Geologic Controls on Apparent Root-Zone Storage Capacity

The water storage capacity of the root zone can determine whether plants survive dry periods and control the partitioning of precipitation into streamflow and evapotranspiration. It is currently thought that top-down, climatic factors are the primary control on this capacity via their interaction with plant rooting adaptations. However, it remains unclear to what extent bottom-up, geologic factors can provide an additional constraint on storage capacity. Here we use a machine learning approach to identify regions with lower than climatically expected apparent storage capacity. We find that in seasonally dry California these regions overlap with particular geologic substrates. We hypothesize that these patterns reflect diverse mechanisms by which substrate can limit storage capacity, and highlight case studies consistent with limited weathered bedrock extent (melange in the Northern Coast Range), toxicity (ultramafic substrates in the Klamath-Siskiyou region), nutrient limitation (phosphorus-poor plutons in the southern Sierra Nevada), and low porosity capable of retaining water (volcanic formations in the southern Cascades). The observation that at regional scales climate alone does not "size" the root zone has implications for the parameterization of storage capacity in models of plant dynamics (and the interrelated carbon and water cycles), and also underscores the importance of geology in considerations of climate-change induced biome migration and habitat suitability. What determines how much water plants can store in their root zone? One school of thought posits that plants "size" the root-zone capacity to survive a drought of a particular return period. In this scenario, plants extend their roots into the subsurface in response to climate drivers (e.g., precipitation magnitude-frequency and atmospheric water demand). This worldview neglects the potential for geology to restrict root access to water. "Bottom-up" limitations on storage capacity have been described at individual field sites, but it has been unclear how to identify geologic limitations at large scales. Here, we introduce an approach that quantifies differences between the climatically expected and locally observed apparent storage capacity, and relate these spatial patterns to geologic substrate. Importantly, we quantify apparent storage capacity via a method that includes water below the upper 1.5 m, within weathered bedrock, which is an important water source in seasonally dry climates and is typically excluded from traditional soil texture databases. We find that geology limits storage capacity at regional scales, and synthesize existing field evidence to hypothesize mechanisms of bottom-up control. Our findings have important implications for water-carbon cycle modeling efforts and the prediction of plant biome migration in response to climate change. Regionally extensive areas of low apparent root-zone storage capacity for a particular climate coincide with particular geologic substrates Hypothesized geologic controls include water storage capacity limitation, nutrient limitation, and toxicity