Background <p>Ecosystem responses to climate variability are heterogeneous and poorly understood. Here, we use protected areas in South America as a natural experiment to isolate climate from land use effects and assess how interannual variability in precipitation and temperature shapes carbon (C) gains across broad environmental gradients. Using three decades of satellite-derived vegetation activity and gridded climate data, we quantify ecosystem responsiveness using two complementary metrics: climate sensitivity (the marginal response of C gains to annual changes in precipitation or temperature) and climate association (the strength of the association between C gains and precipitation or temperature), from hyper-arid deserts to tropical rainforests.</p> Results <p>Our results show how ecosystem dynamics are fundamentally shaped by water availability. Sensitivity to precipitation peaked in semi-arid and dry subhumid sites and declined toward both arid and humid extremes, whereas temperature sensitivity increased toward more humid conditions. Dominant climate control shifted from precipitation in water-limited sites other than the most arid to temperature in humid ecosystems. Semi-arid sites combined the highest climate sensitivity and strongest association together with greater changes and higher variability during dry and warm anomalies.</p> Conclusions <p>Our study advances current understanding through an integrative perspective by (i) quantifying ecosystem responses along two complementary dimensions, climate sensitivity and association; (ii) examining the effects of both precipitation and temperature; (iii) spanning broad environmental gradients across ecosystems ranging from deserts to tropical rainforests; (iv) focusing on South America, a continent that remains underrepresented in global syntheses; and (v) unambiguously isolating climate from land-use effects by restricting the analysis to protected areas. We provide a unified framework to interpret how climate controls on ecosystem functioning reorganize along water availability gradients, highlighting the transitional nature and heightened responsiveness of semi-arid systems. These metrics offer a transparent and scalable first-order assessment of ecosystem responsiveness but should be complemented by process-based studies to elucidate the mechanisms underlying these patterns, including the roles of phenology, radiation, nutrients, and community structure.</p>