a smaller geographic scale, substrate conditions became important in influencing vegetation type. Prairie and oak woodland in the Puget Sound area favored summer drought conditions on the coarse-textured soils found there today and presumably throughout the Holo-cene. The modern forests of the Pacific Northwest are believed to have formed only in the last few millennia when the climate became wetter and solar radiation was reduced. Whitlock (1992, p. 22) concludes, "modern communities are loose associations composed of species independently adjusting their ranges to environmental changes on various time scales."

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  • drew
    What causes millennial scale climate variability?
    5 months ago

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