A Field Demonstration of the Effect on Streamflow of Clearing Invasive Pine and Wattle Trees from a Riparian Zone

SYNOPSIS

Two portable weirs were used to quantify changes in streamflow following clearfelling of a dense stand of self-sown Pinus patula and Acacia mearnsii along a riparian zone on Kalmoesfontein, a SAPPI forest plantation south-east of Lydenburg, Eastern Transvaal. The weirs were set up at positions 500 m apart on the same stream, and used to monitor streamflow levels before and after the clearing of all trees between the weirs to an average distance of 25 m from the stream. Analysis of streamflow data revealed that the clearing operation resulted in a 120% increase in streamflow at the lower weir, equivalent to 30,5 m3 of water per day. Streamflow at the lower weir was less than at the upper weir before clearfelling, but equalled that at the upper weir after clearfelling. This demonstrates that the intervening trees were responsible for the initial difference in streamflow. Two further lines of evidence point to the adjacent trees exerting a strong influence on streamflow. Firstly, a clear daily fluctuation in streamflow was evident at both weirs. This is a consequence of transpiration by trees taking place only during hours of daylight. Secondly, the occurrence of cloudy, rain-free weather led to an increase in streamflow. Such weather reduces the evaporative demand of the air, causing transpiration rates to decline as well.

We conclude that invasive exotic trees should be removed from riparian zones to promote significant streamflow increases from afforested catchments. There is a need for a broader body of information to assess the effects of species, density and age distribution, as well as catchment characteristics, on streamflow responses to clearing invasive trees. This study has demonstrated the suitability of the portable weir technique in capturing such information.

Keywords:

• Streamflow
• riparian zone
• Pinus patula
• Acacia mearnsii
• portable weirs