Assessment and mitigation of forest clearfelling impacts on salmonid receiving waters

Abstract

It was estimated that about 500,000 hectare (ha) of peatland was afforested between the 1950s and 1990s in the UK and 300,000 ha in Ireland. Many of these blanket peat forests are now reaching harvestable age and concerns have been raised about the potential release of phosphorus (P) to the receiving aquatic systems as a result of harvesting. These areas contain the headwaters of rivers, many of which contain Red List species (e.g. salmonids and freshwater pearl mussels), which make them important biodiversity refuges. Despite the fact that the sensitivity of clearfelling upland peat catchments has risen to prominence in recent years in terms of economic and conservational viability, sustainable protection methods are poorly researched and proven. The objectives of this study are to investigate the impacts of forestry clearfelling on the ecology and flow regime of receiving waters, and to assess the performance of buffer zones, phased felling, brash removal and a novel grass seeding method on ameliorating any negative clearfelling impacts. The study was based in the Burrishoole Catchment, Newport, Co. Mayo.

Hydrological, physical, chemical and biological parameters, including rainfall, stream flow rate, pH, temperature, dissolved oxygen (DO), electrical conductivities (EC), P, nitrogen (N), suspended solids (SS), macroinvertebrates and diatoms, were monitored for two years before and one year after clearfelling took place, in two sub-catchments. The results indicated that with the implementation of best management practices (BMPs), peatland forest harvesting activities could (1) have no significant impact on SS concentration in the receiving water; (2) increase catchment water yield, but not increase flood risk; (3) increase P and N concentrations in the study streams; and (4) affect the macroinvertebrate and diatom assemblages in the rivers.

Buffer zones (BZs) have been recommended internationally as a mitigation measure for tackling pollution sources and transport. However, large areas of upland blanket peat were afforested in the UK and Ireland before the importance of the riparian buffer areas was realised. In order to reduce the possible negative impact of harvesting activities on receiving waterbodies, the creation of BZs along receiving water courses prior to the clear-felling of the main plantation has been proposed. In this study, a small BZ, with the effective area of about 0.1 ha, was established and seeded with native grass species, onto which runoff from an upstream forest, with an area of about 10 ha, was spread. One year later, the upstream forest was harvested. The results indicated that the BZ removed 45.3 % of SS, 33.7 % of TON and 17.6 % of total reactive phosphorus (TRP), respectively, in the first year of harvesting.

To reduce nutrient leaching from forest catchments to receiving water, a novel practice - grass seeding clearfelled areas immediately after harvesting - was proposed in this study. It was hypothesised that if the vegetation could quickly recover after forest harvesting, the nutrients would be retained in situ through vegetation uptake. A field trial was conducted to identify the successful native grass species that could grow quickly in the recently clearfelled blanket peat forest. The two successful grass species, Holcus lanatus and Agrostis capillaris, were sown in three harvested blanket peat forest study plots with areas of 100 m2, 360 m2 and 660 m2 immediately after harvesting. Areas without grass seeding were used as controls. One year later, the P contents in the above ground vegetation biomass of the three respective study plots were 2.83 kg P ha-1, 0.65 kg P ha-1 and 3.07 kg P ha-1. These values were significantly higher than the value of 0.02 kg P ha-1 observed in the control plots. The average concentrations of water extractable phosphorus (WEP) in the three study plots were 8.44 mg (kg dry soil)-1, 9.83 mg (kg dry soil)-1 and 6.04 mg (kg dry soil)-1, respectively, which were lower than the value of 25.72 mg (kg dry soil) -1 in the control sites. These results indicate that grass seeding of the peatland immediately after harvesting can quickly immobilise significant amounts of P and warrants additional research as a new BMP following harvesting in the blanket peatland forest to mitigate P release.

To further examine the grass seeding practice, experimental plots with defined boundary conditions were established. In addition, other mitigation approaches, such as whole tree harvesting, were also tested using these plots. Three sets of five treatments were compared as follows: (1) no brash and no seeded grass; (2) brash without seeded grass; (3) brash with seeded grass; (4) seeded grass only and (5) brash mats. The results indicated that (1) the brash mat was a significant source of nutrient release; (2) whole tree harvesting could significantly reduce nutrient release, and (3) grass seeding could be a sustainable practice for nutrient release control after forest harvesting.