Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest

Ecological responses to climatic variability in the Southwest include regio nally synchronized fires, insect outbreaks, and purses in tree demography ( births and deaths). Multicentury, tree-ring reconstructions of drought, dis turbance history, and tree demography reveal climatic effects across scales , from annual to decadal, and from local (<10(2) km(2)) to mesoscale (10(4) -10(6) km(2)). Climate-disturbance relations are more variable and complex than previously assumed. During the past three centuries, mesoscale outbrea ks of the western spruce budworm (Chorisroneura occidentalis) were associat ed with wet, not dry episodes, contrary to conventional wisdom. Regional fi res occur during extreme droughts but, in some ecosystems, antecedent wet c onditions play a secondary role by regulating accumulation of fuels. Interd ecadal changes in fire-climate associations parallel other evidence for shi fts in the frequency or amplitude of the Southern Oscillation (SO) during t he past three centuries. High interannual, fire-climate correlations (r = 0 .7 to 0.9) during specific decades (i.e., circa 1740-80 and 1830-60) reflec t periods of high amplitude in the SO and rapid switching from extreme wet to dry years in the Southwest, thereby entraining fire occurrence across th e region. Weak correlations from 1780 to 1830 correspond with a decrease in SO frequency or amplitude inferred from independent tree-ring width, ice c ore, and coral isotope reconstructions. Episodic dry and wet episodes have altered age structures and species compo sition of woodland and conifer forests. The scarcity of old, living conifer s established before circa 1600 suggests that the extreme drought of 1575-9 5 had pervasive effects on tree populations. The most extreme drought of th e past 100 years occurred in the mid-twentieth century (1942-57). This drou ght resulted in broadscale plant dieoffs in shrublands, woodlands, and fore sts and accelerated shrub invasion of grasslands. Drought conditions were b roken by the post-1976 shift to the negative SO phase and wetter cool seaso ns in the Southwest. The post-1976 period shows up as an unprecedented surg e in tree-ring growth within millennia-length chronologies. This unusual ep isode may have produced a pulse in tree recruitment and improved rangeland conditions (e.g., higher grass production), though additional study is need ed to disentangle the interacting roles of land use and climate. The 1950s draught and the post-1976 wet period and their aftermaths offer natural exp eriments to study long-term ecosystem response to interdecadal climate vari ability.