Multidecadal Timescale

Multidecadal changes for the Pacific Northwest (PNW) are related to the PDO. Taylor and Southards (1997) ( noted a cool, wet period from 1896 to 1914, a warm and dry period from 1915 to 1946, a cool and wet period from 1947 to 1975, and a warm and dry period from 1976 to 1994. Mantua et al. (1997) have shown these periods to be related to changes in the synoptic-scale climate indices that have reversal times during the period 1900-1996 in 1925, 1947, and 1977. The climate regime shifts related to the PDO were first noticed after the 1976 shift because of the correspondence in numerous ecosystem and environmental responses in the PNW (Ebbesmeyer et al. 1991). These responses include variables such as the numbers of goose nests, crab production, mollusk abundance, and the path of returning salmon and salmon catch (see chapter 13). There are suggestions that another climate regime shift may have occurred in the mid-1990s (JISAO CIG 1999).

Given these regional changes, one might expect clear evidence of such climate variability and ecosystem responses in the Andrews ecosystems. However, an unequivocal variability before and after 1976 is not immediately apparent in the values of some variables where it might be expected, such as winter water year precipitation, stream discharge, or in the percentage change of water yield relative to that in the 12 years before 1976. There are, however, suggestions of some interesting multidecadal changes in "cyclic" behavior of parts of the system. As described subsequently, such changes in cone production, the number of peak streamflows per year, and possibly in debris flow frequency need about another 100 years of records to establish the reality or absence of cyclic behavior, but these changes do raise some interesting research questions.

Cone production records from high elevations above about 1000 m for noble fir, silver fir, and mountain hemlock at the AND and in other parts of the Cascade Range commence in 1962. There is some evidence to suggest that temporal patterns of cone production by upper slope noble fir, silver fir, and mountain hemlock in the Oregon and Washington Cascades may be associated with variability in the PDO (figure 19.2). Cones counted on canopy trees on 14 plots in the Cascades show a marked 3-year periodicity from 1962 to 1974, as exemplified by Pacific silver fir (Abies amabilis) in figure 19.2. This was during a period of lower than average PDO (cool phase). For at least the next two decades, this periodicity ends with the 1971-1974 cone production period for Pacific silver fir, noble fir, and mountain hemlock when the north Pacific sea surface temperature rises above normal (PDO positive, warm phase). This loss of this 3-year periodicity may be caused by loss of a trigger, common to these species, needed for cone production. Preliminary analyses indicate that, until the mid-1970s, warmer than average summer temperatures in the Cascades preceded by one year the large cone crops seen in figure 19.2. No warming or cooling can be seen, however, in average summer (June, July, August) temperatures after 1976. After 1976, another change in cone production pattern occurred: There was less synchronicity both among and within species (figure 19.2). If a PDO regime shift in the late 1990s does prove true, a return to the 3-year cyclic cone production will be one test of the PDO/cone productivity relationship. More research is needed to explore potential links between PDO change and cone production response.

Cone production also displays the importance of preexisting conditions at a monthly timescale. In the case of Douglas-fir, a warm sunny dry June 15 months before cone maturation, cool moist March and April 17 and 18 months before cone maturation, and cool moist summer months 25 to 27 months before cone maturation are all associated with increased cone production.

Although no clear evidence of a 1976 climate regime shift is seen in precipitation and stream discharge records at the Andrews Forest, peak streamflows show an interesting pattern. Five-year running means of the number of peak streamflows per year in unharvested, high-elevation basins at the Andrews site between 1952 and 1996, as counted by storm matching techniques, show two complete and similar "cycles" with a period of about 10 years (Jones and Grant 1996). The "cyclic" nature of these data stops at approximately 1976 and is not seen in the later part of the time series.

Another geomorphologic and ecosystem response to PDO climate fluctuations may involve the occurrence of debris flows, rapid mass movements of 100 to greater than 1000 m3 of soil and organic debris down steep headwater stream channels. Snyder (2000) examined the inventory of 91 debris flows occurring between 1946 and 2000 in a 125-km2 study area including the Andrews Forest. Debris flows

Figure 19.2 Relationship between Pacific Decadal Oscillation and cone production of three upper slope conifer species in the Cascades of Oregon and Washington. (Courtesy of Joseph Means)

initiated in unharvested forested areas occurred at a rate of 0.38 events per year in the wet phases of the PDO (before 1976 and after 1994) and only 0.05 events per year in the intervening dry phase. The majority (81%) of events in forested areas occurred in just three storms during this greater-than-50-year period, which raises the issue of whether these records represent the vagaries of storm history or a true PDO signal.

Once again, many future years of data and fairly precise methods of identifying and dating debris flows of the past 150-200 years are necessary to see whether the PDO/debris flow relationship may be established. Preexisting conditions may be important in this possible relationship. No matter what meteorological and clima-tological circumstances occur, debris flows can take place only if the potential debris material has already been rendered into a potentially movable condition.

Some interesting research questions are raised by these data. First, what kind of changes in system subvariables manifest themselves as a result of a multidecadal climate-driving cycle in a system? Can there be changes in system subvariables that show themselves as cyclic but with higher frequencies? Can changes in system subvariables be represented by the absence of a "response cycle" altogether in one phase of the driver cycle? More interesting, what are the complex steps in the system cascade that could give rise to this state of affairs, assuming the answers to these questions are positive. At least three other possibilities exist besides climatic cause and effect. First, there may be some other nonclimatic drivers at work such as land management and road construction. Second, nonclimatic drivers interact with climatic drivers. Third, the ecosystem events are random and there is no cause and effect.

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