Surface‐Atmosphere Carbon Dioxide Exchange and Ecosystem Water Use Efficiency in a Multi‐Functional Perennial Polyculture Agroecosystem

ABSTRACT

Perennial polycultures, with a combination of perenniality and vegetation diversity, have carbon mitigation potential and water conservation benefits, via increased plant carbon uptake and improved ecosystem water use efficiency (EWUE). Few studies have assessed the coupled carbon and water balance in perennial polyculture agroecosystems that are not yet a widespread agricultural practice. We measured net ecosystem carbon dioxide (CO ₂ ) exchange (NEE) and evapotranspiration (ET) using eddy covariance, partitioned measurements into gross primary productivity (GPP), ecosystem respiration (R _ECO ), evaporation (E) and transpiration (T), to assess annual carbon sinks and EWUE in a fruit orchard with diverse forbs and grass understory cover in Southern California's Mediterranean climate. Seasonal trends showed peak CO ₂ uptake and EWUE from late spring through summer. The agroecosystem functioned as a modest yet statistically significant net carbon sink compared to a hypothesised net zero carbon balance (no carbon storage), sequestering 58 g C m ⁻²  year ⁻¹ ( Z (standardised variability) = −2.41, p  = 0.016) in 2022 (January–December) and 174 g C m ⁻²  year ⁻¹ ( Z  = −7.31, p  < 0.001) in 2023 (January–October). Smaller net CO ₂ losses from January to March when trees were dormant underscored the critical role of understory vegetation during winter and early spring in maintaining the net annual carbon sink. Cumulative ET was 664 mm (2022) and 617 mm (January–October 2023) with T dominating E, particularly from April to July during peak tree activity. Monthly EWUE ranged from 1.4 to 4.7 g C m ⁻²  mm ⁻¹ H ₂ O. EWUE stayed above half of the maximum observed value throughout the tree dormant season, beyond October until apricot leaf‐out in April, due to photosynthetically active understory cover. Staggered complementary phenologies between trees and understory cover vegetation can extend the carbon uptake period, making perennial polyculture agroecosystems a net CO ₂ sink with high water‐use efficiency.