Hierarchical effects of tamarix aphylla afforestation in a sand dune environment on vegetation structure and plant diversity

One method of controlling dune encroachment is afforestation, which may result in biodiversity loss because of habitat change and adverse effects of trees on ecosystem functioning. We carried out a study on the effects of planting discrete areas with Tamarix aphylla (L.) Karsten trees, over 50 years ago, in a semiarid dunefield in the northern Negev, Israel. We surveyed the vegetation and sampled litter and soil in five microhabitats formed by this afforestation scheme. Afforestation had spatially and functionally hierarchical effects on vegetation and plant diversity. The strongest effect was associated with land-use change, showing reduced species richness (by 30-50 percent) and litter production following afforestation. The second strongest effect was associated with salt accumulation in T. aphylla leaves and litter, which increases soil salinity under T. aphylla canopies by 4-5, forming "salinity islands,"and leading to over 30 percent decrease in plant cover and aboveground biomass. The assumed effect of trees in blocking solar irradiance and wind was observed only outside canopies and had a weak impact on plant cover. Therefore, afforestation can increase plant diversity at the regional scale, but at the local (microhabitat) scale it has an overall adverse effect on measured ecosystem functions. Study Implications: Afforestation in sand dune environments has multiple effects on native vegetation structure and diversity. The strongest effects are those associated with land-use change, namely loss of mobile sand dune habitats and their unique species. We found that the adverse effects of afforestation on plant diversity are relatively limited in space, and are generally absent in nonafforested plots adjacent to afforested plots. Therefore, the size of the afforested area should be kept to a minimum, preferably forming a mosaic of afforested and nonafforested plots. Planting trees that have adverse effects on their environment (in this case, salt accumulation and excretion) can reduce vegetation cover, aboveground plant biomass, and plant species diversity directly under tree canopies, so such tree species should be avoided.