Influence of wind exposure on needle desiccation and mortality for timberline conifers in Wyoming, U.S.A. [Picea engelmannii, Abies lasiocarpa]

Wind exposure strongly affected needle water relations and mortality in Picea engelmannii and Abies lasiocarpa krummholz during winter at timberline in Wyoming. Windward needles had lower water contents, xylem pressure potentials, and viability, compared to leeward needles. Water content and viability increased for windward needles with height above the snow level. Below the snow surface, needle dehydration and mortality were minimal. Both absolute and relative water content were highly correlated with needle viability (a needle viability of near 50% corresponded to a water content of about 60% of dry weight). Air temperatures did not drop below -29°C in 1980/81, and needle mortality was not correlated with low nighttime needle temperatures. However, dehydration and mortality of wind-exposed needles were associated with low daytime needle temperatures and low cuticular resistance to water vapor loss. A preliminary experiment with excised shoots indicated that summer pre-disposition probably was not the primary cause of winter needle death, and that dehydration and death could have resulted from winter wind exposure alone, perhaps through cuticle abrasion.