Verschuivingen in het voorkomen van Veronica longifolia L., in het bijzonder in Twente

The occurrence of Veronica longifolia in the Netherlands is shifting in 3 respects: / its area tends to expand in southwestern direction; 2 the species is becoming rarer along the rivers from which it has been known of old and, on the other hand, is spreading along some little-used canals; 3 V. longifolia is nowadays most abundant in a type of vegetation (see table i) different from that in which it occurred optimally in the past. The latter two changes have been investigated in Twente (eastern part of the province of Overijssel) and were compared with data from the rest of the Netherlands and from Germany. Principally V. longifolia occurs along the middle reaches of smaller rivers crossing sandy areas, viz. the Vecht with its affluent the Dinkel (prov. of Overijssel) as well as the Dommel (prov. of Noord-Brabant). Flotsam belts at the fringe of the area of inundation constituted its characteristic natural habitat. River canalization and water pollution together have caused rarification of localities of V. longifolia of this kind. Nowadays the species is locally rather abundant along some canals which are in contact with the rivers just mentioned. There its chief type of habitat is the transition zone between the talus and the bank. In some places it is found on the slope as well as – although 'suppressed' and not flowering – on the treaded foot of the canal dike (fig. 2). Both in the natural and the artificial habitat of V. longifolia the accompanying vegetation is dominated by tall herbs, but in the latter case it contains more species of relatively dry environment. Moreover combinations of species occur which are rather unlikely in natural habitats, e.g. plants of dry grassland together with plants of reed swamps. According to Ellenberg's ecological figures V. longifolia is found now in Twente in an environment which on average is drier, more acid and poorer in nitrogen than is generally the case for its habitat in Central Europe. V. longifolia V. longifolia owes the opportunity to extend its occurrence partially to traffic ways (verges of canals, railways and motorways). But besides migration routes of plant species these are often also carriers of hypertrophy and pollution, which favour few species like Glyceria maxima, Phalaris arundinacea and Urtica dioica superseding V. longifolia. It is chiefly along canals out of use that this species gets appropriate settling chances. Management of verges and ditches along railways and motorways aiming at conservation of the indigenous flora will probably lead to further expansion of V. longifolia. In the Netherlands V. longifolia is of rather marginal occurrence. In its habitat taller plants with larger leaves usually dominate, like Filipendula ulmaria or Eupatorium cannabinum. V. longifolia, however, appears to tolerate incidental treading better than these species and also seems to endure drought, mowing and covering with sand more successfully. Along rivers mainly resistance to drought and covering with sand will have favoured its occurrence, while along canals and on verges treading, mowing and drought probably are important factors. On the whole the latter habitat appears to be drier than the former; water pollution and hypertrophy have repelled V. longifolia to a relatively dry niche. Similar shifts are shown by Dianlhus deltoides, Juncus filiform is and Sanguisorba officinalis, which show distribution patterns partially similar to that of V. longifolia. In their original area and habitat they have become rarer, while they are spreading elsewhere, finding (except Juncusfiliformis) more or less different types of habitat especially on motorway and canal verges. Also of many other species their progress and decline is determined by their possibilities to use traffic ways as migration routes and to move from natural to artificial habitats.