Altitudinal change in soil and foliar nutrient concentrations and in microclimate across the tree line on the subtropical island mountain Mt. Teide (Canary Islands)

Mt. Teide (Tenerife, Spain) is a high volcanic island mountain with an unusually low tree line elevation (2000–2100 m). While searching for the causes of this tree line depression, we analysed the concentrations of total N, available P, and salt-exchangeable Ca, K and Mg in three soil horizons, and the foliar N, P, Ca, K and Mg concentrations in six abundant plant species (trees, shrubs, forbs) along a transect from 1400 (1600) to 3100 m a.s.l. The objective of the study was to detect altitudinal trends in soil and plant nutrient status below and above the tree line. For characterising elevational changes in microclimate and hydrology, we also conducted measurements of air and soil temperatures (Ta and Ts), atmospheric water vapour saturation deficit (D), potential evaporation (E) and soil moisture (θ) on 3–4 occasions along the transect.Ta and Ts did not linearly decrease with elevation but were highest at or immediately below the tree line and were relatively low in the closed Pinus canariensis forest at 1600 m. Ts reached maxima at about 70 °C near the tree line. The absence of a linear temperature decrease with elevation was caused by a stable temperature inversion at about 2000 m and by canopy shading in the forest canopy below the tree line. In June, the topsoil (10 cm) dried out almost completely between 1800 and 3100 m, but remained moist in the pine forest at 1600 m. This gradient reflects the transition from the montane cloud belt to the dry alpine belt higher upslope. The subsoil (30 cm) contained >30 vol% of soil water at all elevations even in June except for the uppermost site (3100 m). Potential evaporation increased with elevation despite a decrease in D. We assume that this is mainly due to the air pressure-dependent increase with elevation in the diffusion coefficient for water vapour in air.The concentrations of N and ‘available’ P (after Olsen) in the mineral topsoil were by far smaller than in alpine soils of other humid mountains which is thought to be a consequence of a very dry and biologically inactive topsoil in the semi-arid alpine belt of Mt. Teide. In contrast to many other mountains, foliar N, P and cation concentrations in the plants did not increase with elevation but either remained unchanged, or decreased as in the tree line species P. canariensis. Nevertheless, P. canariensis probably is not limited by nutrient deficiency at the tree line despite rather low N and P needle concentrations. Rather, drought and heat stress effects on seedling establishment are thought to be the causes of the tree line depression.