COMPETITION FOR SOIL-WATER BETWEEN PERENNIAL BUNCH-GRASS (ELYMUS-GLAUCUS BB) AND BLUE OAK SEEDLINGS (QUERCUS-DOUGLASII H-AND-A)

The competition effects of the perennial bunch-grass (Elymus glaucus B .B.) on the growth and survival of the oak seedlings (Quercus douglasi i H. & A.) were investigated. There were four levels of Elymus competi tion, replicated three times. The three densities of Elymus employed w ere zero (control), 50 (Low-'L'-), 116 (Medium-'M'-) and 199 (High-'H' -) plants m(-2). Rates of soil water depletion, stomatal conductance, transpiration, shoot elongation and leaf expansion rates were measured between 23 March and 26 May 1988. Rates of soil water depletion, stom atal conductance and transpiration differed amongst the treatments and were higher in the control for the duration of the experiment. Shoot elongation rate (SER) and leaf expansion rate (LER) of blue oak seedli ng were directly related to soil water potentials. Zero values of LER rates for all treatments were observed at soil water potentials lower than -1.91 MPa, and concurrent reductions of stomatal conductance indi cated stomatal closure due to the soil water deficit. In the control t reatment, transpiration alone was not high enough to deplete soil mois ture and to reduce LER of the oak seedlings. Leaf dessication occurred first in the II and M treatments (53% of seedlings dessicated) and tw o weeks later in the L treatment (37% dessicated) when the soil water potential was approximately -4.0 MPa. The number of reproductive tille rs and seed dry weight indicated that Elymus plants were under water s tress from April 25 and concluded on May 25 with an early summer dorma ncy in all treatments. Data indicated that light intensity of 50% of a mbient did not limit the development of oak seedlings. The results sug gested that density of the perennial bunch-grass Elymus glaucus lower than 50 plants m(-2) could allow survival and successful establishment of blue oak in understories.