Earth's critical zone includes plants as well as the heterogeneous near-surface layers into which they are rooted: the soil, saprolite, and weathered bedrock. It is within the critical zone (CZ) that water is stored and then released to streams and withdrawn by vegetation, highlighting its importance to humans, ecosystems, and the functioning of the Earth system in general. The research presented here is motivated by the global-scale challenges of mapping and predicting subsurface critical zone structure and understanding how that structure impacts water routing and storage for terrestrial ecosystems. It addresses these challenges by shedding light on inter-related ecohydrologic puzzles presented at the regional-scale across natural landscapes in the Eel River watershed: What explains the persistence and dominance of a particular species of oak, Quercus garryana, across savannas typified in the dry season by rolling golden hills mantled by senesced annual grasses? What explains the sharp ecotone that extends for hundreds of kilometers between this oak savanna community and a dense evergreen forest in a region of similar climate? Why did plant communities in this region fare better than others in the face of recent extreme drought?

By studying Quercus garryana's ecophysiology, I show that the oak is extremely water-limitation tolerant, which explains its ability to persist where a thin subsurface critical zone provides limited water storage capacity. Through sapflow monitoring on mature trees inhabiting an upslope position in the Central belt melange of the Franciscan, I reveal that the oaks maintain high rates of transpiration throughout the summer dry season, even as pre-dawn water potentials dropped to very low levels (below -3 MPa). At this site, Douglas fir has not encroached upon the oak groves like it has elsewhere throughout their mutual range. This is presumably due to Douglas fir's lower water limitation tolerance, and anticipates Quercus garryana's likely persistence in a warmer, drier future along western North America.

Through geospatial analysis coupled with intensive hillslope- and catchment-scale hydrological observation, I show that under a similar climate, adjacent landscapes across an extensive region of the Northern California Coast Ranges are either evergreen forest or deciduous oak savannah depending on their subsurface lithology. This is due to lithologically controlled contrasts in the extent of bedrock weathering and water storage capacity and thereby plant‐-available moisture in the summer dry season: a thick subsurface critical zone stores ample moisture and supports evergreen forest, whereas an adjacent thin subsurface critical zone sheds wet season rains and sustains savannah. The thick subsurface critical zone occurs in the modestly deformed shales and sandstones of the Coastal belt of the Franciscan; the thin subsurface critical zone occurs in the intensely deformed mud-matrix melange of the Central belt of the Franciscan. An important difference between the rock types is the thickness of the weathered bedrock layer; in contrast, the soil layer is of similar thickness. The thicker weathered bedrock at the Coastal belt site results in a larger rock moisture reservoir, i.e., unsaturated water residing within the weathered bedrock, which is primarily responsible for sustaining summer transpiration after shallow soils have dried. Extensive drilling was used to characterize the subsurface weathering extent and water storage dynamics, and was complemented by point--scale precipitation monitoring, catchment-wide remotely sensed evapotranspiration, and stream gauging to determine annual water budgets. Finally, late--summer dry season pre-dawn water potential observations in the dominant trees at both sites were used to compare subsurface water availability and show that where the weathered bedrock was thicker and hosted more rock moisture, the forest was less water stressed.

The observation of plant-available water in the dry season being mediated by the water storage capacity of the critical zone motivated a new hypothesis for how the subsurface can regulate plant response to drought: where annual rainfall reliably exceeds subsurface storage capacity, plant productivity and water use in summer should be insensitive to winter rainfall variability, because the subsurface water storage is replenished in wet and dry years alike. A simple ecohydrologic model for water storage in seasonally dry, Mediterranean climates predicts that storage will be replenished most reliably where the average precipitation is high relative to the subsurface storage capacity and winter evapotranspiration, and where year-to-year variability in precipitation is low. In order to test whether this storage-capacity limited behavior indeed decouples plants from inter-annual rainfall variability, running winter water balances were calculated for all unimpaired and generally undisturbed rain-dominated Mediterranean USGS catchments, using runoff, gridded precipitation and remotely sensed evapotranspiration as input data. These water balances revealed that sites in the Northern California Coast Ranges exhibited storage-capacity limitation, unlike most of the rest of the state. At all of the sites in the Northern California Coast Ranges, the consistent summer plant water supply resulted in plant insensitivity to rainfall variability, as revealed by remotely sensed summer plant greenness. Such sites are inherently resilient to meteorological drought, helping to explain the lack of mortality in the region in the extreme 2011-2016 drought.