Canopy architecture as a partial control over ecosystem water-use efficiency in a high-elevation, subalpine forest

PRATER, M.R., J. HU, S.P. BURNS, J.P. SPARKS, K. SPARKS and R.K. MONSON

Canopy architecture can play a major role in patterns of carbon and water exchange.  We have observed different patterns of ecosystem water-use efficiency in our eddy flux record depending on the direction of the forest fetch at the Niwot Ridge AmeriFlux site.  We investigated the degree to which these differences were dependent on the magnitude and distribution of leaf area index (LAI).  Leaf area distribution was determined by destructive harvest of Pinus contorta (lodgepole pine), Picea engelmannii (Engelmann spruce) and Abies lasiocarpa in the subalpine, coniferous forest.  Allometric regressions were utilized to estimate the leaf area in 1-meter vertical increments for 547 trees around the tower.  The trees west of the tower, dominated by spruce and fir, had greater overall leaf area (LAI of 3.8 ± 0.3 compared to 3.1 ± 0.3).  However, trees east of the tower, composed mostly of pine, had a greater percentage of leaf area (55%) in the upper third of the canopy compared to trees in the west (42%).  Needle carbon isotope ratios indicated that both canopy position and species had significant effects on water-use efficiency.  Greater allocation to sun leaves in the pine-dominated forest rather than to lower canopy shade needles, as in the spruce-fir forest, partly explain the observed differences in ecosystem-level water-use efficiency between these forest communities.