Actual and potential transpiration and carbon assimilation in an irrigated poplar plantation
Hyun-Seok Kim (1, 2), Ram Oren (1) and Thomas M. Hinckley (3)
1. Nicholas School of Environmental & Earth Sciences, Duke University, Durham, NC 27708-0328, USA / 2. Corresponding author () / 3. College of Forest Resources, University of Washington, Seattle, WA 98195-2100, USA / Received May 15, 2007; accepted October 4, 2007; published online February 1, 2008
Summary
We examined the tradeoffs between stand-level water use and carbon uptake that result when biomass production of trees in
plantations is maximized by removing nutrient and water limitations. A Populus trichocarpa Torr. × P. deltoides Bartr. & Marsh. plantation was irrigated and received frequent additions of nutrients to optimize biomass production. Sap
flux density was measured continuously over four of the six growing-season months, supplemented with periodic measurements
of leaf gas exchange and water potential. Measurements of tree diameter and height were used to estimate leaf area and biomass
production based on allometric relationships. Sap flux was converted to canopy conductance and analyzed with an empirical
model to isolate the effects of water limitation. Actual and soil-water-unlimited potential CO2 uptakes were estimated with a canopy conductance constrained carbon assimilation (4C-A) scheme, which couples actual or potential
canopy conductance with vertical gradients of light distribution, leaf-level conductance, maximum Rubisco capacity and maximum
electron transport. Net primary production (NPP) was about 43% of gross primary production (GPP); when estimated for individual
trees, this ratio was independent of tree size. Based on the NPP/GPP ratio, we found that current irrigation reduced growth
by about 18% compared with growth with no water limitation. To achieve maximum growth, however, would require 70% more water
for transpiration, and would reduce water-use efficiency by 27%, from 1.57 to 1.15 g stem wood C kg–1 water. Given the economic and social values of water, plantation managers appear to have optimized water use.
Keywords:
gas-exchange, gross primary production, leaf area index, leaf water potential, light-use efficiency, net primary production,
soil water, water-use efficiency.
