The vascular plant flora of shell gravel deposits on the Aland Islands, SW Finland - Community structure in relation to calcium

Plants are referred to as calciphilic (calcicole) or acidophilic depending on their relation to lime. We censused the vascular plant flora on seven shell gravel deposits from the Littorina Age and in two reference areas with naturally high species richness on the angstrom land Islands, assuming that a higher proportion of calciphilic plants would be found on the shell gravel deposits than elsewhere. Both the total number of plants and the number of calciphiles showed a significant positive correlation with the calcium (Ca) concentrations of the soils, but the highest number of species and proportions of calciphilic species were not found on shell gravel, but at intermediate to high Ca-levels in areas affected by man and domestic animals. We did not observe a strong influence of Ca on the species distribution patterns, otherwise than possibly indirectly via a raised pH-value. On the contrary, the strongest explanatory environmental variable seemed to be phosphorus, P. This in turn raises further questions: Should we rather use the word alkaliphilic? Are plants Ca-dependent or Ca-tolerant? Could the whole story be about P and not Ca? When switching the analytical focus from sites to species, Ca-neutral species were on average found at more sites than moderately or strongly calciphilic species. In a species-based ordination, the calciphiles also clustered significantly differently from the Ca-neutral species, both when analysing all species simultaneously and when analysing a subset of the most widely distributed species. The exact meaning of this is hard to evaluate, but it may give a hint that many calciphiles could be poor generalists and that the Ca concentration not always was sufficiently high at all shell-gravel sites. It also demonstrates the functionality of dividing species into these four groups with regard to Ca dependency, especially if the Ca-neutral group solely consists of acidophilic species. This is also in support for a continuing use of calciphilic plants as indicators for rough assessments or initial screenings of plant diversity in applied conservation.