The American Southwest's ecosystems are significantly influenced by decadal climatic variability, with mesoscale disturbances such as fires, insect outbreaks, and droughts shaping ecological responses. Tree-ring reconstructions reveal climate effects across scales, from annual to decadal, and local to mesoscale. Climate-disturbance relationships are complex, with mesoscale outbreaks of the western spruce budworm linked to wet, not dry, conditions. Regional fires occur during extreme droughts, but antecedent wet conditions can regulate fuel accumulation. Interdecadal changes in fire-climate associations align with shifts in the Southern Oscillation (SO) over the past three centuries. High fire-climate correlations during specific decades (e.g., 1740–80 and 1830–60) reflect periods of high SO amplitude, while weak correlations from 1780–1830 correspond with reduced SO frequency or amplitude. Episodic dry and wet episodes have altered age structures and species composition of forests. The 1575–95 drought had pervasive effects, while the mid-20th century drought caused broadscale plant dieoffs and shrub invasion. The post-1976 shift to the negative SO phase brought wetter cool seasons, leading to a surge in tree-ring growth, possibly indicating improved recruitment and rangeland conditions. The 1950s drought and post-1976 wet period offer natural experiments for studying long-term ecosystem responses to interdecadal climate variability. The Southwest's ecosystems are influenced by interannual and interdecadal climate variability, with El Niño and La Niña conditions affecting precipitation and fire activity. Insect outbreaks, such as the western spruce budworm, are linked to wet periods, not droughts. Regional wildfires are synchronized with extreme droughts, with fire frequency and severity influenced by antecedent moisture conditions. The 1950s drought caused widespread vegetation dieoffs, while the post-1976 wet period led to increased tree growth and recruitment. The post-1976 climate change complicates interpreting vegetation trends, as shrub invasions may be due to drought, overgrazing, or CO₂ enrichment. Climate appears to regulate exotic plant invasions, with wetter winters encouraging the spread of Eurasian red brome. The Southwest's ecosystems are shaped by complex interactions between climate and land use, with long-term studies essential for understanding these dynamics.The American Southwest's ecosystems are significantly influenced by decadal climatic variability, with mesoscale disturbances such as fires, insect outbreaks, and droughts shaping ecological responses. Tree-ring reconstructions reveal climate effects across scales, from annual to decadal, and local to mesoscale. Climate-disturbance relationships are complex, with mesoscale outbreaks of the western spruce budworm linked to wet, not dry, conditions. Regional fires occur during extreme droughts, but antecedent wet conditions can regulate fuel accumulation. Interdecadal changes in fire-climate associations align with shifts in the Southern Oscillation (SO) over the past three centuries. High fire-climate correlations during specific decades (e.g., 1740–80 and 1830–60) reflect periods of high SO amplitude, while weak correlations from 1780–1830 correspond with reduced SO frequency or amplitude. Episodic dry and wet episodes have altered age structures and species composition of forests. The 1575–95 drought had pervasive effects, while the mid-20th century drought caused broadscale plant dieoffs and shrub invasion. The post-1976 shift to the negative SO phase brought wetter cool seasons, leading to a surge in tree-ring growth, possibly indicating improved recruitment and rangeland conditions. The 1950s drought and post-1976 wet period offer natural experiments for studying long-term ecosystem responses to interdecadal climate variability. The Southwest's ecosystems are influenced by interannual and interdecadal climate variability, with El Niño and La Niña conditions affecting precipitation and fire activity. Insect outbreaks, such as the western spruce budworm, are linked to wet periods, not droughts. Regional wildfires are synchronized with extreme droughts, with fire frequency and severity influenced by antecedent moisture conditions. The 1950s drought caused widespread vegetation dieoffs, while the post-1976 wet period led to increased tree growth and recruitment. The post-1976 climate change complicates interpreting vegetation trends, as shrub invasions may be due to drought, overgrazing, or CO₂ enrichment. Climate appears to regulate exotic plant invasions, with wetter winters encouraging the spread of Eurasian red brome. The Southwest's ecosystems are shaped by complex interactions between climate and land use, with long-term studies essential for understanding these dynamics.