Observed Effects

So far, few studies have demonstrated a correlation between global change and change in lichen habitat. The study by van Herk et al. [1] was the first and only one reported in the meta-analysis by Parmesan and Yohe [10] in their study of 'globally coherent fingerprint of climate change impacts across natural systems'. The lichen study was based on a long-term (22 a) monitoring involving all the 329 epiphytic and terrestrial lichen species occurring in the Netherlands and were considered in relation to their world distribution. The investigation focussed on the exposed wayside trees in the province Utrecht in the Netherlands. The research was initially started to document changes resulting from changes in sulphur dioxide air pollution levels. When the levels dropped, the effects on the lichens were clearly visible. However, the pattern was disturbed by a new emergent air pollution problem ammonia from increasingly intensive cattle farming. As different lichens show different responses to this pollutant, the lichen monitoring was continued for a different purpose, viz. a detailed mapping of the areas with problematic ammonia pollution. Changes between 1995 and 2001, however, could not be explained in terms of air pollution variables alone. Analysis, however, showed a positive correlation with temperature, oceanity and nutrient demand, indicating a recent and significant shift towards species preferring warmer circumstances, independent from, and concurrent with changes due to nutrient availability. In short, warmth-loving, oceanic lichens are expanding and boreal lichens are diminishing.

The lichens that are expanding most dramatically are those with the green algae Trentepohlia as their photobiont. As these lichen species (i.e., the myco-biont) belong to different unrelated taxonomic groups and the effect has been observed in different ecosystems (exposed trees, forests), Aptroot and van Herk [11] argue that it seems likely that the effect of the global warming is, in fact, directly related to the alga, and all lichens with this alga can profit from the expansion of their photobiont. The process as described here is continuing and probably even accelerating. A recent study by van Herk [12] shows that most of the recent changes can be now attributed to global warming (see Figs. 1 and 2).

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