© 1996 Heron Publishing—Victoria, Canada
Growth indices and stomatal control of transpiration in Acacia koa stands planted at different densities
Frederick C. Meinzer (1), James H. Fownes (2) and Robin A. Harrington (2)
1. Hawaii Agricultural Research Center, 99-193 Aiea Heights Drive, Aiea, HI 96701, USA / 2. Department of Agronomy and Soil Science, University of Hawaii at Manoa, Honolulu, HI 96822, USA / Received December 6, 1995
Summary
We examined the influences of selected environmental variables on stomatal behavior and regulation of transpiration in 26-month-old
Acacia koa Gray (koa) stands planted at spacings of 1 × 1 m or 2.5 × 2.5 m and grown without irrigation. Field measurements were made
during recovery from an extended 60-day dry period with only 38 mm of precipitation. Biomass and leaf area were also measured
at 3-month intervals over the first 24 months after planting and again following completion of the transpiration (T) and stomatal conductance (gs) measurements at about 26 months after planting. Transpiration was measured as sap flow through intact branches by a heat
balance method.
After a 22-day period during which 130 mm of rain were recorded, average T was substantially higher in the 2.5 × 2.5 m stand on both a leaf area and ground area basis even though leaf area index was
about 3.5 times higher in the 1 × 1 m stand. After an additional 25 mm of rain during an 8-day period, T was still slightly higher on a leaf area basis in the 2.5 × 2.5 m stand but was about 3 times higher on a ground area basis
in the 1 × 1 m stand. A strong stomatal response to humidity limited the increase in T with increasing evaporative demand. Values of gs in koa phyllodes were comparable to those reported for leaves of other mesic tropical forest trees, but were several times
higher than those reported for Acacia species native to arid and semi-arid regions. The 1 × 1 m planting yielded three times more biomass per unit ground area
than the 2.5 × 2.5 m planting. However, greater stand density, which resulted in more rapid depletion of soil water between
rainfall inputs, was associated with lower growth efficiency and lower radiation conversion efficiency.
Keywords:
biomass productivity, crown conductance, growth efficiency, leaf area index, radiation conversion efficiency, stomatal conductance,
vapor pressure deficit, wind speed.
