The principal goal of the project is to improve the understanding of the dynamics of species composition and size hierarchy in plant stands. Mortality and fertility are strongly correlated with size of individuals. In plant stands the size of individuals varies considerably due to size-specific differences in growth rates. Size-specific growth is mainly affected by competition for resources among plants. For example, resource acquisition and utilization of individuals are modified with increasing competition. Additional stress due to parasite infection or air pollutants (ozone) increases the carbon costs for defense and repair processes.
Ozone reduces growth and yield in crops. However, rather little is known about the effects of ozone on grassland systems. These systems are characterized by the composition of species, competition among individuals and repeated defoliation. An effect of ozone on all three aspects is to be expected. First, legumes are generally more sensitive than grasses. Thus, ozone may affect species composition in grassland. Secondly, competition results in individuals which are at least partially shaded. Low irradiance might increase susceptibility to ozone, as less assimilates are available for defense and repair processes. However, the lower transpiration rate of shaded leaves might reduce ozone uptake. Thirdly, defoliation reduces the availability of resources. This might increase the internal competition for resources which are used for growth and defense, respectively.
The two Medicago sativa cultivars Apica and Team were used in the experiments. Team was reported to be more ozone-tolerant than Apica (Renaud et al. 1998, Can. J. Bot. 76: 281-289).
The aim of the project was:

• to investigate the effect of ozone on the parameters of leaf photosynthesis for the two cultivars,
• to quantify carbon allocation and use in respiratory processes after short term ozone exposure,
• to qualify the effect of repeated defoliation on ozone-tolerance and
• to qualify the competitive ability and defense strategies of the two cultivars grown in monoculture and mixture.

Experiment 1
Individual plants were grown on sand in pots (5 cm diameter, 35 cm long) placed in growth cabinets adjusted to 22/18 °C day/night temperature, 75 % RH, 400 µmol m^–2 s^–1 PFD during a 16 h photoperiod. Plants with about ten leaves on the main stem were used for ozone exposure. Treatments were charcoal filtered air (cfg), cfg+100 ppb O₃ (mean of the 16 h exposure) and cfg+200 ppb O₃. Afterwards, light and CO₂ response on leaf photosynthesis was measured on the 4^th and 5^th youngest leaf of the main stem.

Experiment 2
Individual plants were propagated with stem cuttings. Plants were grown in growth cabinets adjusted to 22/18 °C day/night temperature, 75 % RH, 400 µmol m^–2 s^–1 PFD during a 14 h photoperiod. The δ ¹³CO₂/¹²CO₂ of the air was –12.3 ‰. For each genotype, randomly selected individuals, which had about ten leaves on the main stem, were placed in a second growth cabinet where δ of the air was –43.3 ‰. In this cabinet, plants were exposed to cfa or cfa+209 ppb O₃ (± 9 SD) for one photoperiod. Dark respiration of shoots and roots was measured during the following 10 h using an open gas exchange system with infrared gas analyzer and continuous-flow isotope-ratio mass spectrometer. Afterwards, plants were harvested, oven-dried and analyzed for total C and C isotope composition. In this way, all carbohydrates assimilated during one photoperiod were labeled (C_new), and their contribution to the dark respiration (R_new) was analyzed.

Experiment 3
Plants were established in pots (18 x 18 x 18 cm) on a sand/soil mixture. Each pot contained 12 plants of a single cultivar or a 1:1 mixture of both cultivars (Fig. 1). Pots were arranged outdoor to form stands (3 x 1.3 m, including border plants) of monoculture or mixed culture. Half of the stands were cut either two (2 cut interval) or three (3 cut interval) times during the vegetation period (4.5 cm cutting height) (Table 1). After the first cut of the 3 cut interval, selected pots were transferred into four growth cabinets where they formed small stands. Growth conditions were: 22/18 °C day/night temperature, 75 % RH, 400 µmol m^–2 s^–1 PFD during a 16 h photoperiod and charcoal filtered air. After 1 w, two growth cabinets were supplied daily with ozone during 8 h in the middle of the photoperiod (Fig. 2). After 6 w, plants were transferred back to their original place.
During the growth periods, shoots of a single pot were harvested weekly for growth analysis. At the end of each growth period, plants of a single pot of each treatment were washed free from the soil and separated into individuals. The number of nodes, leaves and branches, stem length and leaf area was recorded. Plants were dissected into leaves, stems and roots and fresh mass was weighed. Afterwards, plant material was stored at –20 °C. 

• After short term ozone exposure, TEAM showed less visible injuries on leaves than APICA.
• Short term ozone exposure resulted in an accumulation of recently assimilated C in leaves and less export to roots. Accumulation tended to be stronger in APICA than in TEAM.
• With comparable leaf injuries, TEAM showed less reduction in photosynthesis than APICA.
• Shoot respiration rate of recently assimilated C was not significantly affected in TEAM, but decreased in APCIA with leaf injury. After ozone exposure, respiration rate of older C tended to be higher in TEAM, but not in APICA (Lötscher & La Scala 2004).
• APICA showed a higher relative competitive ability than TEAM. Ozone affected the competitive ability of APCIA so that, relative to the O₃-free treatment, APICA was more competitive in the 3 cut interval and less competitive in the 2 cut interval.