Altitudinal change in needle water relations of Pinus canariensis and possible evidence of a drought-induced alpine timberline on Mt. Teide, Tenerife

The alpine timberline on Mt. Teide, Tenerife, Canary Islands, Occurs at about 1000 m lower elevation than that of continental mountains of similar latitude. We tested the hypothesis that edaphic and/or climatic drought are major causes for the timberline depression on this high subtropical island. Comparative measurements of soil water content, needle water status, stomatal conductance, carbon isotope signature, and foliar N concentration were conducted in mature trees of the timberline species, Pinus canariensis Chr. Smith ex. DC, along an elevational transect (upper montane forest at 1600 m to the timberline at 2 100 m) in the wet and early dry seasons. The topsoil (0- 10 cm) desiccated completely in the dry season at the timberline but retained a significantly higher soil moisture at 1600 m where cloud cover is frequent. Daily maximal stomatal conductance of pine needles decreased significantly from 1600 to 2 100 in in the wet season which coincided with it higher water vapour saturation deficit of the air and a drier topsoil at 1800 and 2 100 m compared to 1600 m. The delta(13)C value of sun-fit pine needles increased by 4.5parts per thousand, km(-1) from 1600 to 2 100 m, which is a greater increase with elevation than has been found in mountains on continents; this may indicate partial stomatal closure in high elevation pines. Daily minima of needle water potential in the dry season increased from -1.0 to -1.5 MPa at 1600 m to -2.0 to -2.5 MPa at 1800 and 2 100 in. We conclude that mature P. canariensis trees at the timberline are seasonally affected by edaphic and probably climatic drought which contradicts the opinion that drought stress is principally of low importance in alpine timberline environments. However, neither drought nor frost stress are likely to cause a critical reduction in vitality and growth rate of mature pines because the uppermost pine trees grow up to 14 in in height and have a vital appearance. The carbon sink limitation hypothesis is not applicable to the isolated pine trees at the timberline because self-shading and, consequently, lowered root zone temperatures do not occur. We forward the 'multiple limitations at the seedling stage hypothesis' in order to explain the timberline depression as resulting from a failure of P. canariensis to regenerate successfully at high elevations.