EXPERIMENTAL STUDIES ON THE COMPETITIVE BALANCE BETWEEN 2 CENTRAL-EUROPEAN ROADSIDE GRASSES WITH DIFFERENT GROWTH FORMS .1. FIELD EXPERIMENTS ON THE EFFECTS OF MOWING AND MAXIMUM LEAF TEMPERATURES ON COMPETITIVE ABILITY

The weakly-competitive grass, Puccinellio distons (Jacq.) Parl., and t he highly competitive grass, Elymus repens (L.), coexist on roadsides in Central Europe which are regularly mowed. The effect of mowing on t his existence was evaluated in situ at the roadside and in experimenta l field plots under non-limiting water and nutrient conditions. In fou r experimental garden plots, tussocks of P. distons were transplanted into monocultures of E. repens early in the growing season. After esta blishment, two of the plots were mowed and two left unmowed. In the un mowed plots, P. distans was quickly overtopped and died, while in mowe d plots, P. distons was able to persist throughout the growing season. Despite persistence in the mowed plots, the number of tillers of P. d istons declined throughout the growing season as numbers of tillers of E. repens increased. It was concluded that mowing could enhance coexi stence in situations of unlimited nutrients and water where the faster growing and aggressive E. repens would exclude P. distans without mow ing. At the roadside, six plots were established at a site containing both species early in the growing season. Weekly mowing was performed on three of the plots while the others were left unmowed. In mid-july, when the grasses were beginning to senesce and had produced seedheads , foliage area for P. distans was significantly higher in the mowed th an the unmowed plots while the reverse was true for E. repens. While P . distons had higher foliage area in the mowed plots, it was able to p ersist to seedhead production in the unmowed plots. Simulations conduc ted with a multispecies canopy photosynthesis model indicated that red uctions in carbon gain for P. distons due to shading by foliage of E. repens did not correspond well with foliage area for P. distans at the start of experiment or in the mowed and unmowed plots in mid-july. In both the mowed and unmowed plots, the portion of foliage consisting o f P. distans increased with closeness to the roadway and corresponded inversely with soil depth, At soil depths of greater than 15 cm, P. di stons did not occur. More effective exploitation of shallow soil may s hift the competitive balance toward P. distons and be a significant fa ctor in the coexistence of these two species in the shallow soils at t he roadside. Differences in leaf temperatures at the roadside that mig ht result from different leaf widths were also evaluated. The narrower -leaved P. distans was hypothesized to have lower midday leaf temperat ures at sites close to the asphalt and perhaps be closer to the temper ature optimum for photosynthesis during warm summer days. In situ leaf temperature measurements made with small thermocouples attached to in tact leaves, however, were not significantly different for the two spe cies, and coexistence was not likely to be affected by leaf temperatur es.