Early Survival and Growth of Six Afforestation Species on Abandoned Cropping Sites in Irrigated Drylands of the Aral Sea Basin

Afforestation is known as a low-input measure to rehabilitate marginalized irrigated croplands with a shallow, saline groundwater table. This study assessed the potential of extending afforestation to sites long-term abandoned from cropping in the lower reaches of the Amu Darya River, Central Asia. Tree survival and establishment were monitored during two growing seasons following afforestation with six tree species at two abandoned cropping sites. The sites, characterized by soil salinity and different depth to the groundwater table, received deficit irrigation of 154 mm yr−1. Afforestation was feasible with Elaeagnus angustifolia, Ulmus pumila, Morus alba, and Populus nivea x tremula given their survival rates of 75–91% on both sites after two years. N2-fixing E. angustifolia was assessed as most promising among all species evidenced by highest survival and largest above-ground biomass increment (up to 904 kg ha−1 yr−1). The principal species of the native floodplain forest, Populus euphratica and Salix nigra, failed to establish, showing survival rates <19%. Kaplan-Meier survival curves revealed differences in tree survival between the sites, indicating the necessity of a site-specific evaluation of the afforestation species. Both current plant-available water and soil salinity in the root zone significantly affected tree survival, with distinct differences in the stress tolerance among the species. Afforesting long-term abandoned cropland is associated with higher risks and lower outputs than tree planting in marginal agricultural areas, implying the necessity of a site-dependent evaluation including cost-benefit analyses of afforestation as opposed to natural re-vegetation of abandoned sites.