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Clusters and environmental variables
Our results showed that species clusters differed in terms of analysed environmental variables. Generally, temperature-related variables (BIO1, BIO3, BIO4 and BIO9) were the key factors responsible for differentiation between the clusters. Elevation and variables connected with the terrain sculpture (WI, TI, TRI and MRVBF) were among the most important topographic variables separating species clusters. The influence of the geological variables (related to the bedrock) was relatively weak, but significant in some cases.
It seems to us that analysed clusters can be divided into major groups reflecting their distribution and characteristic derived from environmental variables. In case of climatic variables Luzula arcuata and Carex rupestris clusters can be considered as most cold tolerant, while remaining groups seem to prefer areas with moderate (e.g. Anthoxanthum odoratum) and mild climatic conditions (Puccinellia maritima, Rumex longifolius). This differentiation seem to be shaped mainly by the mean temperature.
There is, however, another interesting example of climatic differentiation present within the analysed dataset. This is Saxifraga aizoides cluster, which is separated by a variable designed to quantify temperature seasonality (coefficient of variation calculated from monthly means). Values of this variable were the lowest in case Saxifraga aizoides group, suggesting best fit to areas with the lowest temperature oscillations during the year.
It is clear that the relative frequency of arctic species followed also temperature gradient: being the highest in Luzula arcuata, Carex rupestris and Bistorta vivipara and intermediate or low in remaining groups apart from Puccinellia mariti...

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...rticularly important in terms of climate change and its effects on plant distribution. It seems that spatial patterns of clusters dominated by arctic species (Luzula arcuata, Carex rupestris) are more threatened than those with significant proportion of boreal and temperate species. Callaghan et al. (2004) stressed that arctic species will be most vulnerable to the climate change. They argued that ecological amplitude of arctic taxa will narrow and abundance decrease during climate warming.
At present there is no match between the distribution of protected areas and distribution of the clusters with highest proportion of threatened species. Proposals were made in 2008 to establish several protected areas but they still did not pass legislation process. It seems therefore that mechanisms are still not in place to ensure effective protection of the Icelandic flora.

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