Two decades of riparian woodland water vapor and carbon dioxide flux responses to environmental variability

Authors: Gallo, Erika; Scott, Russell - Russ; Biederman, Joel

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2024
Publication Date: 7/5/2024
Citation: Gallo, E.L., Scott, R.L., Biederman, J.A. 2024. Two decades of riparian woodland water vapor and carbon dioxide flux responses to environmental variability. Agricultural and Forest Meteorology. 355. Article 110147. https://doi.org/10.1016/j.agrformet.2024.110147.
DOI: https://doi.org/10.1016/j.agrformet.2024.110147

Interpretive Summary: Riparian woodlands are small yet important features of drylands because they provide a wide range of ecosystem services. However, it is unclear how and why water and carbon fluxes have changed over time in these types of semiarid ecosystems. We address these knowledge gaps using 21 years of eddy covariance data from a velvet mesquite riparian woodland in southern Arizona. A unique feature of these riparian woodlands is that deeply rooted vegetation can access two sources of water: 1) deep groundwater and 2) shallow soil water sourced from precipitation. Access to groundwater resulted in annual evapotranspiration that was 1.9 to 5.4 times greater than precipitation, leading to significant net carbon uptake. Over the long-term, availability of both deep groundwater and shallow soil water increased; and we observed an increase in woodland photosynthesis and respiration. The greatest increases in photosynthesis and respiration occurred during the times of year with the greatest soil water availability. The impact of increasing atmospheric CO2 concentrations on photosynthesis was only detected when the dominant tree species was dormant. Groundwater subsidized evapotranspiration during the driest parts of the year, and water use shifted to shallow soil water during periods of rainfall. Access to groundwater made this riparian woodland highly resistant to climate variability at annual and seasonal time scales.

Technical Abstract: Riparian woodlands occupy a small area of global drylands but are hotpots for carbon and water cycling because groundwater supplements a small moisture supply from precipitation (P). Despite their regional importance, it is unclear if and how climate variability alters water and carbon fluxes in these ecosystems, and how ecosystem drivers vary across annual and seasonal scales. Here we use 21 years of eddy covariance measurements to understand land-atmosphere controls on carbon and water fluxes of a semiarid riparian mesquite (Prosopis vetulina) woodland with year-round access to deep groundwater and highly variable summer and winter rainfall. Access to groundwater supplemented evapotranspiration (ET) that exceeded precipitation (ET:P range 1.9 to 5.4), making this riparian forest a substantial carbon sink (367 ± 83 g C m-2 yr-1). Contrary to general expectations of regional climatic drying, there was a shift to more favorable water conditions as P and soil moisture increased over time. Annual gross ecosystem production (GEP) and respiration (Reco) increased at the same rate (~9 g C m-2 yr-2) due to GEP and Reco increases during the wettest periods of the year. Growth year separation based on GEP phenology and regression models show that water availability and antecedent phenophase fluxes control seasonal ET and GEP, with CO2 fertilization detected only during winter dormancy, the least-active phenophase. The major Reco driver during the spring and summer was GEP, and this coupling intensified following the onset of summer rainfall. Groundwater subsidies support ET during the dry growing season and decouple water and carbon. These results highlight the dynamic nature of water and carbon cycling in semiarid riparian woodlands, the value of groundwater subsidies to buffering ecosystem responses from the interannual effects of climate variability, and the importance of two water sources in driving seasonal ecosystem responses.