There is evidence to suggest that part of the recent changes in the lichen flora of the Netherlands is attributable to an increase in temperature. Changes which have occurred over the last 22 years were studied in detail, and were subjected to a statistical treatment by comparing the change of species to their latitudinal distribution and to ecological determinants.
All 329 epiphytic and terrestrial lichen species occurring in the Netherlands were considered in relation to their world distribution. Arctic-alpine/boreo-montane species appear to be declining, while (sub)tropical species are invading. The proportion of increasing species is by far the largest among the wide-tropical lichens (83%), and smallest among the arctic-alpine/boreo-montane lichens (14%). None of the wide-tropical species was found to decrease, while 50% of the arctic-alpine/boreomontane species show a decline.
Long-term monitoring of the epiphytic lichen flora in the province of Utrecht from 1979 onwards shows that the total number of taxa present increased from 95 in 1979 to 172 in 2001, while the average number of taxa per site increased from 7·5 to 18·9. The rate of increase was greatest by far between 1989 and 1995. The majority of the species (152 taxa or 85%) show a gross increase, only 17 species (10%) show a decrease.
A detailed analysis of these data using multiple regression suggests global warming as an additional cause for recent changes, next to decreasing SO⊂2 and increasing NH⊂3. Changes appear to be correlated initially (1979-1995) only with toxitolerance and nutrient demand. Changes between 1995 and 2001, however, appear positively correlated to both temperature and nutrient demand, indicating a recent and significant shift towards species preferring warm circumstances, independent from, and concurrent with changes due to nutrient availability.
This is the first paper reporting long-term floristic changes for lichens that appear to be correlated significantly with increasing temperatures. We suggest that future lichen monitoring programmes also pay attention to effects of climatic change, instead of focusing on air pollution effects only.