EXPERIMENTAL STUDIES ON THE COMPETITIVE BALANCE BETWEEN 2 CENTRAL-EUROPEAN ROADSIDE GRASSES WITH DIFFERENT GROWTH FORMS .2. CONTROLLED EXPERIMENTS ON THE INFLUENCE OF SOIL DEPTH, SALINITY AND ALLELOPATHY

Monocultures and 1:1 mixtures of the highly competitive grass Elymus r epens (L.) Gould and the low competitive grass Puccinellia distans (Ja cq.) Parl. were grown under controlled conditions in a growth chamber at four different soil depths and under various water and nutrient con ditions in order to evaluate whether shallow soil depth could be the d etermining factor in causing a stable coexistence of the two species, or dominance of P, distans, as recently observed in the shallow soil a reas of Middle European road shoulders. As expected, above- and belowg round biomass production of monocultures of both species was strongly reduced by shallow soils under all treatments. However, with decreasin g soil depth, the aboveground biomass fraction of P. distans was found to markedly increase relative to that of E. repens in the mixtures, p articularly if the plants were grown under water-stress conditions. Al though biomass production of both species increased with high nutrient supply, nutrient levels did not affect the competitive relationship o n shallow soils. Apparently, P. distans which naturally develops a rat her extensive fine root system in the top 10-15 cm of soil is better a ble to effectively use the limited resources of shallow soil than the normally deeper rooted E. repens. In consequence, E. repens cannot pro duce enough aboveground biomass to effectively shade and ultimately ex clude P. distans. Hydroculture experiments with both species at four d ifferent salinity levels revealed that biomass production of E. repens was more affected by salt stress than that of the facultative halophy te P. distans. Experiments are also presented which show that allelopa thic effects of E. repens, which have been equivocally discussed in th e literature, do not seem to affect biomass production of P. distans. It is concluded that shallow soil conditions are sufficient to create the observed patterns of co-occurrence at the roadside, but that commo n road treatments like winter salt application or frequent mowing of t he road shoulders further enhance the competitive ability of P. distan s in the shallow soil areas close to the pavement.