UC Berkeley

Abstract Water availability governs ecosystem productivity, yet estimates of vegetation sensitivity to water can differ greatly depending on whether the sensitivity is examined spatially or temporally. In particular, the spatial sensitivity is often reported to be much stronger than temporal sensitivities, leading to highly uncertain projections of ecosystem responses to future climate change when using space‐for‐time substitution. The large difference between spatial and temporal sensitivities remains unexplained. Prior research, however, primarily relied on precipitation as the water availability proxy, whereas vegetation responds to soil moisture. Here, we combined satellite estimates of vegetation productivity with soil moisture data across water‐limited ecosystems of the continental United States (CONUS) to identify a convergent sensitivity of productivity to water availability. Using precipitation, we show that temporal sensitivity is 66% lower than spatial sensitivity overall. Our analysis identified the cause of the difference to be primarily driven by the seasonal variability of water availability, rooting depth, and soil properties. When using soil moisture instead of precipitation, we observed widespread convergence in the spatial and temporal sensitivities—that is, the two sensitivities became much more similar in magnitude across all water‐limited ecosystems within CONUS. These results show that overlooking soil hydrology can inflate perceived discrepancies between spatial and temporal vegetation sensitivities, leading to biased projections of ecosystem dynamics under future hydro‐climatic change. Plain Language Summary Water is essential for plant growth, especially in dry regions. To understand and predict how ecosystems respond to climate change, scientists often study vegetation responses to water over time at specific sites, but this approach is geographically limited. A common alternative—“space‐for‐time substitution”—analyzes vegetation–water relationships across multiple locations. Yet the two methods often disagree, creating uncertainty in predicting how ecosystems will respond to future climate conditions. While most studies use precipitation to represent water availability, we focused instead on soil moisture, the water plants actually use. We quantified the strength of vegetation responses across both space and time and investigated the reasons behind space‐time differences. Surprisingly, much of the difference stems from relying on precipitation as the water availability proxy. Contrary to prior assumptions that vegetation constraints drive weaker temporal responses, we found that soil type, rooting depth, and seasonal water variability play larger roles. Most importantly, when using soil moisture instead of precipitation, spatial and temporal responses converged. Our study highlights the role of soil hydrology—how water moves and is stored in soil—in shaping vegetation responses. Our findings suggest soil moisture as a more informative indicator of water stress, and caution against relying solely on precipitation in ecosystem studies. Key Points Vegetation responds more strongly to precipitation across space than time, adding uncertainty in ecosystem projections under climate change The space‐time discrepancy is largely due to differences in seasonal water variability, rooting depth, and soil properties Vegetation sensitivity to soil moisture converges across CONUS drylands, underscoring soil hydrology's role in ecosystem response