Comparison of water-use by alien invasive pine trees growing in riparian and non-riparian zones in the Western Cape Province, South Africa

Abstract

Self-established stands of alien invasive pine trees are common in many parts of South Africa and elsewhere. They mainly invade non-riparian settings but sometimes invade riparian habitats. There are clear visual differences in the physical attributes of trees that occupy riparian and non-riparian zones. We have little information whether the differences between trees growing in these contrasting habitats reflect their water-use. The goal of this study was to establish the water-use of alien invasive pines growing adjacent to and away from a perennial stream, and to determine the driving factors behind the variations. The study was conducted in a self-established 20-year old mixed pine forest occupied by roughly equal proportions of Pinus pinaster and Pinus halepensis. Individual tree transpiration rates were measured using the heat pulse velocity (HPV) sap flow method. Evapotranspiration (ET) from entire stands was determined from the surface energy balance equation using sensible heat flux data collected using a boundary layer scintillometer and measurements of the available energy (net radiation – soil heat flux). A simple two-layer model in which the stand ET was calculated as the algebraic sum of the outputs from transpiration (E) and soil evaporation sub-models was evaluated at the two contrasting sites. Annual transpiration and ET rates were higher in the riparian zone at 980 and 1417 mm compared to 753 and 1190 mm, respectively in the non-riparian area. The model predicted stand transpiration fairly accurately for both sites (average R2 44 > 0.75), but was less accurate for evapotranspiration (average R2 45 < 0.70) due to the difficulties in simulating soil evaporation. No significant differences in sap velocities were found between trees at the two sites so the greater water-use of trees in the riparian zone was due to the larger basal area per stem. Based on the measured transpiration data we conclude that self-sown pine stands growing in riparian zones use at least 36% more water than those occurring in non-riparian settings justifying the high priority given to clearing invasive trees in riparian zones.