Wind Effects on Needles of Timberline Conifers: Seasonal Influence on Mortality

In the Rocky Mountains, USA, year—round wind exposure has been correlated with widespread needle dehydration and death in winter for alpine timberline conifers. Needle death may be due solely to the drying effects of winter wind or may also result from predisposition to winter injury associated with suboptimal summer growth conditions. Our experiments showed that in the lower timerbline ecotone in southwest Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind—exposed needles of flagged trees and surface needles of krummholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged ≥75% for needles unprotected by snow, regardless of wind exposure. Snow covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind—exposed needles of Picea engelmannii occurred at ≤—4.5 MPa water potential (≤60% relative water content). Cuticular resistance of wind—exposed needles declined from 100—250 ks/m in autumn to ≤30 ks/m by midwinter. Experimentally reversing the windward—leeward orientation of small, flagged trees in early winter resulted in lower xylem pressure potentials and needle viabilities for newly wind—exposed (originally leeward) compared to newly sheltered (originally windward) shoots. Also, sheltering exposed branches from winter wind on flagged trees in the lower timberline ecotone (3200 m) increased mean overwinter needle survival from near 0 to ≥50% in both Picea engelmannii and Abies lasiocarpa. Scanning electron micrographs of dehydrate wind—exposed needles collected in March at this site showed little cuticular surface wax, probably because of windborne ice crystal abrasion. However, winter dehydration and death in both experimentally and naturally wind—sheltered needles at higher elevation may have been due to inadequate needle maturation during summer, which could act to exclude flagged trees from the upper timberline ecotone.