Allelopathic Effects of Non-Native Prosopis juliflora and Native Prosopis cineraria Trees on Native Plant Germination and Growth in Iran

Allelopathic compounds secreted by plants have significant effects on natural ecosystems, especially on the structure of plant communities. In particular, allelopathic effects of non-native plants can have major negative impacts in their introduced regions. This study aimed to evaluate and compare the allelopathic effects of two Prosopis (Neltuma) tree species (native (Prosopis cineraria) and non-native Prosopis juliflora) on the germination and growth (radicle and plumule length) of herbaceous plants and trees commonly found in riverine forests in Iran. ‎We hypothesized that soils and tissue extracts from non-native P. juliflora trees would have stronger inhibitory allelopathic effects on native plant species than native P. cineraria trees in Iran. Seeds of native trees (Acacia tortilis and Acacia oerfota) and ground flora species (Taverniera spartea and Pennisetum divisum) were grown in soils collected from under the canopy and the canopy edge of P. cineraria and P. juliflora trees and soils collected in open areas away from any trees (control). In an additional experiment, sterile soils were treated with powdered extracts made from the roots, bark and leaves of P. juliflora and P. cineraria at different concentrations (0, 2, 4, 6 and 8% (W/V)) and the same native species were grown in them. The results showed a general positive (facilitation) effect of P. cineraria on native species germination and growth, and the opposite (inhibitory allelopathic effects) for P. juliflora, especially in comparison to control soils. The release of inhibitory allelopathic compounds (especially from leaves) may be one of the mechanisms promoting P. juliflora invasions (spread) and negative impacts on native plant communities in the region. Our results provide experimental evidence to inform sustainable forestry and restoration planning by identifying plausible risks of planting invasive non-native tree species and highlighting the ecological benefits of native species for riverine forest restoration.