Minimum cuticular conductance and cuticle features of Picea abies and Pinus cembra needles along an altitudinal gradient in the Dolomites (NE Italian Alps)

Winter desiccation is believed to contribute to stress in coniferous trees growing at the treeline because cuticular conductance increases with altitude. To test whether winter desiccation occurs in high-altitude conifers of the Dolomites (NE Italian Alps), we measured minimum cuticular conductance (g(min)) needle wettability (contact angle) and cuticle thickness in Picea abies (L.) Karst. and Pinus cembra L. needles from December to August. Samples were collected from adult trees along an altitudinal gradient from valley bottom (1050 m a.s.l.) to the treeline (2170 m a.s.l.). The treeline site is one of the highest in the area and is characterized by a generally low wind exposure. Altitude had no effect on g(min), in either species. In P. abies, large seasonal variations in g(min) were recorded but no changes were related to needle age class. Pinus cembra had a low g(min) and appeared to be efficient in reducing needle water losses. There was a significant increase in g(min) with needle aging in P cembra growing at low altitude that could be related to a shorter needle longevity compared with P. abies. High contact angles (> 110-120degrees) suggested the presence of tubular epicuticular waxes on needles of both species. Contact angles were higher (low wettability) in high-altitude needles than in low-altitude needles. By the end of winter, there was no difference in contact angles between needles in the windward and leeward positions. Wax structures transformed toward planar shapes as demonstrated by the decrease in contact angle from winter to summer. In both species, the cuticle was thicker in needles of high-altitude trees than in needles of low-altitude trees and there was no correlation between gmin and cuticle thickness. Because desiccation resistance did not decrease with altitude in either species, we conclude that they are not susceptible to winter desiccation at the tree line.