Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA

In a mixed desert shrub community we removed and added shrub canopies to ex amine above- and belowground influences of 3 species of shrubs on islands o f soil fertility and the survival of transplanted Ambrosia dumosa seedlings . Soils sampled under shrubs in the wet season had higher pH, water content , organic matter, and both total and mineralizable nitrogen than soils in a djacent open areas, confirming a widely established pattern in arid lands. However, we also found species differences in soil parameters. Soils under Coleogyne ramosissima had highest pH, soils under A. dumosa had highest wat er content and nitrogen mineralization rates, and soils under Larrea triden tata had lowest water content. Soils sampled under shrubs in the dry season , 7 months after experimental shrub removal, maintained higher organic matt er and total and mineralizable nitrogen content than adjacent open soils, b ut pH and water were altered by shrub manipulations. Species differences pe rsisted only in soil water levels (A. dumosa soils were driest). Over a 1-y ear period, transplanted A. dumosa seedlings had highest survivorship in sh rub removal and open treatments and died most rapidly under control shrubs of all 3 species, suggesting that shrubs had a strong negative effect on se edling survival, even in the presence of higher organic matter, nutrients, and (initially) higher water content of fertile islands. Our results sugges t that nurse plants and islands of soil fertility have the potential to fac ilitate growth of other species by nutrient additions, but that the net eff ect of nurse plants can be negative due to shading and/or root competition.