After almost 40 years of experience in wetland restoration in Central Europe in which vegetation changes have been monitored by means of permanent plots or vegetation maps, some light can be shed on the intrinsic dynamics of such ecosystems, showing the limits of restoration and constraints in its manipulation. Sometimes such constraints in the restoration process can be identified, mostly being constraints in nutrient availability or in the water regime, but unexpected changes can also be the result of intrinsic species fluctuations or invasive species. Unexpected vegetation developments are sometimes undesired, can be very persistent and may indicate that environmental conditions are not suitable for target communities. Unexpected developments also illustrate the limits in restoration ecology. Very often the restoration process simply proceeds along successional pathways we did not anticipate. Theories about such alternative pathways can be explored using prediction models, such as cellular automata, which can handle the results of biomonitoring very efficiently. Biomonitoring during 40 years, however, has also shown that a certain amount of unpredictability has to be taken for granted, both in natural wetlands and in areas under restoration.

Summary1. The effects of nutrient enrichment on wetland vegetation may depend on the responses of different plant species to nutrient supply over several years, in waterlogged or flooded soils, and under either nitrogen-or phosphorus-limited conditions. However, most growth experiments comparing species from differently productive sites have focused on their short-term responses to variation in N supply. In this study we investigated whether increased N or P supply affects plant growth differently, whether these effects differ between the first and second year of growth, and whether they are modified by the water regime. 2. Plants of 16 wetland species were grown during two seasons in tubes with sand under full light. Treatments combined three nutrient levels (low N and P, high N, high P) with three water regimes (constantly wet, periodically aerated, periodically flooded). 3. In the first year, shoot biomass was enhanced by high N supply, particularly in species from nutrient-rich sites; this was associated with reduced shoot P concentration. In the second year, shoot biomass was generally enhanced by high P supply and reduced by high N supply; responses to high P were strongest in species with low shoot biomass and high N concentration but unrelated to the productivity of the species' sites. 4. The total biomass produced during both years was smaller at high N supply than at high P supply. A smaller fraction of the N and P supply was recovered in high-N plants, and these plants allocated less biomass to roots than those grown at high P supply or low N and P supply. 5. Periodic flooding reduced biomass production and nutrient recovery, but hardly influenced the effects of nutrient supply on plant growth. Species from wet meadows were affected more by flooding than species from fens in the first season, but not in the second season. 6. We propose that high N supply reduced second-year growth because strong P limitation increased below-ground nutrient losses from plants, whereas high P supply enhanced second-year growth by improving N retention in plants. Our results therefore suggest that N and P enrichment may have quite different effects on wetland vegetation.

Abstract. Short‐term field experiments are often used to predict and evaluate long‐term management effects. Based on a mowing experiment in two calcareous fens near Lake Neuchâtel, Switzerland, we investigated whether short‐term treatment effects (i.e. during the first four years) were confirmed by long‐term results (13 ‐ 14 yr). Plots were mown in summer or in winter or left unmown. The main long‐term trends in overall species composition (based on percentage cover estimates) were already observable in the first four years: mown and unmown plots diverged, whereas summer‐cut and winter‐cut plots remained similar. At the individual species level, however, short‐term and long‐term treatment effects differed considerably: many species whose abundance seemed affected by treatments during the first four years showed no response in the long term, and vice versa. These discrepancies were similar when based on cover estimates or on counts of shoots. Species responses did actually depend on the time scale considered. Short‐term and long‐term treatment effects on species richness were similar (i.e. a decrease in unmown plots), although only long‐term effects were significant. Treatment effects on the above‐ground biomass varied considerably, and short‐term trends (lower biomass in unmown plots) differed from long‐term trends (higher biomass in unmown plots). Our sites showed little overall change in species composition during the period investigated, and treatment effects were low compared with other similar experiments. If study sites are less stable or treatment effects more drastic, a short‐term evaluation is expected to be even less reliable. Knowledge on species dynamics at a site may help to choose the adequate spatial and temporal scale of investigation, and thus increase the efficiency of management experiments.

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