Discussion

Present knowledge of climate variability and ecosystem response at a range of timescales provides a variety of answers to some of the guiding questions of this volume. New questions also emerge. Sometimes, it is difficult to specify the most important timescale at which causes and consequences are operating. For example, is forest fire occurrence at the Andrews LTER site more related to a seasonal scale or to a decadal to century scale? What is the interaction, if any, or relative importance between scales? A schematic representation of the characteristic timescales of some of the ecosystem responses to climatic disturbances helps to conceptualize the temporal variability (figure 19.3).

Preexisting conditions appear to be particularly important at the shorter timescales considered in this chapter. Important preexisting conditions can occur because of natural and/or anthropogenic-derived variability. Natural factors, such as fuel buildup, emphasize preexisting conditions with respect to fire frequency at the century scale. The need for suitable antecedent soil moisture conditions and potentially movable debris as a precursor for debris flow is another example. Swanson et al. (1998) also note that the preexisting condition of the geography of controls on debris flow occurrence causes some headwater streams to experience repeated, severe disturbance, whereas others may never have debris flows. Anthropogenic factors, such as forest management practices, may also be regarded as establishing preexisting conditions. By far the largest area of windthrow in the Bull Run basin in the northern Cascades of Oregon over a 100-year period, for example, was found to have occurred only after forest harvesting began in 1958 (Sinton et al. 2000).

The situation regarding preexisting conditions may be different at the longer timescales. The speed with which plant communities can be altered at the millennial scale in the PNW region, as represented by vegetation changes at Little Lake, implies that the exact nature of the preexisting communities is less important at this scale. The vegetation history in the PNW suggests that the nature of the vegetation existing previous to a climate change plays a minor role in determining the type of ecosystem response in terms of the new vegetation community that takes over a given location. For example, the Little Lake pollen record near 14,500 years b.p., which shows relatively rapid changes from spruce to Douglas-fir and back again (Grigg and Whitlock 1998), gives little evidence, except possibly that related to seed availability, that the later vegetation affected the type of the newer vegetation.

A consideration of longer timescales leads investigators to examine the timing of ecosystem response. Neglecting, for the moment, disturbance- and succession-related vegetation change, as far as the forests of the PNW are concerned, evidence suggests that climate-induced vegetation change can show response to climatic episodes at timescales of as little as 500 to 1000 years (Whitlock 1992). Paleoeco-

Figure 19.3 Characteristic timescales of some of the ecosystem responses to climatic disturbances.

logic records reveal the relatively ephemeral nature of modern communities. Modern forests represent an association that has existed for less than 3-6 millennia, and in the Cascade Range only a few generations of the forest dominants have been present in some sites at this timescale (Sea and Whitlock 1995). Species apparently have responded individualistically to Holocene environmental changes rather than as whole communities, and in the process, plant associations have been dismantled and reformed at a millennial pace.

There is no doubt that climate change and variability in the ecosystems of this region go far beyond an individual cause and result. There is almost always a cascade of resulting effects. Nakamura et al. (2000) explicitly employed the cascade concept for short-term events. They have established common sequences of events related to a cascade of hydrological and geomorphological events associated with the floods at the Andrews Forest. A biotic example of a cascade of events acting through time is evident in the effect of a windstorm, which may topple small trees or patches of trees. In some cases, at the edge of clear-cuts, the toppling event may be followed by drought that favors local eruptions of bark beetles, who emerge from the fallen trees to attack nearby live trees (Powers et al. 1999). At a longer timescale, Long et al. (1998) suggest that throughout the Holocene, changes in both vegetation and fire frequency were controlled by climate in finally determining the species composition and distribution of Coast Range forests. Studies from this region also show that besides cascading effects, extra factors can act as additional forcing functions alongside climate forcing. Factors such as soil texture and humans both causing, and suppressing, fires are examples. This is not a surprising conclusion, but it does emphasize the continued need to establish the importance of climate relative to other kinds of ecosystem forcing.

In this region we find examples at the quasi-quintennial and the multidecadal scales where the event, such as climate stage of ENSO, and the response, such as stream discharge, return to their "original" state by the time of the next event. This might not be true if vegetation or other environmental conditions have changed in the meantime. For example, since stream discharge is affected by water use by the vegetation, a lagged response in vegetation to specific climate variability may produce a lagged response in stream discharge. Furthermore, the concept of vegetation communities being "loose associations composed of species independently adjusting their ranges to environmental changes on various timescales" (Whitlock 1992, p. 22) suggests that at the century and millennial timescales there are likely to be no identical past analogs to the ecosystem at any point in time. It is unlikely that an ecosystem will return to its "original" state at this longer timescale, and the concept of "original" state itself has little meaning.

Acknowledgment The material in this chapter is based on studies related to the H. J. Andrews Experimental Forest Long-Term Ecological Research program, which is supported by the National Science Foundation and the USDA Forest Service Pacific Northwest Research Station.

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