Increasing Vascular Plant Richness on 13 High Mountain Summits in Southern Norway since the Early 1970s

We describe changes in vascular plant richness on 13 high mountain summits based on a historical study performed approximately 40 years ago. A summit is defined as the uppermost 30 m of the mountain tops. The altitudes of the summits range from 1512 to 1814 m. Data from neighboring climatologic stations showed higher mean values for July temperature (+0.7 °C) and January temperature (+1.5 °C), and mean annual precipitation had increased from 714 to 764.7 mm (7.1%) for the period from 1970 until 2009 compared with average data for the normal period (1961–1990). The total "top flora" had during this period increased by 19 taxa. On average, the increase in taxa richness was 18.9 ± 8.4 per summit, representing an average increase of 90.2%. Woody species had an average upward movement of 7.3 m during the study period. The ongoing upward shift of common, ubiquitous alpine species has resulted in a floristic homogenization of the mountain summits, and thus increasing α-diversity was accompanied by decreasing β-diversity. The use of recorded plants as temperature indicators showed that average summer temperatures had increased by approximately 1.3 °C during this period. Several of the newly established species indicate that the climate has become more humid. We assume that the recorded floristic changes are the result of ongoing climatic changes.