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Physical and hydrological impacts of blanket bog afforestation at Bad a' Cheo, Caithness: the first 5 years
1 Midlothian, Scotland
This study was established to investigate the effects of blanket bog afforestation on the physical properties of the peat and on the quantity and timing of runoff. Three afforestation treatments, representing the options for planting on very deep blanket bog, were used: Sitka spruce (Picea sitchensis), given phosphorus (P) and potassium (K) fertilizers at planting, lodgepole pine (Pinus contorta), given no fertilizer until P was needed to relieve deficiency in year 3, and a 50:50 mixture of the two species, given P at planting. Afforestation treatments reduced annual runoff by 7 per cent compared with an unploughed, unplanted but drained control. Runoff was reduced in spring and summer but not in autumn or winter. Compared with the drained control, peak flows were increased by afforestation while the baseflow component of total flow was reduced. Sediment accumulated at a similar rate (0.016 kg dry matter per metre of drain per year) in the afforested treatments and the drained control. Afforestation lowered the water table significantly compared with the control, but there were no differences among the three treatments. Peat water content was reduced within 4 weeks by the ploughing operation that preceded planting in the afforestation treatments. In the first 5 years the undisturbed ground between plough ridges subsided by 11 (±2) cm while the tops of ridges and the bottoms of furrows subsided by 18 (±2) cm and 5 (±1) cm, respectively. Subsidence increased with proximity to a drain, from 9 (±1) cm, 10 m from the drain edge, to 16 (±2) cm, 0.1 m from the drain edge. Even in the control treatment, the perimeter drain caused a subsidence of 2 (±1) cm 10 m away, and 11 (±5) cm 0.1 m from the edge. The rate of subsidence slowed down with time and, after the third year, further subsidence only occurred in summer. Peat depth markers showed that subsidence of the ground surface was caused by consolidation of peat throughout its thickness, rather than solely by shrinkage of the drained layer. Half of the subsidence near the centre of the afforested plots was caused by compression of peat at depths of over 1.5 m. Drainage system design and maintenance needs to take account of the differential subsidence beneath planted and unplanted ground. Where drains run from planted onto unplanted ground they need to be deep enough to allow for a 0.6° reduction in gradient in the first 5 years, increasing to 1° or more subsequently. When considering pre-commercial felling to restore peatland habitats, areas of pre-thicket age first rotation forest can be considered candidates because, at this stage, the peat has not been altered to a degree which would hinder rewetting and consequent habitat development.
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  J. Holden, P. J. Chapman, and J. C. Labadz Artificial drainage of peatlands: hydrological and hydrochemical process and wetland restoration Progress in Physical Geography, March 1, 2004; 28(1): 95 - 123. [Abstract] [PDF]
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