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Title: Impact Of Conifer Harvesting And Replanting On Upland Water Quality
Author: B Reynolds C Neal
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_417, Representation ID: 118, Object ID: 1756
Many parts of the UK uplands have acidic and acid sensitive soils reflecting the inability of the bedrock to weather at a sufficient rate to counteract both the acidity generated within the soils,. and the impacts of acidic atmospheric pollution. These upland areas have also been a major focus for the development of conifer.plantations since the turnof the century. It is believed that conifers can enhance the capture of atmospheric acid and other pollutants, thereby increasing their impacts on soils and fresh waters, although the scale of the increase remains uncertain: Many ,upland plantation forests are now reaching. the age for harvesting and replanting and the amount of such activities will increase over the next decade. Harvesting and replanting practices may,lead to an increased acidification of stream water from: 1. Accelerated nitrate leaching from felled sites during the first 1 to 3 years post felling caused by disruption of the nutrient cycle accompanied by enhanced rates of organic matter mineralisation and nitrification. 2. Export of base cations in harvested productsand uptake in to the replanted forest leading to soil base cation depletion on the decadal to centennial timescale.. The magnitude of this particular impact on water quality is hard to gauge as there,-are poorly. quantified mechanisms which can partially. or wholly offset the acidification. These include an .. increase in base cation weathering .beneath the trees and enhanced atmospheric base cation inputs due to scavenging by the forest canopy. The environmental importance and vulnerability of upland aquatic ecosystems in the UK, the lack of detailed scientific understanding of the effects.of harvesting and the imminence and scale of harvesting and replanting operations, means that the actual impacts need to be assessedurgently. This report contains the findings of an extensive five-year research project designed to meet this need. Its-remit was ato assessthe impacts of conifer harvesting and replanting on upland stream water quality with a view to identifying best practice ameliorative management strategies and the development of a model and guidelines for environmental impact assessmenta. The project has been extensive, covering a large spectrum of detailed water quality information a_ from paired catchment manipulation experiments, a regional survey, long-term monitoring and a synthesis of the findings from other UK studies of forest harvesting impacts. The results show that with harvesting there is a disruptiorrof the biogeochemistryof the-forest ecosystem and a change in the physical characteristics of each site which results in water quality changes across a range of timescales. The initial biogeochemical response to felling may last for up to three years and is manifest by variable increases in stream water concentrations of nitrate (typically from less than one up to ten mg-NC and /l) and potassium (typically from less than one up to about two mg/l) together with, at the small scale, phosphate (typically from less than 0.02 up to-O.06 mg-P/l) and ammonium (typically from less than 0.2 up to about 0.6 mg-NHJ) for catchments with gley soils; -The physical response to felling is seen in 1) a reduction in aerosol capture leading to reduced sea salt and pollutant loads to catchments, and 2) an increase in runoff because of reduced evapotranspiration losses. The duration and -intensity of the changes are determined by the proportion of the catchment area felled, the method of harvesting, .the rapidity of re-vegetation and the soil type. R and D Technical Report P2 1I V Detailed statistical analysis reveals that the biogeochemical and physical changes associated with forest harvesting affect acidity, aluminium and base cation levels within the soil solution due to their interaction with the soil matrix. In the upper soils where aluminium is tightly bound, the interaction involves base cations and hydrogen ions. However, in the lower soils, aluminium leaching and cation exchange reactions linked to weathering of base cations are involved. Across all the scales of cat and rent monitoring, there is a balance between increased stream acidification due to nitrate generation following felling and decreased stream acidification because of reductions in strong acid anion concentrations (and possibly increased base cation concentrations due to enhanced weathering associated with carbon dioxide production). This is probably the most important finding of the project. Thus: in thevast majority of cases, when set against other temporal variations in water quality, the net acidification effect of felling is hard to discern at the catchment level. The only potential cause for concern lies with acidity and aluminium production following felling and even here only a very limited number of cases are involved. Apart from small (5-10 ha) first order catchrnents, Acid Neutralisation Capacity (ANC) changes in response to forest harvesting are usually much less than 30 pEq/l, even where the entire catchment has been felled. Thus overall, the felling responses for these and the twenty or more trace elements measured in this study do not pose a water quality problem in relation to statutory limits as, except perhaps at the very local scale, the concentrations are too low. Potentially, there are other considerations beyond the scope of this project such as chemical thresholds to protect biodiversity and conservation interests and the development of more appropriate and representative environmental diagnostics for assessing the biological impacts of acidification and land use change. Regional predictions of water quality sensitivity based on maps of soil and geological attributes are already available with regards to stream acidification and forest planting strategies. It therefore seems reasonable to use these as templates for determining vulnerable areas with regards to felling. However, an analysis of observed catchment hydrochemistry in relation to acid sensitivity predictions has identified -a large number of outlier values at both high and low stream water ANCs. This results ti-om the highly heterogeneous nature of upland catchments. In order to capture this variability and improve predictions at the appropriate catchtnent scale, an interim modification to the existing Environment Agency Wales map is recommended, whereby the sensitivity classification is extended to incorporate the areas of intermediate sensitivity containing a high proportion of the low ANC outliers. For the longer term, our firm recommendation is to use regional acid sensitivity maps based on direct measurements of water quality and biology as the current alternatives are too crude a representation at the scale of interest for assessing the potential impact of forest harvesting operations. As ANC changes in response to forest harvesting are usually much less than 30 pEq/l, even where the entire catchment has been felled, this value can be used as a precautionary threshold to separate sensitive from nonsensitive areas. As with other approaches, the threshold value should be applied relative to the minimum values observed in the stream rather than the average. With the introduction of more sophisticated GIS systems it is appropriate that harvesting impacts should be judged in relation to the proportion of a catchment felled rather than to coup size as at present. R and D Technical Report P211 vi Practical ways of minimising potential water quality impacts associated with forest harvesting are essentially contained within the Forests and Water Guidelines (FC; 4993) and other !agood practicea manuals (eig. Nisbet. et al., 1997): The following are highlighted for consideration. specifically in relation to forest harvesting: 1. Early re-vegetation of the site should be encouraged to minimise the nitrate pulse and the associated potential acidification. 2. The effects of felling should be diluted out by phasing the felling operations within a catchment. This will be effective because: a) there will be dilution by runoff from the standing crop remaining in the catchment and b) where there are areas felled two or more yearspreviously, these will contribute less acidic water than both the remaining standing crop and the recently clearfelled area. 3. Care should be taken a) to minimize the impacts of extreme events associated with high flows and b) to prevent localised forestry activities introducing intern and tent-point sources. of pollution. In these respects, we endorse the consistent application of the existing guidelines. The longer term impacts associated with,the introduction of second and subsequent generations of plantation forestry are- linked to the overall balance between (1) base cation .losses from biomass export; (2) accelerated- base cation leaching losses at harvesting ,and (3) base cation .. inputs fi-om the atmosphere,and soil/bedrock. Indeed, this aspect may well provide the critical determining factor over the sustainability of repeated forestry cycles both in terms ofnutritionalrequirements and water quality impacts. An analysis of calcium cycling within mature forest stands and a dynamic water quality modelling,study indicates that the calcium budget is finely balanced, lying within the errors of many of the measured parameters; whole tree harvestingwill deplete the base cation store to a greater extent than.stem-only harvesting. Thus, the long term: impacts of forestry cycles cannot be assessed accurately and this aspect remains a major upland environmental management-issue requiring further research. Within our study, the end.product has been straightforward, practical solutions to issues of great scientifid complexity. However, the ,benefits of good environmental management ultimately depend on forward looking co-operation between forestry interests and the aenvironmental regulators. All parties should be encouraged to combine their considerable practical expertise and experience in addressing the environmental issues related to commercial plantation forestry in the uplands. The value of this approach can be the success of this project. R and D Technical Report P211 vii R and D Technical Report P211 . .. VII1
Publisher: Environment Agency
Subject Keywords: Water quality; Rivers; Trace metals; Forestry; Acidification; Uplands; Aluminium: anc; Alkalinity; Harvesting; Replanting,- conifers
Extent: 152
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