Annual Cycle of Leaf Water Potential in Picea Engelmannii and Abies Lasiocarpa at Timberline in Wyoming

Leaf water potential of Picea engelmannii and Abies lasiocarpa was measured throughout the year in the Medicine Bow Mountains, Wyoming. Winter-exposed needles of trees at timberline (3,300 m) exhibited severe water stress during the winter (-30 to -35 bars water potential) while winter-protected needles of trees at timberline and needles of trees at lower elevations (2,990 m) exhibited less stress (-18 to -20 barswater potential). The winter stress of the timberline trees appeared to be the result of the interaction of primarily two factors, the strong winds which increased transpirational losses and the cold soil which inhibited absorption of water. Leaf water stress appears to be a significant limiting factor of tree growth at timberline in the central Rocky Mountains.