Factors limiting the distribution of Digitaria adscendens and Eleusine indica on coastal sand dunes in Japan

Abstract Fate of Digitaria adscendens and Eleusine indica seedlings under field conditions and their responses to salt spray, drought and nutrients were experimentally investigated in order to evaluate the possible mechanisms controlling the different distributions of the two species in coastal sand dune areas. Salt spray produced no apparent positive effect on the growth or survival of both species. Seeds of each species germinated well in the field, although 80% of E. indica seedlings died during a summer drought and surviving seedlings neither grew nor bore fruit. The mortality of D. adscendens seedlings due to the drought was less than 10% and the survivors mostly bore fruit by the end of the growth season. No major difference in the sublethal water saturation deficit was noticed between the two species. However, D. adscendens individuals extended their roots into the deep sandy soil to a much greater extent in water‐stressed conditions than in well watered conditions, whereas E. indica showed no such behavior. Additional watering in a dune environment did not help the growth of E. indica seedlings, but additional nutrients had a markedly stimulatory effect. D. adscendens maintained its growth and fruition with much smaller amounts of nutrients than E. indica . Soil nitrogen content at a site where E. indica and D. adscendens were distributed sympatrically was higher than that at a site where only D. adscendens was present. Based on these findings, it is proposed that E. indica seedlings are unable to become established because of their lower resistance to summer drought and the poor nutrient conditions present in a coastal rear sand dune habitat.