Does tree harvesting in streamside management zones adversely affect stream turbidity? - preliminary observations from an Australian case study

Purpose: In Australia, farmers and natural resource managers are striving to enhance environmental outcomes at farm and catchment scales by planting streamside management zones (SMZs) on farms with trees and other perennial vegetation. Lack of sound information on and funding for establishing and managing trees in SMZs is hindering wide-scale adoption of this practice. Australian Codes of Forest Practice discourage or prevent harvesting of trees in SMZs of perennial streams. One concern is the potential effect of tree harvesting in SMZs on delivery of sediment to adjacent streams. The aims of this paper were to summarize the literature relevant to this concern and, in one case study in an agricultural context, to determine the effects on turbidity of harvesting a SMZ plantation.

Materials and methods: Information was sourced from published studies that reported on impacts of tree harvesting inside SMZs. In addition, a study was conducted in Tasmania, Australia, to evaluate the water quality benefits of a SMZ and the effects of tree harvesting in this zone. This case study consisted of a 20-year-old Eucalyptus nitens pulpwood plantation in a SMZ of an intermittent stream that was harvested according to the state Code of Forest Practice. A machinery exclusion zone immediately adjacent to the stream limited machinery traffic. Ground cover and water quality pre- and postharvesting were measured to identify the major sources of sediment in this headwater catchment, and to determine the effect of tree harvesting.

Results and discussion: Literature indicates that tree harvesting in SMZs is an accepted practice in the USA, New Zealand, and Germany, if conducted carefully, i.e., using best management practices (BMPs). Negative effects of this practice on water quality, in- and near-stream habitat, and biodiversity have been recorded, but these effects were generally minor or transitory. Tree harvesting in the Tasmanian study resulted in minimal mineral soil exposure and increased surface roughness. Postharvesting turbidity levels in streamflow were similar to preharvest levels (< 2.5 nephelometric turbidity units exiting the catchment). Much more significant sources of sediment were a road, a dam that was accessible to cattle, and a cultivated paddock. These sources led to turbidities of c. 300 NTUs in a dam immediately below these points and above the harvested stream reach during a storm in late June 2009. In-stream dams, installed many years earlier to store water for stock and irrigation, acted as very effective sediment traps.

Conclusions: The SMZs and other BMPs used in agroforestry landscapes are effective at protecting water quality. Forest harvesting operations can be conducted in SMZs without increasing stream turbidity, if existing BMPs are followed.