Document: ANN-3-32-42

Water transport in large northern Rocky Mountain conifers is not always limited.

SALA, A.* ¹, E.V.CAREY ^1,2 and R.M.CALLAWAY ¹

The University of Montana, Missoula MT 59812 USA ¹
University of Minnesota, Saint Paul, MN 55108-6112 USA ²

Abstract:
Evidence suggests that as trees grow older and larger, hydraulic conductance decreases causing stomatal closure and low photosynthesis rates. This "hydraulic limitation hypothesis" has been proposed as one of the mechanisms responsible for the decline of productivity of old trees and forests. If water transport becomes limited in large trees, then leaf area-based whole-tree water use should decline with tree size. We tested this hypothesis for two pairs of Rocky Mountain conifers: whitebark pine (Pinus albicaulis) and subalpine fir (Abies lasiocarpa) in the subalpine zone, and Douglas-fir (Pseudotsuga menziesii) and western larch (Larix occidentalis) in low elevation forests of western Montana during July 1996. We measured whole tree water use with sapflow techniques in 9-20 trees of different sizes of the four species. At the end of the season we harvested all trees to measure biomass allocation and leaf area. Daily integrated sapflow on a leaf area basis declined significantly with tree size for whitebark pine (r² = 0.46, p = 0.03) and western larch (r² = 0.45, p = 0.02) but not for subalpine fir and Douglas-fir. Decreased leaf area-based sapflow for large pines and larches were related to significant increases of leaf to sapwood area ratios as tree size increased. Constant leaf area-based sap flux densities with tree size in firs were related to either constant or decreasing leaf to sapwood area ratios as trees grew larger. Our results for the two firs indicate that water supply to foliage does not always become limited as trees increase in size. They also indicate that water transport capacity of trees of different sizes is related to their corresponding leaf to sapwood ratios.

Keywords: Water transport, Conifers, Pinus, Abies, Pseudotsuga, Larix

This abstract is being presented at: 4:30 PM in session:
Oral Session #34: Water Relations in Trees.