Interpretation of altitudinal gradients in South Central Norway based on vascular plants as environmental indicators

The main aim of this study is to quantify environmental differences along altitudinal gradients on the basis of different sets of plant indicator systems and recorded vascular plants within 100-m altitudinal bands. Two areas are included in the study, and in total they include an altitudinal span from sea-level up to 2400 m a.s.l. The applied indicator systems are the six values defined by Ellenberg et al. [Ellenberg, H., Weber, H.E., Düll, R., Wirth, V., Werner, W., Paulissen, D., 1991. Zeigerwerte von Pflanzen in Mitteleuropa. Scripta Geobotanica 18, 1–248], eight Raunkiaer life forms, the respiration values of Dahl [Dahl, E., 1998. The phytography of Northern Europe. Cambridge University Press], and the snow indicator values of Odland and Munkejord [Odland, A., Munkejord, H.K., 2008. Plants as indicators of snow layer duration in southern Norwegian mountains. Ecological Indicators 8, 57–68]. Mean indicator values are calculated for each altitudinal band, and they show significant linear increasing or decreasing trends, significant unimodal (quadratic) trends or no significant altitudinal trend. The altitudinal variation in mean indicator values is discussed in relation to general environmental conditions such as bedrock, soil characteristics, topography, summer temperature, light, snow and distribution of major vegetation types. The main differences between the altitudinal bands as shown by a PCA analysis indicate that the highest bands, representing the high alpine zone (more than 1800 m a.s.l.) strongly differ from the others. The altitudinal gradient is mainly associated with variation in temperature, nitrogen indicators and some of the Raunkiaer life forms (primarily the Phanerophytes, Nanophanerophytes, Geophytes and Therophytes). The two tested temperature indicator systems tested were highly linearly correlated, but the altitudinal variation in respiration value followed the general temperature lapse rate better than the Ellenberg temperature indicator value. Indicators of high soil moisture have their optima at intermediate altitudes. The uppermost bands in Aurland show lower mean temperature indicator values and a higher snow indicator value compared to bands at the same altitude in Jotunheimen. This may partly be explained as a result of the Massenerhebung effect. Relationships between altitudinal distribution patterns of some vascular plants in Aurland and their Ellenberg temperature indicator value indicate that the values should be revised to better suit Norwegian conditions.