Looking to improve water quality? Plant more trees

Posted 09 May 2020

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Dr Philip Smethurst, as a soil and water scientist at the CSIRO, has been studying the effects on water quality of plantation forestry in streamside management zones on Tasmanian farms. His conclusions? If all farmers were using trees for water quality management, drastic improvements to water quality could be made for the greater good. Plantations in riparian zones improve water quality, create almost non-detectable effects on stream flows, and neither planting nor harvesting along stream sides increased contaminants. In fact, not only do plantations improve water quality, but they also decrease evapotranspiration overall for the adjacent pastures.

“Worldwide, it's recognized that water quality coming out of farmed areas, both cropping land and grazing land, presents water quality problems. A very sensible sort of solution is to keep the stock out of the stream, or manage the sediments that might be washing off the cropland soil, by having a stream side buffer, also called a stream side management zone or area.

“You need a fence to keep the stock out, as well as vegetation in that space and surface roughness to capture any overland flow that's coming off the upland areas of the farm. If you can do that, it is well known worldwide that you’ll see water quality improvements.”

Applying the research locally

Dr Smethurst explains how he set up his research experiment by looking at two different farms in Tasmania. What he discovered surprised him.

“We chose to work at low-input, extensive agriculture sites, where it would be easiest to detect an increase in contaminants due to forestry.

“Concerns were mainly around doing plantation forestry right next to streams. “Plantation forestry involves some soil disturbance at both the harvesting stage, when you actually have a plantation to harvest, and whenever you establish a new plantation. So, cultivation and harvesting are the main disturbances that present risks to stream water quality when using streamside zones of plantations. We focused on both of those phases.

“We did the plantation establishment phase on the Willowbend Farm in the south of the state with Chris White and Private Forest Tasmania. We also studied water quality during harvesting of a 20-year-old plantation that was right next to the Pet River, which is part of the Burnie water supply in the Northwest of the state.”

“But what we actually found were further improvements to water quality by using streamside management zones containing plantations, despite disturbances duringboth plantation establishment and harvesting.

“We saw no increase in sediment delivery to the stream compared to the other sorts of practices that were going on around the farm, which are most importantly roadside drainage and direct stock access. Both processes lead to water contamination.”

Water quality improvement

Drawing conclusions from his research, Dr Smethurst explains that, if all farmers were using trees for water quality management, drastic improvements to water quality could be made on-farm and for the broader community.

“Generally, it's a very good practice for improving water quality that comes out of small headwater streams that drain a farm. If every farmer in the catchment did it, or a large proportion of those farmers, you would have substantial improvements in the water quality of the larger streams that flow through farms and delivery into estuaries and seas.”

While the results might be surprising to some, Smethurst says it’s all about following the Forest Practices Code that has been set up to serve as a guideline.

“If you follow the Forest Practices Code, you'll be right. The machinery that's available and practices available these days allow you to work very close to streams without causing much disturbance at all.”

But what about water losses due to usage by pastures and trees?

For micro-climate, Smethurst also says that early findings point to more good news.

“There's good evidence developing that, even in Tasmania, plantations decrease evapotranspiration overall from adjacent pastures.

“There's light, water and nutrient competition going on close to the plantation, but further out, if the configuration of the plantation is appropriate, you'll see decreased wind speeds that lead to decreased evapotranspiration from the pastures, and better water use, water retention, and water use efficiency can result.”

Smethurst goes on to explain that the worry for water use by plantations next to streams should be of little concern.

“Some people might be concerned about water use by the plantations in the streamside management zone leading to reduced stream flows. We measured that here and found an almost non-detectable effect.”

Reducing tunnel erosion

Stream side plantations may also be useful in dealing with tunnel erosion, Dr Smethurst explains. In regions of Tasmania receiving 650 to 800 millimeters of annual rainfall, dispersive soils are common, which can cause tunnel erosion, leading to holes in the soil that both farm machinery and stock can sink into, and which therefore presents a safety issue. But stream side plantations can help.

“The science is not fully in on this because it's a very long term issue, but I've seen examples, two here in Tasmania and one in Victoria, where those farmers who did put plantations in these areas thought it was a very useful practice to make use of land affected by tunnel erosion.”

“[A plantation] stabilizes the tunnel erosion and tends to dry out the landscape, which helps ease the problem.”

Dr Philip Smethurst is a soil and water scientist at CSIRO.

His work at both plot and catchment scales improves our understandingof plantation and agricultural production, catchment water management, and codes of practice.

Using advanced research methods to gather data including real-time sensor technologies, Smethurst seeks to provide situational awareness and forecasting for practice change.

With operationally-focused research and development experience in the forest plantation sector, Dr Smethurst has led assessmentsof codes of practice for plantation forestry in all Australian states and territories.

• Smethurst P, Petrone K (2010) Streamside management zones for protection of water quality. Fact sheet for managers and policy-makers #13 Landscape Logic Commonwealth Environmental Research Facilities Hub, University of Tasmania, Sandy Bay, Tasmania, Australia
• Smethurst, P. J. (2019). Hydrological indicators of flow in headwaters for assessing farm management impacts: Streamside forestry management case study. Ecological Indicators, 98, 627-633. DOI: 10.1016/j.ecolind.2018.11.048
• Smethurst P, Neary D (2010) Farm-scale sediment sources: tree harvesting, cattle and roads. Fact sheet for managers and policy-makers #4, Landscape Logic Commonwealth Environmental Research Facilities Hub, University of Tasmania, Sandy Bay, Tasmania, Australia
• Neary DG, Smethurst PJ, Baillie BR, Petrone KC, Cotching WE, Baillie CC (2010) Does tree harvesting in streamside management zones adversely affect stream turbidity? - Preliminary observations from an Australian case study. Journal of Soils and Sediments 10:652-670. http://link.springer.com/article/10.1007%2Fs11368-010-0234-2?LI=true#page-1, DOI: 10.1007/s11368-010-0234-2
• Neary DG, Smethurst PJ, Baillie B, Petrone C (2011) Water Quality, Biodiversity and Codes of Practice in Relation to Harvesting Forest Plantations in Streamside Management Zones. CSIRO Report, 99p. (https://www.fs.fed.us/rm/pubs_other/rmrs_2011_neary_d004.pdf)
• Smethurst P, Petrone K, Neary D (2012) Understanding the effectiveness of vegetated streamside management zones for protecting water quality. In Lefroy T, Curtis A, Jakeman T, McKee J (eds)Landscape Logic: Pattern, People and Process in Landscape Management. CSIRO, Collingwood, Australia, pp. 51-67 (http://www.publish.csiro.au/pid/6769.htm)
• Smethurst PJ, Petrone KC, Langergraber G, Baillie CC, Worledge D, Nash D (2014) Nitrate dynamics in a rural headwater catchment: measurements and modelling. Hydrological Processes 28:1820–1834. DOI: 10.1002/hyp.9709 http://onlinelibrary.wiley.com/doi/10.1002/hyp.9709/pdf