Plantation water productivity (PWPWOOD) and not water-use efficiency (WUE) as the measure of commercial plantation yield improvement: a review

Global demand for forest products is ever-increasing, creating competition for water between downstream water users and commercial forest producers. Tree production should therefore aim at the effective use of water by producing maximum tree biomass from water used with the least total evaporative losses. A ratio of accumulated biomass to transpiration (T) known as the water use efficiency (WUE) is a common technique used to determine commercial forest plantation productivity. This review argues that WUE does not account for total plantation water use (ET), transpiration efficiency of trees (TE) and harvest index (HI, the tree stemwood, which is the most profitable component of a tree). This research suggests using PWPWOOD which is defined as the maximum amount of wood produced, also referred to as plantation yield (TE × HI) from a given volume of ET (T + canopy interception + soil water evaporation). Improving PWPWOOD requires that TE and HI are increased while ET losses are kept to a minimum. Practical interventions to improve plantation yield (TE and HI) are discussed in detail while techniques to minimise ET are discussed to a lesser extent. PWPWOOD has been shown to be a better measure and found to improve commercial forest plantation productivity as indicated by different case studies presented in this review.