The gypsophilous scrub communities of the Ebro Valley (Spain)
Loidi, Javier; Fernández-González, Federico
A phytosociological revision of the gypsophilous scrub vegetation of the Ebro Valley using all the available data is made. For the analysis of the relevés set (initially 258) computer ordination methods, such as CA and PCA, have been applied in order to establish a relationships model between the different community-types. Along the analysis process, several groups have been distinguished and successively removed until we reached to a residual set of 178, which has been interpreted as homogeneous and representative of the nucleus of the main association. Part of this last set constitutes an example of biased sampling which favourises the distinction of two groups of relevés due to dominance of two plants. The comparison of both groups with other taken from the same area but without that bias reveals, when plotted together, the real significance of the two former groups as the cloud of the last one intermingles with the first ones of them. Discussion about the meaning and syntaxonomical recommended status for each group is also made and expressed in the accepted syntaxonomy; in it a new subassociation is described: Helianthemo thibaudii-Gypsophiletum hispanicae helianthemetosum rotundifolii. It is argued that typified syntaxa, such as associations and sub associations, in addition to a clear floristical characterisation, should have an ecological (climatic, edaphic) scope and often a phytogeographical significance. Ecotonic communities and transitional stages of internal succession could be expressed by means of variants and facies respectively.