The response in water yield to the thinning of Pinus radiata, Pinus patula and Eucalyptus grandis plantations

Abstract

The paired catchment method was used to test for the effects of thinning on the water yield in three afforested catchments in South Africa, namely, Biesievlei, Jonkershoek, 98% afforested with Pinus radiata (three thinnings); Westfalia catchment D, 100% afforested with Eucalyptus grandis (two thinnings); and Cathedral Peak CII that was 74% afforested with Pinus patula (one thinning). During and after two separate thinnings, each of which removed roughly one third of the stems in a maturing P. radiata plantation in the Biesievlei catchment, annual streamflow increased by between 10 and 71% (19–99 mm). These increases persisted for three and two years after the thinning, respectively. A final thinning in the same catchment removed only 22% of stems at an age of 28 yr. The following years (1977 and 1978) were wetter than average, and reductions in annual streamflow of 26 and 55% were recorded in these two years. At Westfalia catchment D and Cathedral Peak CII, the hydrological trends were entirely dominated by the rapidly declining streamflow caused by the developing E. grandis and P. patula plantations respectively. Any savings in water use that may have resulted from the thinning of these plantations were insufficient to affect the downward trend in annual streamflow. Thinnings may have had a minor effect of delaying or reducing the desiccation of these catchments but such effects could not be assessed due to natural variability and the limited resolution of the paired catchment method. The trends in total water yield from the catchments were generally mirrored in the dry season streamflow, and there were no strong indications that thinning effects are linked to a particular season.