- The effect of soil moisture, orange matter content, and texture on annual species diversity and aboveground phytomass was studied along a gradient of mobile to stabilized sand dunes in the northern Sharon Plains, Israel.138 vegetation samples of 30 by 30 cm and 22 soil samples, each of four layers (soil surface, 10, 20 and 30 cm), were chosen along this gradient.Field capacity was found to be highly influenced by organic matter. Species diversity showed a positive correlation with soil moisture and aboveground phytomass increased together with soil moisture and organic matter.The relation between number of species and aboveground phytomass is described as an arched curve, with a broad range (ca. 150–350 gm2) of intermediate phytomass in which species number is high. Below this range species productivity is determined largely by the small number of species adapted to higher degrees of environmental stress. Above this range the number of species decreases although plant production increases.