Science Spin March 2009 : Sponsored Content - COFORD

Quantification of erosion and phoshorous release from peat soil forest catchment

A study undertaken by NUI Galway, the Marine Institute Newport, and Coillte, and funded by COFORD, is now providing new insights into the nature of these releases and their duration and verifying the efficacy of the current best practice advocated by the Forest Service for harvesting in these areas.

Wet soils and high exposure require that most of the western plantations are managed on a clearfelling and replanting silvicultural system. This requires that the plantations are harvested in blocks when trees reach a marketable size. Harvesting removes the merchantable part of the tree but leaves branches and foliage behind. It is in the latter that most of the nutrients in the tree are located and as the foliage and fine branches decay, nutrients are released which can be flushed into nearby watercourses, if not absorbed by vegetation.

The effect of nutrient enrichment, if not controlled, can have serious impacts on salmonids (trout and salmon) and also on Ireland's longest living creature, the freshwater pearl mussel. These effects are particularly serious in the upper reaches of nutrient poor catchments where biological activity is usually phosphorus limited. In these cases nutrient enrichment can cause excessive algal growths, resulting in dissolved oxygen fluctuations, and disruption of the ecosystem.

Similarly ground disturbing operations associated with harvesting such as road building and timber extraction can lead to the release of silt in the form of peat or fine mineral soil particles.

Silt can have an adverse effect on aquatic flora and fauna. Light penetration can be reduced affecting overall productivity, fish feeding and migration. Fine sediment can affect fish respiration and also damage spawning areas by physically covering and blocking spawning gravels.

The forest industry is currently working to operational guidelines, produced by the Forest Service, to ensure that harvesting impacts are avoided. These guidelines were drawn up in 2000 and were based on what was perceived at the time as best practice, and while supported by scientific research, the effect of their implementation had not been tested.

Study

To gain a deeper understanding of the impacts of harvesting operations and to test the efficacy of the Forest Service guidelines, a study was carried out by NUI Galway, the Marine Institute and Coillte in the Burrishoole catchment, Co Mayo. The study site ( 20 ha area) was located within a lodgepole pine plantation, established in 1971, on blanket peat that receives an annual rainfall in excess of 2000 mm (Figure 1). The site is drained by a first order stream that receives the flows from the ploughed furrows, mainly via collector drains and discharges directly to the salmonid Shrahrevagh River.

The study site was divided in two parts: the downstream part of 10.5 hectares was clearfelled in early autumn 2005 and the upstream part of 7.2 hectares was left intact. Two instrumented stations (Figure 2) were established - one just upstream (US) and the other just downstream (DS) of the area that was clearfelled. The instrumentation at each station included a flume, water-level recorder, rain gauge, automatic water sampler and a multi-probe (Figure 3) for measuring physico-chemical water variables.

The rainfall flows and water quality variables at the two stations were recorded every five minutes for about 1 year pre-clearfelling, during clearfelling and harvesting, and for 19 months post-clearfelling. During flood events, water samples were taken hourly at the US and DS stations for suspended sediment (SS) and phosphorus (P) analysis. The study site was clearfelled by Coillte and their contractors in accordance with the Forest Service Guidelines, with no on-site work being carried out during wet weather.

The SS and phosphorus concentrations during the study period indicate that the effects of clearfelling and harvesting are longer lasting with regard to phosphorous release than suspended sediment release. However, in this catchment, the dilution ratio available in the Shrahrevagh River of about 24 - based on the ratio of the area of the upstream river catchment to the clearfelled area - maintained the average P concentrations in the water downstream of the confluence of the river and study stream at less than 10µg TRP/l - an acceptable concentration. This indicates that knowledge of areas, flows and nutrient concentrations at strategic locations in a catchment could provide a rational and quantitative basis for determining the optimum size of clearfelling areas to have minimal effects on receiving waters.

In summary, the Forest Service Guidelines provide good guidance for sustainable forestry. The results demonstrate that the current guidelines and measures implemented in the salmonid Burrishoole catchment research study were effective in protecting water quality.

In addition to the guidelines other measures are being taken that will provide lasting protection for watercourses in the future. As old plantations are being felled they are being redesigned to incorporate unplanted buffer strips, native riparian woodland and open space that will develop into permanent riparian zones and will be managed for both water protection and biodiversity enhancement.

[Authors: Michael Rodgers and Liwen Xiao]

ACKNOWLEDGEMENTS

The authors gratefully acknowledge COFORD, EPA, NPWS and Coillte for funding the project.

(The full report of the work can be accessed by clicking here)

For further information, contact:
Dr Michael Rodgers
Senior Lecturer in Civil Engineering
National University of Ireland, Galway
email. michael.rodgers@nuigalway.ie