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A letter to the Welsh Assembly on the hydrology of Newborough
In relation to the hydrology the issues at Newborough, the following evidence should be taken into account.
The Countryside Council for Wales (CCW) claim that the forest is having an adverse impact upon water levels in the warren, and for this reason, parts of the forest should be removed. What constitutes a significant impact on the warren and how this compares to other hydrological factors such as: " digging of drainage ditches north of the rock ridge, successional development of vegetation and soils leading to increased evapotranspiration, morphological change, past changes in stage state of the small lake in the Warren and the possible effects of nutrient enrichment (Stratford et. al., 2009) " is uncertain.
Long term the " humid dune slack system at Newborough is drying out (Stratford et. al., 2009)". The initial assumption was that the forest was responsible for wide scale lowering of the water table observed anecdotally (Jennings, 1990; Betson et. al., 2002), and this assertion was supported by the conclusions of an earlier debate (Kirby et. al., 1991) about the adverse effects of upland conifer forests on river flows and water supplies in Mid Wales. However, recent research has found this earlier conclusion is largely incorrect. The situation is more complex, and changes as the forest matures (Marc and Robinson, 2007).
"Much of the variation of the water levels at Newborough can be explained by decadal- and seasonal-scale patterns of rainfall (Robins et. al., 2009)" and water level records, show that few of the dune slacks had water levels which would have qualified as humid slack at the time the SAC was designated (Robins et. al., 2009; Stratford et. al. 2006, 2007, 2009). From 2004, rainfall has been above average, and humid conditions have returned to some parts of the forest and to the warren, including slacks next to the forest.
Although there has been some felling and thinning of the forest, the return to more humid slack conditions at Newborough is dependant upon the rainfall pattern, and parts of this pattern include periods of dry slack conditions. The desire to increase further the area of humid slack should be seen as aspirational and assessed alongside the natural rainfall pattern. It is unlikely to be achieved by large scale felling (Robins et. al. 2009; Stratford et. al., 2007, 2009).
Although the forest is not responsible for the long term drying of the warren, the forest does lower water tables below the forest, whilst water levels recover quickly moving into the warren (Taylor and Low, 2008; Hollingham, 2008). The annual differences, however, are negligible, because the lowering of the water table under the forest also results in a reduction in drainage under the forest (Hollingham, 2008). The greatest difference in water levels between forest and warren will be most evident under a large area of rapidly growing forest (Marc and Robinson, 2007).
In conclusion, the forest is not responsible for the long term drying out of the warren and "it is unlikely that removal of the trees would reverse this trend" (Stratford et. al., 2009)". However, the forest has some effect on the water levels underneath it; therefore the forest should be pulled back around 50-100m from the warren as a precaution to prevent any impact on the neighbouring warren.
This could be used as an opportunity to measure the effects of the forest, as recommended by the Centre of Ecology and Hydrology (CEH) hydrological review team (Stratford et. al., 2006, 2007). To assess the effects of tree age and thinning, parts of the forest should be felled and allowed to regenerate. Forestry Commission Wales should use the results to evaluate the effects of stocking rates and thinning regimes on water levels along the warren forest boundary; whilst CCW should use the results along with other research into the other significant hydrological factors present at Newborough, in formulating their management plans.
Copyright © Martin Hollingham