ABSTRACT:

Multi-year droughts are known to negatively impact Mediterranean montane forest productivity. However, the role of subsurface moisture storage capacity remains unclear. It is generally thought that larger root-zone moisture storage capacity leads to greater drought resistance, particularly where moisture storage across years can maintain vegetation water use. An emerging hypothesis is that smaller moisture storage capacity, that is replenished despite below-average precipitation, leads to greater forest drought resistance. To examine how forest productivity varied across landscapes with distinct moisture storage capacity during a multi-year drought, we leverage three pine forest sites within 4.5 km in southern Sierra Nevada, California with similar climate and species (i.e., ponderosa or Jeffrey pine) but distinct root-zone moisture storage capacities (310, 590, 970 mm). At each site, we compared new tree-ring chronologies and estimated moisture storage dynamics using a water balance approach. During California's 2012–2015 drought, the site with the largest moisture storage capacity had the greatest decline in growth, while the site with the smallest storage maintained or exceeded average growth throughout the drought. Forest dieback also corresponded with larger moisture storage capacity. Together, these findings reveal a forest drought vulnerability paradox: larger moisture storage capacity can amplify rather than buffer pine growth suppression during multi-year drought likely because trees acclimated to abundant moisture supplies are more vulnerable to incomplete replenishment.

ABSTRACT:

Chemical weathering and the subsequent porosity it generates controls soil development, water cycling, and nutrient availability. However, predicting spatial variability in chemical weathering remains challenging due to complex interactions between dominant drivers including climate, topography, water cycling, and vegetation dynamics. Here, we leverage neighboring ridgetops with the same lithology but distinct topographic relief, vegetation, water cycling, and bedrock fracturing to investigate if and how compound weathering drivers reflect the observed chemical weathering profiles. Specifically, within a central coast California site underlain by metagreywacke sandstone, we drilled four boreholes (6-40 m deep), collected rock cores for geochemical and physical analyses, and conducted hydrological monitoring during water year 2023 on a ridgetop with oak trees and one with only grasses. Despite these distinct environmental and geomorphic conditions, we observed strikingly similar chemical weathering magnitudes within the shallow subsurface (< 5 m) between ridgetops. These near-surface similarities likely reflect legacy effects of widespread tree cover during the cooler and wetter Pleistocene climate. Chemical weathering differences emerged most strongly within the fractured bedrock (5-25 m), emphasizing the importance of fracture density for deep chemical reactions. Additionally, chemical depletion extended to the stream channel elevation within both ridgetops, coinciding with the present-day water table and indicating dominant groundwater control on chemical weathering initiation. These findings have widespread implications for predicting subsurface weathering and associated ecosystem processes across water-limited landscapes, where paleoenvironmental legacies may overprint shallow chemical weathering patterns while fracture density mediates deep weathering extent.

ABSTRACT:

Quantifying the spatiotemporal variability in evapotranspiration is critical to accurately predict vegetation health, groundwater recharge, and streamflow generation. Differences in hillslope aspect, the direction a hillslope faces, result in variable incoming solar radiation and subsequent vegetation water use that influence the timing and magnitude of evapotranspiration. Previous work in forested landscapes has shown that equator-facing slopes have higher evapotranspiration due to more direct solar radiation and higher evaporative demand. However, it remains unclear how differences in plant functional groups (i.e., grasses and trees) influence evapotranspiration and water partitioning between hillslopes with opposing aspects. Here, using field-based measurements of soil water storage and oak tree and grass transpiration, and remotely-sensed evapotranspiration and normalized vegetation difference index, we quantified evapotranspiration and subsurface water storage deficits between a pole-facing and equator-facing hillslopes with contrasting vegetation types within central coastal California. Our results suggest that cooler pole-facing slopes with oak trees have higher evapotranspiration than warmer equator-facing slopes dominated by grasses, which is counter to previous work done in landscapes with singular vegetation types. Our water storage deficit calculations indicate that the pole-facing slope has a higher subsurface storage deficit and a larger seasonal dry down than the equator-facing slope. This aspect difference in subsurface water storage deficits may influence subsequent deep groundwater recharge and streamflow generation. In addition, larger root-zone storage deficits on pole-facing slopes may reduce their ability to serve as hydrologic refugia for oaks during periods of extended drought. This research provides a novel integration of field-based and remotely-sensed estimates of evapotranspiration required to properly quantify hillslope-scale water balances. These findings emphasize the importance of resolving hillslope-scale vegetation structure within Earth system models, especially in landscapes with diverse plant functional groups.

ABSTRACT:

The structure of the critical zone is a product of feedbacks between hydrologic, climatic, biotic, and chemical processes. Ample research within snow-dominated systems has shown that aspect-dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, more research is needed to understand the role of microclimates on critical zone development within rain-dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the critical zone of a north-facing and south-facing slope within a first-order headwater catchment located in central coastal California. We combined terrain analysis of vegetation distribution and topography with field-based soil pit characterization, geophysical surveys and hydrologic measurements between slope-aspects. We observed thicker soil profiles, higher shallow soil moisture, and denser vegetation on north facing slopes, which matched previously documented observations in snow-dominated sites. However, average topographic gradient and saprolite thickness were uniform across our study hillslopes, which did not match common observations from the literature. These results suggest dominant processes for critical zone evolution are not necessarily transferable across climates. Thus, there is a continued need to expand critical zone research, especially in rain-dominated systems. Here, we present four non-exclusive, testable hypotheses of mechanisms that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects. Specifically, we propose both past and present ecohydrologic functions must be taken into account to understand what shaped the present day critical zone.