Winter Water Relations of Red Spruce on Mount Monadnock, New Hampshire

Abstract

With the pressure chamber, the water potential of red spruce stems (Picea rubens) was measured in four altitudinal zones, including timberline, on Mount Monadnock, New Hampshire, during two winters. Average water potential over all zones and dates was —1.14 (sd @= 0.57, range —0.1 to —4.0) MPa for 545 stems from 91 trees. More variation in water potential occurred within single trees than between juvenile and adult trees, and than among altitudinal zones or among seasons. Temperature data from maximum—minimum thermometers and from thermistors implanted into trunks and soil show that bulk water flow periodically occurs in winter. The slow decline in water potential of branches excised in winter, and scanning electron micrographs of plugged stomates, indicate that high diffusion resistances maintain needles within the relatively high levels of water potential measured in this study. Our results disagree with traditional concepts, support the conclusions of Slatyer (1976) and Marchand and Chabot (1978), and indicate that the mechanisms behind the variability in water status of modular parts of single trees must be understood before the causes, timing, and consequences of winter death in timberline trees can be unraveled.