Ecophysiology of an urban citrus orchard

Characterizing the carbon and water economy of non-commercial urban citrus orchards can help determine their value in urban settings. This includes provisions of urban ecology and ecosystem services, such as shade, conservation of biodiversity, and carbon sequestration, under space and water limitations that may be particularly suitable in the semi-arid Mediterranean regions. We carried out canopy-scale eddy covariance (EC) measurements of net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET), partitioned these fluxes to their components using chamber-based soil fluxes, and combined them with carbon stock to provide a first approximation of the apparent ecosystem carbon turnover rate (τ_eco). The urban orchard switched from a carbon sink of 0.6 μmol m⁻² s⁻¹ to a carbon source of 0.2 μmol m⁻² s⁻¹ between winter and summer, with a first approximation of the annual carbon storage capacity of ∼75.3 g m⁻² and a total carbon accumulation over its 40 year life span of ∼3014 g m⁻². Carbon accumulated predominantly below ground (67 % of total), and soil CO₂ effluxes showed low sensitivity to temperature (Q₁₀ ∼1.6) and therefore also to climate warming, but also a fast C turnover rate (∼5.4 y), and therefore sensitivity to disturbances. The rates of ET increased from 0.40 to 1.25 mmol m⁻² s⁻¹ between the wet and dry seasons, and was ∼50 % of a similar nearby commercial orchard. Partitioning of the ecosystem carbon and water fluxes indicated high canopy water use efficiency (11.7 μmol CO₂/mmol H₂O; during the peak activity period). We demonstrate the potential of urban citrus orchards with low supplemental irrigation (50 % compared with commercial orchards in the area) to store significant amounts of carbon with high transpiration efficiencies. The results will help decision making in regard to urban tree planting and the ecological management of urban green spaces and community use in water-limited environments.