Forests and surface water eutrophication and
sedimentation
Dr Simon Harrison, Dr Stephen
Hutton, Prof. John O’Halloran, Dr
Guillaume Juhel, Mr Paul Phelan Mr
Tad Kirakowski
University College Cork
Dr Mary Kelly-Quinn, Dr Michael
Bruen, Dr Ted Farrell, Dr JanRobert Baars, Dr Robert
Cruikshanks, Dr Ronan Matson, Mr
James Johnson
University College Dublin
Introduction
Conifer afforestation of catchments is known to pose a risk to
the chemical and biological integrity of receiving waters:
- Low pH (mid and mature phase)
- High sediment loading (planting and harvesting)
- High concentrations of plant nutrients and other dissolved
and particulate substances (planting and harvesting)
- Low light and temperature (mature phase)
Although all these effects have been documented at local scales,
there has been few attempts to assess the wider scale risk of
commercial conifer afforestation to stream water quality.
The impact of a particular plantation will depend on many
factors including:
- Geology, geomorphology and soil type.
- Small scale ‘random’ factors such as the nature of a
particular planting or felling operation, rainfall patterns, water
drainage from a site.
Research question:
What is the risk posed by afforestation to receiving waters?
- Assess the relative impact of forestry on water quality, in
relation to other land uses.
- Effective mitigation measures for current and future plantations
Sampling
200+ streams draining upland catchments across the Republic
were sampled three times in 2007-8:
• Sampling was restricted to 1st & 2nd order upland streams
•‘Forest’ sites were streams that flowed through conifer
plantations
•‘Control’ sites were streams that flowed through
neighbouring unforested moorland
• Forested catchments were either mature plantations
without felling or mature plantations with felling
• Catchments were chosen in peat and podsol/lithosol
(poorly drained mineral soil) catchments
Chemical parameters: Water samples taken 3x from each stream.
Comprehensive range of parameters analysed, including plant
nutrients, heavy metals, DOC, suspended solids, pH, alkalinity.
Sediments Sediment collected using a modified Surber sampler.
Sample sieved at two fractions; coarse (250-1000μm) and fine
(50-250μm), dried and weighed.
Macroinvertebrates: 3 multi-habitat kick samples taken from
each stream on one occasion.
Diatoms: Diatoms scraped from stone surfaces taken from each
stream in summer.
Sampling Locations
101 Peat catchment streams
88 Podsolic lithosolic streams
Results – water chemistry
Mean water chemistry parameters (1)
Peat catchments
Podsol lithosol catchments
Mean water chemistry parameters (2)
Peat catchments
Podsol lithosol catchments
Mean DOC concentration in streams vs percentage
catchment felled within last 5 years
% catchment felled
Mean total ammonia concentration in streams vs
percentage catchment felled within last 5 years
% catchment felled
Mean total phosphorus in streams vs percentage
catchment felled within last 5 years
% catchment felled
Criteria for impact assessment:
Draft European Communities Environmental Objectives
(Surface Waters) Regulations 2008: these define ‘high’ and
‘good’ water quality status according to a suite of chemical
and biological parameters:
Mean MRP (mg P/l)
High status = ≤ 0.025 mg P/L
Good status = ≥ 0.025 ≤ 0.035 mg P/l
Failed status = >0.035 mg P/l
Mean Ammonia (mg N/L)
High status = ≤ 0.040 mg N/L
Good status = ≥ 0.065 ≤ 0.040 mg N/l
Failed status = > 0.065 mg P/l
Impact assessment Phosphorus (SRP)
Phosphorus water quality status of streams in data set
Peat catchments
SRP concentration
Podsol lithosol catchments
Status
Control
Mature
Clearfelling
Control
Mature
Clearfelling
>0.035 mgP/l
Failed status
0.0
0.0
14.0
0.0
0.0
0.0
0.025-0.035 mgP/l
Good status
0.0
0.0
9.3
0.0
0.0
0.0
<0.025 mgP/l
High status
100.0
100.0
76.7
100.0
100.0
100.0
Impact assessment Total Ammonia
Total Ammonia water quality status of streams for the
different catchment soil types
Peat catchments
Total Ammonia
concentration
Status
>0.065 mgN/l
Podsol lithosol catchments
Control
Mature
Clearfelling
Control
Mature
Clearfelling
Failed status
0.0
4.0
18.6
0.0
0.0
2.6
0.04-0.065 mgN/l
Good status
3.0
20.0
32.6
3.2
16.7
5.1
<0.04 mgN/l
High status
97.0
76.0
48.8
96.8
83.3
92.3
Results - Macroinvertebrates
Small Stream Risk Scores
Peat catchments
Podsol lithosol catchments
Invertebrate metrics
Peat catchments
Podsol lithosol catchments
TDI
Trophic Diatom Index
Conclusions
 Clear impacts of forestry operations on stream water chemistry
and sediment bedload.
 Elevated levels of key eutrophication and sedimentation
parameters seen in streams draining peat catchments, but not
mineral-soil podsol lithosol catchments.
 High percentage of streams draining peat catchments subject
to clearfelling fail water chemistry standards.
 Benthic macroinvertebrate and diatom metrics reflect reduced
water quality in streams draining forests planted in peatsoil
catchments.
..Don’t plant or re-plant conifer trees on peat!