Effects of broadleaf woodland cover on streamwater chemistry and risk assessments of streamwater acidification in acid-sensitive catchments in the UK
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Date
2007Author
Gagkas, Zisis
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Abstract
Acidification of surface waters has been recognised as the major water quality
problem in the UK uplands. The adverse effects of conifer afforestation on
streamwater chemistry and ecology are well documented in acid-sensitive
catchments and have mainly been attributed to the enhanced deposition of
atmospheric pollutants onto conifer canopies (the “scavenging effect”). Currently,
international and national policies promote the expansion of native broadleaf
woodland in the UK. Pollutant deposition onto broadleaf canopies is considered less
than onto the more aerodynamically rough conifers, but there is concern that largescale
broadleaf planting could delay the recovery of acidified waters or lead to
further acidification in most sensitive areas. However, there has been limited
investigation of the influence of broadleaf woodland cover on streamwater chemistry
in the UK.
To investigate the effect of woodland cover 14 catchments with different (0-78%)
percentages of broadleaf woodland cover were identified in representative acidsensitive
areas in north-western and central Scotland (Glen Arnisdale and Loch
Katrine area) and northern and south-western England (Ullswater area and Devon)
using spatial datasets in a GIS. Streamwater was sampled at high flow from the
catchment outlets in winter and spring 2005 and 2006 and was analysed for major
cations, anions and trace metals using standard methods. The number of samples
ranged from two in the Glen Arnisdale catchments to 10 in the Loch Katrine area
catchments which were sampled more intensively.
Significant positive correlations were found between percentage broadleaf woodland
cover and streamwater NO3 (rs = 0.51) and soluble Al (rs = 0.64) concentrations. The
greater NO3 leaching to streamwater in the three most forested catchments (> 50%)
was probably due to enhanced N deposition onto woodland canopies and nitrification
by alder in the Ullswater area forested catchments. Streamwater NO3 concentrations
equalled or exceeded non-marine SO4 in the above catchments indicating that NO3
was the principal excess acidifying ion in catchments with greater woodland cover.The woodland effect on streamwater chemistry in the study catchments was masked
to some extent by variability in acid deposition climate and soil type composition.
Seasalt inputs were found to be a more important control than woodland cover for
streamwater chemistry in the maritime Glen Arnisdale catchments.
A risk assessment of acid-sensitivity in the study catchments was conducted by
calculating streamwater critical load exceedances using the Steady-State Water
Chemistry (SSWC) and First-order Acidity Balance (FAB) models and modelled
pollutant deposition for 1995-97 and 2002. Critical loads were exceeded by 0.01 to
1.74 keq H ha-1 yr-1 in two catchments which had woodland covers > 50% and in the
Devon control catchment. The remaining 11 study catchments were assessed to be
not at risk of acidification, probably due to significantly reduced non-marine S
deposition from 1986 to 2001, but seasalt inputs to the Glen Arnisdale catchments
might cause acidic streamwater episodes. Acid-sensitivity was also assessed using
macroinvertebrates sampled in 11 of the study catchments and the results generally
agreed with the critical load assessments. More detailed estimates of the
enhancement of dry S and N deposition onto birchwoods in the Loch Katrine area
catchments using calculated roughness length within FRAME showed that it posed
no risk for streamwater acidification in these catchments because of the high rainfall
environment. However, in acid-sensitive areas of the UK with lower rainfall and
closer to major pollution sources, enhanced pollutant scavenging by broadleaf
woodland canopies could pose a greater risk of acidification to freshwaters.
The finding that almost all study catchments with woodland covers less than 30% are
well protected from acidification suggests that this is a sensible threshold value for
use in risk assessments of the effects of broadleaf woodland planting conducted
within the Forests and Water Guidelines. The results of a sensitivity analysis of the
Guidelines’ methodology, conducted using parameters such as numbers and timing
of streamwater sampling, different runoff estimates and critical acid neutralising
capacity values, showed that the Guidelines should be able to protect sensitive
freshwaters from acidification in areas where broadleaf woodland is expanding.