Question: Is quantile regression an appropriate statistical approach to estimate the response of fen species to single environmental factors? Background: Data sets in vegetation field studies are often characterized by a large number of zeros and they are generally incomplete in respect to the factors which possibly influence plant species distribution. Thus, it is problematic to relate plant species abundance to single environmental factors by the ordinary least squares regression technique of the conditional mean. Location: Riparian herbaceous fen in central Jutland (Denmark). Methods: Semi‐parametric quantile regression was used to estimate the response of 18 plant species to six environmental factors, 95% regression quantiles were chosen to reduce the impact of multiple unmeasured factors on the regression analyses. Results of 95% quantile regression and ordinary least squares regression were compared. Results: The standard regression of the conditional mean underestimated the rates of change of species cover due to the selected factor in comparison to 95% regression quantiles. The fitted response curves indicated a general broad tolerance of the studied fen species to different flooding durations but a narrower range concerning groundwater amplitude. The cover of all species was related to soil exchangeable phosphate and base‐richness. A relationship between soil exchangeable potassium and species cover was only found for 11 species. Conclusion: Considering the characteristics of data sets in vegetation science, non‐linear quantile regression is a useful method for gradient analyses.

No abstract

The effects of reduction and cessation of sheep grazing on salt-marsh vegetation were studied on a formerly intensively grazed salt marsh in northern Germany. Plant species cover was recorded in 45 permanent plots from 1992 to 2000. In 1995, physical and chemical soil parameters were analysed. Results of Redundancy Analysis (RDA) indicated that salinity and the depth of anoxic conditions below the surface were the most important soil factors related to the spatial vegetation pattern. Furthermore, plant species distribution was influenced by present and past grazing intensity, by soil grain size and nitrogen content.Vegetation changes over 9 yr were analysed by non-linear regression. The cover of Aster tripolium, Atriplex portulacoides, and, to a lesser extent, Artemisia maritima and Elymus athericus increased due to reduced grazing pressure, whereas the cover of Salicornia europaea decreased. After a strong increase in the first years Aster decreased 2 to 6 yr after abandonment. In the mid salt-marsh zone Puccinellia maritima was replaced by Festuca rubra. The cover of Puccinellia, Festuca, Suaeda maritima, Glaux maritima and Salicornia fluctuated strongly, probably due to differences in weather conditions and inundation frequency. Species richness per 4 m 2 generally increased while vegetation evenness decreased during the study period. Only in the high salt marsh abandoned for 9 yr did the number of species decrease slightly. Thus far, cessation of grazing did not lead to large-scale dominance of single plant species.

Question: Is quantile regression an appropriate statistical approach to estimate the response of fen species to single environmental factors? Background: Data sets in vegetation field studies are often characterized by a large number of zeros and they are generally incomplete in respect to the factors which possibly influence plant species distribution. Thus, it is problematic to relate plant species abundance to single environmental factors by the ordinary least squares regression technique of the conditional mean. Location: Riparian herbaceous fen in central Jutland (Denmark). Methods: Semi-parametric quantile regression was used to estimate the response of 18 plant species to six environmental factors, 95% regression quantiles were chosen to reduce the impact of multiple unmeasured factors on the regression analyses. Results of 95% quantile regression and ordinary least squares regression were compared. Results: The standard regression of the conditional mean underestimated the rates of change of species cover due to the selected factor in comparison to 95% regression quantiles. The fitted response curves indicated a general broad tolerance of the studied fen species to different flooding durations but a narrower range concerning groundwater amplitude. The cover of all species was related to soil exchangeable phosphate and baserichness. A relationship between soil exchangeable potassium and species cover was only found for 11 species. Conclusion: Considering the characteristics of data sets in vegetation science, non-linear quantile regression is a useful method for gradient analyses.

Abstract. The effects of reduction and cessation of sheep grazing on salt‐marsh vegetation were studied on a formerly intensively grazed salt marsh in northern Germany. Plant species cover was recorded in 45 permanent plots from 1992 to 2000. In 1995, physical and chemical soil parameters were analysed. Results of Redundancy Analysis (RDA) indicated that salinity and the depth of anoxic conditions below the surface were the most important soil factors related to the spatial vegetation pattern. Furthermore, plant species distribution was influenced by present and past grazing intensity, by soil grain size and nitrogen content. Vegetation changes over 9 yr were analysed by non‐linear regression. The cover of Aster tripolium, Atriplex portulacoides, and, to a lesser extent, Artemisia maritima and Elymus athericus increased due to reduced grazing pressure, whereas the cover of Salicornia europaea decreased. After a strong increase in the first years Aster decreased 2 to 6 yr after abandonment. In the mid salt‐marsh zone Puccinellia maritima was replaced by Festuca rubra. The cover of Puccinellia, Festuca, Suaeda maritima, Glaux maritima and Salicornia fluctuated strongly, probably due to differences in weather conditions and inundation frequency. Species richness per 4 m2 generally increased while vegetation evenness decreased during the study period. Only in the high salt marsh abandoned for 9 yr did the number of species decrease slightly. Thus far, cessation of grazing did not lead to large‐scale dominance of single plant species.