Securing Biodiversity in Breckland - European Commission

the status of Deschampsia flexuosa. This plant is highly palatable to rabbits, and only two plants wererecorded before the war; today it is widespread and occurs as an abundant component of grassland"(Crompton 1971). A comprehensive and detailed report of vascular plants at Foxhole Heath preparedby S.M. Walters following site visits in 1950 and 1953 (Tansley and Watt, undated) did not includeDeschampsia flexuosa; however in 1971 Eric Duffey noted a small patch of this grass “which I had notseen before during the period we were there working on the fauna” (i.e. during the mid 1950s).Within Breckland, D. flexuosa is particularly associated with sites where an organic horizon hasaccumulated overlaying the mineral sand. In contrast Festuca ovina may occasionally be found closeto D. flexuosa dominated swards, but in patches retaining mineral soil with minimal or no overlyingorganic material (e.g. on track verges with disturbance; P. Dolman pers. obs.). Thus, the increase in D.flexuosa is compatible with reduced grazing intensity, the accumulation of nutrients through reducedmanagement and with the effects of nitrogen deposition.Pitcairn, Fowler and Grace (1991) quantified change in three Breckland heathlands (Cavenham,Knettishall, Tuddenham) and concluded that reductions in the cover of Calluna vulgaris was at leastpartly attributable to increased nitrogen deposition. However, changes occurred over a period thatalso experienced greatly reduced grazing pressure.For calcareous grass-heath vegetation in Breckland (e.g. CG7), changes in composition noted byRodwell by the 1970s (in contrast to the composition described by Watt in the 1940s and 1950s)included a reduction in abundance and diversity of both bryophytes and lichens, an overall increasein grass and an increase in particular grass species that are more competitive with high levels ofnutrient availability or moisture, such as yellow oat-grass Trisetum flavescens and Yorkshire fogHolcus lanatus (Rodwell 1992). These changes are compatible with the effects of nitrogen deposition.Potential management techniques to mitigate effects of nitrogen depositionLeguminous herbs including birds-foot trefoil Lotus corniculatus and black medic Medicago lupulinacan fix large volumes of nitrogen within grassland ecosystems (e.g. 30-100 kg ha -1 yr- -1 , Marrs et al.1983), similar or greater than rates of nitrogen deposition. Wells et al. (1976) suggested thatintensive selective rabbit grazing can effectively exclude nutritious legumes from the sward and thatthis was important in reducing nutrient accumulation in lichen rich CG7 vegetation on raw chalk soilsof the Porton Ranges, Wiltshire.In addition, it is highly likely that disturbance of vegetation and root mats by scraping and burrowingby rabbits and the concentration of dung and urine in latrine areas where vegetation is killed-off,both favour leaching of volatilised nitrogen at least during winter rainfall. Thus rabbits may be key tomaintaining low nutrient status in grass-heaths. Schwinning and Parsons (1996) showed that nitrogenconcentrations in urine exceed the ability of grassland swards to utilise and capture the nitrogen,leading to losses through volatilisation and leaching. However, experimental research quantifyingnitrogen budgets and fluxes in different grass-heath management systems is lacking.Enhanced levels of grazing to remove biomass can contribute to mitigation, but grazing cannot initself prevent deleterious changes in vegetation composition with enhanced nitrogen availability. Soildisturbance treatments involving rotovating, ploughing, sub-soiling, or turf stripping and removal ofsurface organic material, are probably the most effective ways of mitigating nitrogen deposition.101