Reliability of Ellenberg indicator values for moisture,nitrogen and soil reaction: a comparison with field measurements

Abstract. Ellenberg indicator values for moisture, nitrogen and soil reaction were correlated with measured soil and vegetation parameters. Relationships were studied through between‐species and between‐site comparisons, using data from 74 roadside plots in 14 different plant communities in The Netherlands forming a wide range. Ellenberg moisture values correlated best with the average lowest moisture contents in summer. Correlations with the annual average groundwater level and the average spring level were also good. Ellenberg N‐values appeared to be only weakly correlated with soil parameters, including N‐mineralization and available mineral N. Instead, there was a strong relation with biomass production. We therefore endorse Hill & Carey's (1997) suggestion that the term N‐values be replaced by ‘productivity values'. For soil reaction, many species values appeared to need regional adjustment. The relationship with soil pH was unsatisfactory; mean indicator values were similar for all sites at pH > 4.75 because of wide species tolerances for intermediate pH levels. Site mean reaction values correlated best (r up to 0.92) with the total amount of calcium (exchangeable Ca²⁺ plus Ca from carbonates). It is therefore suggested that reaction values are better referred to as ‘calcium values'. Using abundance values as weights when calculating mean indicator values generally improved the results, but, over the wide range of conditions studied, differences were small. Indicator values for bryophytes appeared well in line with those for vascular plants. It was noted that the frequency distributions of indicator values are quite uneven. This creates a tendency for site mean values to converge to the value most common in the regional species pool. Although the effect on overall correlations is small, relationships tended to be less linear. Uneven distributions also cause the site mean indicator values at which species have their optimum to deviate from the actual Ellenberg values of these species. Suggestions for improvements are made. It is concluded that the Ellenberg indicator system provides a very valuable tool for habitat calibration, provided the appropriate parameters are considered.     

Soil, biomass, and management of semi-natural vegetation – Part II. Factors controlling species diversity

Using a wide range of conditions and plant community types, species diversity was investigated in relation to edaphic and non-edaphic site conditions, management, and biomass characteristics. Both standing biomass and aboveground production were investigated, and their effects compared. Three taxonomic assemblages were studied: (1) vascular plants only, (2) bryophytes also included, (3) terrestrial lichens included as well. Using a multivariate approach, both species richness and evenness could best be explained when only vascular plants were considered, emphasizing the role of taxonomic restrictions. The models best explaining species richness merely required abiotic conditions. This supports recent theories emphasizing the importance of an environmental regulation of the pool of (adapted) species from which the actual species are recruited. Explanatory soil properties were moisture and pH (both unimodal), and the soil available N:P ratio. Plots with large perimeter:area ratios had significantly more species than those with low ratios, indicating the importance of consistency in quadrat shape. Hump-shaped species richness relationships could be identified for both standing biomass and productivity, but they explained only a small part of the variation and were apparent only if soil and management effects were not accounted for. Unimodality (and notably the decreasing phase) was most pronounced when using maximum standing biomass, suggesting that the key factor is competition for light. At intermediate levels of standing biomass, the positive effects of habitat productivity and the negative effects of standing biomass itself are in balance, and high species numbers may be expected. When soil or management variables were allowed in the models, hump-shaped biomass relationships were no longer confirmed, suggesting that such relationships may arise from the covariation of biomass with other factors. Management explained a much larger part of the variation than the hump-shaped biomass relationship, suggesting that mowing and hay removal (both showing independent positive effects) regulate species richness in ways other than solely through the control of maximum standing biomass. Significantly higher evenness values were obtained at sites with low maximum biomass values, but only for the vascular plants. In addition, species evenness was positively related to the frequency of mowing. The numbers of both rare and endangered species were strongly curtailed by high standing biomass values, suggesting that these species are more susceptible to competitive exclusion than others. Through direct as well as indirect effects, management is confirmed to be beneficial not only for general species richness, but also for the occurrence of rare and endangered species.