Lithologically Controlled Subsurface Critical Zone Thickness and Water Storage Capacity Determine Regional Plant Community Composition

Abstract

Explanations for distinct adjacent ecosystems that extend across hilly landscapes typically point to differences in climate or land use. Here we document—within a similar climate—how contrasting regional plant communities correlate with distinct underlying lithology and reveal how differences in water storage capacity in the critical zone (CZ) explain this relationship. We present observations of subsurface CZ structure and groundwater dynamics from deep boreholes and quantify catchment‐wide dynamic water storage in two Franciscan rock types of the Northern California Coast Ranges. Our field sites have a Mediterranean climate, where rains are out of phase with solar energy, amplifying the importance of subsurface water storage for periods of peak ecosystem productivity in the dry season. In the deeply weathered (~30 m at ridge) Coastal Belt argillite and sandstone, ample, seasonally replenished rock moisture supports an evergreen forest and groundwater drainage sustains baseflow throughout the summer. In the Central Belt argillite‐matrix mélange, a thin CZ (~3 m at ridge) limits total dynamic water storage capacity (100–200 mm) and rapidly sheds winter rainfall via shallow storm and saturation overland flow, resulting in low plant‐available water (inferred from predawn tree water potential) and negligible groundwater storage that can drain to streams in summer. This storage limitation mechanism explains the presence of an oak savanna‐woodland bounded by seasonally ephemeral streams, despite >1,800 mm of average precipitation. Through hydrologic monitoring and subsurface characterization, we reveal a mechanism by which differences in rock type result in distinct regionally extensive plant communities under a similar climate.

Drainage from the Critical Zone: Lithologic Controls on the Persistence and Spatial Extent of Wetted Channels during the Summer Dry Season

“…By coupling our understanding of the subsurface hydrology of our sites (Dralle et. al, 2016; Druhan et al, ; Hahm et al, ; Kim et al, ; Link et al, ; Oshun et al, ; Rempe & Dietrich, ; Rempe et al, ; Salve et al, ) with wetted channel observations at the beginning and end of the summer dry season, our study demonstrates the connection between critical zone structure and the extent and duration of wetted channels.…”

Low Subsurface Water Storage Capacity Relative to Annual Rainfall Decouples Mediterranean Plant Productivity and Water Use From Rainfall Variability

“…We rejected these explanations because (i) there are consistent, large seasonal swings in EVI at storage-capacity-limited sites with diverse plant communities (see Figure 11 of Hahm et al, 2019), and (ii) greenness across a wide range of plant communities has been shown to be significantly correlated with water storage (Yang et al, 2014). We rejected these explanations because (i) there are consistent, large seasonal swings in EVI at storage-capacity-limited sites with diverse plant communities (see Figure 11 of Hahm et al, 2019), and (ii) greenness across a wide range of plant communities has been shown to be significantly correlated with water storage (Yang et al, 2014).…”

“…The subsurface CZ at Dry Creek consists of a shallow soil (~40 cm) developed on a relatively thin saprolite and weathered bedrock zone that transitions to low porosity, low conductivity fresh parent material at only~2-m depth (Dralle et al, 2018;Hahm et al, 2019;Lovill et al, 2018). The subsurface CZ at Dry Creek consists of a shallow soil (~40 cm) developed on a relatively thin saprolite and weathered bedrock zone that transitions to low porosity, low conductivity fresh parent material at only~2-m depth (Dralle et al, 2018;Hahm et al, 2019;Lovill et al, 2018).…”

“…The subsurface CZ at Dry Creek consists of a shallow soil (~40 cm) developed on a relatively thin saprolite and weathered bedrock zone that transitions to low porosity, low conductivity fresh parent material at only~2-m depth (Dralle et al, 2018;Hahm et al, 2019;Lovill et al, 2018). This can be explained by the subsurface CZ-structure imposed cap on plant-available water storage (Hahm et al, 2019). Further precipitation generates widespread overland flow, and S does not increase, resulting in limited summer water availability in the summer.…”

“…Elements of this mechanism have emerged in ecohydrologic models and field studies (Fellows & Goulden, 2016;Hahm et al, 2019;Link et al, 2014;Milly, 1994;Porporato et al, 2004;Sayama et al, 2011;Smith et al, 2011;Stephenson, 1990;Zanardo et al, 2012), yet to our knowledge the role of storage-capacity limitation in mediating plant productivity and water use sensitivity to precipitation variability at landscape scales remains unexplored. Here we explore instead whether low subsurface plantavailable water storage capacity-relative to precipitation-can decouple plant water supply (and, consequently, plant productivity) from precipitation anomalies, including droughts.…”