ABSTRACT:

CODE AVAILABLE: https://github.com/erica-mccormick/storage-dynamics

MANUSCRIPT ABSTRACT: Large-scale plant mortality has far-reaching consequences for the water and carbon cycles. The role of belowground root-zone water storage (RWS) on extreme water stress remains uncertain. It has been proposed that the RWS capacity, Smax, can determine ecosystem vulnerability to drought, however, incorporating information about RWS into prediction of vegetation dynamics and mortality has been limited due to the challenge of quantifying RWS at large scales. Here, we present a mass-balance framework for assessing forest resilience to year-to-year variability in precipitation, including megadroughts, by quantifying RWS. We use the relationship between RWS and annual precipitation to evaluate the sensitivity of woody ecosystems to precipitation variability by classifying them as either capacity-limited, where RWS is nearly constant annually and set by Smax, or precipitation-limited, where RWS varies annually based on precipitation amount. We applied this framework to seasonally dry forests and savannas in California and found that approximately 16-23% of the state's total biomass is found in precipitation-limited locations where plants commonly rely on carryover of moisture from one year to the next. These precipitation limited areas experienced disproportionately high rates of mortality in recent drought. In contrast, approximately 51-58% of the state's biomass is found in capacity-limited locations and thus experiences annually reliable moisture supply. Using precipitation projections for the next century, the model framework reveals a tipping point by which 5,163 km2 (27 Tg aboveground carbon) of forest and savanna could transition from stable to unstable moisture supply. An additional 11,950 km2 (55 Tg aboveground carbon), where moisture supply is already annually unstable, is projected to experience increased water stress, due to additional years where precipitation is not sufficient to refill moisture deficits generated in dry years. This framework provides a novel approach for assessing vulnerability of RWS, and thus woody ecosystems, to climate change.

ABSTRACT:

In the past several decades, field studies have shown that woody plants can access substantial volumes of water from the pores and fractures of bedrock. If, like soil moisture, rock moisture serves as an important source of plant-available water, then conceptual paradigms regarding water and carbon cycling may need to be revised to incorporate bedrock properties and processes. Here we present a lower-bound estimate of the contribution of bedrock water storage to transpiration across the continental United States using distributed, publicly available datasets. Temporal and spatial patterns of bedrock water use across the continental United States indicate that woody plants extensively and routinely access rock moisture for transpiration across diverse climates and biomes. Bedrock water access is not confined to extreme drought conditions. On an annual basis in California, the volumes of bedrock water transpiration exceed the volumes of water stored in human-made reservoirs, and woody vegetation that accesses bedrock water accounts for over 50 per cent of the aboveground carbon stocks in the state. Our findings indicate that, like soil moisture, rock moisture is a critical component of terrestrial water and carbon cycling.

CODE AVAILABLE ON GITHUB: https://github.com/erica-mccormick/widespread-bedrock-water-use
FOR MORE INFORMATION, SEE WEBPAGE: https://erica-mccormick.github.io/widespread-bedrock-water-use/

ABSTRACT:

For code, see GoogleColab notebooks:
Part 1: Sr and Dmax: https://colab.research.google.com/drive/1-AntWqpQlb_2_HGV-R6yzgrZbTc5-Ygu?usp=sharing#scrollTo=LFvuxVJf1YkM
Part 2: Figure calculations: https://colab.research.google.com/drive/1z7Q4iFa97cuzfTiBK8xgX5BNA9Xl3JXc?authuser=0#scrollTo=iOztkTVg3cq2