Subsurface storage capacity controls on tree ring sensitivity to precipitation

Abstract Tree growth sensitivity to precipitation varies dramatically across Mediterranean climate regions, but the mechanisms controlling this variation remain poorly understood. We investigated how subsurface water storage capacity mediates the relationship between winter wet season precipitation and annual tree ring width across seasonally dry ecosystems of the western United States. We analyzed tree ring chronologies from the International Tree-Ring Data Bank combined with climate databases and a root zone water storage capacity dataset encompassing both soil and bedrock storage. We categorized seasonal water storage at tree-ring sites into precipitation-limited, intermediate, and storage-capacity-limited groups based on the ratio of net winter precipitation to root zone storage capacity. Our central hypothesis was that sites where winter precipitation consistently exceeds storage capacity would show weaker precipitation-growth coupling than sites where precipitation inconsistently fills available storage. Using Spearman rank correlations between ring width and winter precipitation, we found that storage-capacity-limited sites indeed showed significantly weaker precipitation-growth coupling compared to precipitation-limited sites. This pattern suggests that when subsurface storage is consistently filled, trees have access to similar amounts of water each year regardless of precipitation variability, buffering growth against climate fluctuations. Conversely, where storage is inconsistently filled, tree growth remains sensitive to precipitation variation. These findings have important implications for understanding drought resilience in Mediterranean ecosystems and for paleoclimate reconstruction using tree rings. In storage-capacity-limited environments, tree rings may not reliably record precipitation variability, potentially limiting their utility for drought reconstruction. Our results emphasize the critical role of subsurface storage dynamics in mediating plant responses to climate variability.