Afirst attempt is made towards a synopsis of the general features and differentiation of roadside phytocoenoses in temperate Europe. The evident similarity between synchorological variation of roadside vegetation and the general chorology of Europe is demonstrated with plant communities dominated by species of Poaceae, Apiaceae, and Asteraceae. A more detailed differentiation along the geographical-ecological cline between the North Sea and the Mediterranean Sea is represented in modifications of the sequence of coenoses within the roadside profile.

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The weakly-competitive grass, Puccinellia distans (jacq.) ParI., and the highly competitive grass, Elymus repens (L.), coexist on roadsides in Central Europe which are regularly mowed. The effect of mowing on this existence was evaluated in situ at the roadside and in experimental field plots under non-limiting water and nutrient conditions. In four experimental garden plots, tussocks of P. distans were transplanted into monocultures of E. repens early in the growing season. After establishment, two of the plots were mowed and two left unmowed. In the unmowed plots, P. distans was quickly overtopped and died. while in mowed plots. P. distans was able to persist throughout the growing season. Despite persistence in the mowed plots, the number of tillers of P. distans declined throughout the growing season as numbers of tillers of E. repens increased. It was concluded that mowing could enhance coexistence in situations of unlimited nutrients and water where the faster growing and aggressive E. repens would exclude P.distans without mowing. 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 than the unmowed plots while the reverse was true for E. repens. While P. distans had higher foliage area in the mowed plots, it was able to persist to seedhead production in the unmowed plots. Simulations conducted with a multispecies canopy photosynthesis model indicated that reductions in carbon gain for P. distans 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 of P. distans increased with closeness to the roadway and corresponded inversely with soil depth. At soil depths of greater than 15 em, P. distans did not occur. More effective exploitation of shallow soil may shift the competitive balance toward P. distans and be a significant factor in the coexistence of these two species in the shallow soils at the roadside. Differences in leaf temperatures at the roadside that might result from different leaf widths were also evaluated. The narrowerleaved P. distans was hypothesized to have lower midday leaf temperatures at sites close to the asphalt and perhaps be closer to the temperature optimum for photosynthesis during warm summer days. In situ leaf temperature measurements made Bot. Acta 109 (1996) 441-448 © GeorgThiemeVerlag Stuttgart-NewYork with small thermocouples attached to intact leaves, however, were not significantly different for the two species, and coexistence was not likely to be affected by leaf temperatures.