Exotic invasive trees affect germination and growth of understory plants via soil modifications

Biological invasions may affect diversity in different ways. In the case of exotic invasive plants, as soon as they arrive to a new ecosystem they may directly compete for resources with native plants, make the native environment toxic for native plants or modify soil properties in a way that hinder native plants. The exotic invasive trees, Ailanthus altissima and Robinia pseudoacaica, are listed as 20 of the most harmful exotic invasive species in Spain, but their mechanisms of impact are still not clear. In this study we investigated the extent to which these exotic invasive trees may affect understory native plants via soil modifications. In a riparian forest, we collected soils around exotic (A. altissima and R. pseudoacaica) and native trees (Populus alba) trees. Half of the soils were supplemented with activated carbon (AC) to reveal possible effects via the release of phytotoxic compounds from the invasive trees to the soil. We added seeds of native herb species (target species), Trifolium repens and Dactylis glomerata, to pots containing the different soils. We moistened the soils to favor seed germination. The emergence of plants was counted daily, and the aerial biomass reached by the plants was also measured. Independently of the addition of AC, compared to native soils, those of the exotic invasive trees negatively affected the target species. The biomass of T. repens and the emergence speed of D. glomerata was lower in A. altissima soils than in native soils. The emergence speed of T. repens was affected by both exotic soils. The effects produced by the exotic trees may be attributed to changes in soil properties but not to the release of phytotoxic compounds.