Audorff, V: Vegetation ecology of springs: ecological, spatial and temporal patterns, BayCEER-online, 5, 1-131 (2009) [Link]

Acidification is a phenomenon, which affected the forested catchments of the northern hemisphere severely over recent decades. Acidic depositions depleted the buffering capacities of soil and groundwater, what lead to an impairment of forests, headwaters, and lakes. Even though the depositions were reduced considerably since the early 1990s, the recovery of catchments was found to occur time-delayed. The grade of recovery was found to vary significantly between regions.

Biomonitoring is an appropriate tool to detect spatial and temporal patterns of ecosystem alterations, such as acidification and recovery. However, to know the interrelationships between organisms and their environment is an indispensable precondition for the identification of indicator species. The complexity of ecosystems and ecological processes hampers this quest oftentimes. Springs provide a natural setting that minimises such constraints. Compared to other habitat types, external factors are less relevant, which makes it easier to relate changes in species abundances to changes in their environment. Studying this species-environment relationship, here the response of plant species to the acidification of the spring waters was of particular interest.

In a survey of five regions in Central Europe - taking spatial, hydrophysical as well as hydrochemical parameters of the springs into account - it was clearly shown that the species composition of springs is essentially determined by the spring water chemistry, and more precisely by the gradient of acidity and nutrient availability. This connection was reflected by spatial patterns within and between the regions. These patterns provide useful ecological information about spring water quality and in return about the acidity status of their forested catchments.

Including catchment traits - like bedrock, climatic parameters, and forest vegetation - in the analyses, these emerged to be relevant for the species composition of springs, but less than the spring water chemistry. A path analysis showed that the catchments affect the vegetation of springs not directly, but indirectly via the determination of spring water quality. Hence, the catchments are a part of the functional chain, which is driven by the atmospheric depositions.

The pH-value was found to represent the gradient of acidity and nutrient availability best. It can serve as a proxy measure that can be related to species occurrence and to species dynamics respectively, aiming to identify indicator species for assessing the status and alterations of spring water quality.

With the aim to delineate niche optima and amplitudes, which in return can serve as indicator values, the realised niches of spring-inhabiting species were modelled with respect to pH. The niche attributes were found to be a matter of sampling scale. Larger plot sizes (grain) weakened the species-environment relationship, what consequently resulted in broader niche amplitudes. In contrast, the grain did not influence the species’ pH optima.

Monitoring approaches that target to assess processes in time, such as acidification and recovery, are dependent on the response time of indicator species to changes in their environment. Investigating an interval of four consecutive years, inter-annual variability of the species composition could not be attributed to changes in the acidity of the spring waters. Looking at single species, bryophytes did not show a higher sensitivity to the inter-annual variability of the environment than vascular plants. Actually, only a minority of all species featured abundance changes which were significantly correlated to variations in spring water acidity. Our results suggest that the species inertia retards the vegetation dynamics of forest springs. A delayed or long-term integrating response of potential indicator species must be considered when evaluating their indicator suitability. In conclusion, the biomonitoring of spring water acidification or recovery is expedient only for longer time intervals.

In a nutshell, the vegetation of springs is closely related to the hydrochemical traits of the spring waters, in particular to a gradient of acidity and nutrient availability. Individual species as well as whole plant communities are suitable indicators which allow for the monitoring of the acidity status of forested catchments. The results of this study contribute to a better understanding of the species-environment-relationships, and in return to an improvement of indicator systems.

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