Scottish Natural Heritage Commissioned Report No. 925 A review of the influence of ombrotrophic peat depth on the successful restoration of bog habitat COMMISSIONED REPORT Commissioned Report No. 925 A review of the influence of ombrotrophic peat depth on the successful restoration of bog habitat For further information on this report please contact: Malcolm Fraser Scottish Natural Heritage Silvan House 3rd Floor East 231 Corstorphine Road EDINBURGH EH12 7AT Telephone: 0131 3162629 E-mail: [email protected] This report should be quoted as: Lindsay, R.A. & Clough, J. 2016. A review of the influence of ombrotrophic peat depth on the successful restoration of bog habitat. Scottish Natural Heritage Commissioned Report No. 925. This report, or any part of it, should not be reproduced without the permission of Scottish Natural Heritage. This permission will not be withheld unreasonably. The views expressed by the author(s) of this report should not be taken as the views and policies of Scottish Natural Heritage. © Scottish Natural Heritage Year 2016. COMMISSIONED REPORT Summary A review of the influence of ombrotrophic peat depth on the successful restoration of bog habitat Commissioned Report No. 925 Project No: 15222 Contractor: Richard Lindsay and Jack Clough, University of East London Year of publication: 2016 Keywords Peat; depth; extraction; consent; minimum; residual; bog; ombrotrophic; restoration. Background A number of raised bogs in Scotland currently have planning consent for commercial peat extraction. All of these consents are now subject to periodic review through the Renewal of Mineral Permissions (“ROMP”) process. Whereas in earlier times such consents imposed relatively few conditions in relation to after-use, expectations are now increasingly focused on restoration of the original raised bog habitat and conditions are thus being imposed accordingly, both on consents subject to ROMP and also on any new consents which may be granted. Bog peat is formed when the living, peat-forming surface is no longer able to draw on the underlying mineral sub-soil, or on the mineral-enriched groundwater table, and is thus wholly dependent upon direct precipitation inputs for its supply of water and solutes – in technical terms it becomes ‘ombrotrophic’. If a peatland site is to be restored to ombrotrophic bog following commercial peat extraction, a residual layer of peat must remain in order to form a barrier between the mineral-enriched conditions of the sub-soil with its associated water table, and the peat-forming vegetation newly-established on the bare peat surface after extraction ceases. In some cases the condition imposed in relation to this residual peat layer has been to require retention of ‘an average minimum peat depth of 0.5 m’. Both the origins and likely efficacy of this condition do not appear to have been subject to scrutiny at any point. This report seeks to assess the concept of ‘an average minimum peat depth of 0.5 m’ as a requirement for restoration of ombrotrophic bog vegetation on commercially cut- over bog systems, based on available published material. Main findings There appears to be no published scientific literature, nor any official guidance, which recommends use of an ‘average minimum residual peat depth of 0.5 m’ for restoration of bog habitat; Typically, lowland raised bogs tend to have basal layers of fen peat which are, on average, a little under 2 m deep and thus if the residual peat thickness is less than 2 m there is a strong possibility that any restoration efforts will necessarily begin on a mineral- enriched fen-peat layer; i The vast majority of literature concerned with residual peat depths for peatland (not necessarily bog) restoration has in the past emphasised the need for a minimum residual peat thickness of 0.5 m provided the peat is strongly humified (at least H7 on the von Post scale); There has also been consensus in the past that neither the extraction process itself, nor the drainage system, should cut into the mineral sub-soil; If the peat is not so strongly humified, it has been recommended in the past that the residual peat layer should be at least 1 m deep; Evidence from sub-peat layers of raised bogs in the UK indicates that many sites have somewhat variable sub-peat deposits and thus it is not reasonable to proceed on the basis that a peat bog is underlain by impermeable deposits unless a detailed survey of sub-peat deposits has been undertaken; Strongly humified peat shrinks and cracks more readily than less humified peat and thus even where a minimum layer of 0.5 m of strongly humified has been recommended in the past, it has also been recommended that this peat layer should be covered with at least a 20-30 cm layer of ‘top-spit’ material to minimise the possibility of drying out and cracking; Shrinkage due to drying during the last stages of peat extraction and prior to the water table being raised across the site as part of the restoration programme will mean that a residual peat depth of 0.5 m at cessation of extraction will be less than this by the time the restoration programme is established; Although a great many restoration schemes have been undertaken in the UK, Northern Germany and Canada, none of these has yet established an ombrotrophic bog vegetation, the dominant vegetation generally being a ‘poor-fen’ type which is a pre- cursor for establishment of bog vegetation; All these restoration programmes have been undertaken on areas where the residual peat thickness has generally been somewhat less than 2 m deep and thus are likely to be influenced to greater or lesser degrees by proximity to, or establishment in, fen peat deposits, enhanced still further by any water which irrigates the surface vegetation having been in contact with the mineral sub-soil through cracks in the peat; Under natural conditions, such poor-fen vegetation can persist for 200-300 years, suggesting that the restoration programmes which are currently dominated by poor-fen vegetation may remain in this state for a considerable period into the future; A major review of spontaneous re-vegetation recovery on milled peat sites in Estonia indicates that a minimum residual peat thickness of 2.3 m is required if ombrotrophic bog vegetation is to establish successfully without a poor-fen phase; Experimental work based on the Estonian recommendation of 2.3 m for the residual peat depth indicates that on a former milled site with a residual peat depth of 2.5 m it is possible to re-establish ombrotrophic bog habitat directly. For further information on this project contact: Malcolm Fraser, Scottish Natural Heritage, Silvan House, 3rd Floor East, 231 Corstorphine Road Edinburgh, EH12 7AT. Tel: 0131 3162629 or [email protected] For further information on the SNH Research and Technical Support Programme contact: Knowledge and Information Unit, Scottish Natural Heritage, Great Glen House, Inverness, IV3 8NW. Tel: 01463 725000 or [email protected] ii Table of Contents Page 1. INTRODUCTION 1 1.1 Background 1 1.1.1 Planning context 1 1.1.2 The ecological basis of a ‘minimum peat depth’ 2 1.2 Scope of the Review 6 1.3 Objective of the Review 7 1.3.1 Primary question 7 1.3.2 Sub-questions 7 1.3.3 Key definitions 7 2. METHODS 8 2.1 General principles 8 2.2 Evidence search 8 2.2.1 Search strategy 8 2.2.2 Search terms used 8 2.2.3 Databases and search engines used 9 2.2.4 Study inclusion/exclusion criteria 9 2.2.5 Selection of references for assessment 10 2.3 Critical appraisal 10 2.3.1 Categorisation of publications 10 2.3.2 Evaluation of publications 10 2.3.3 Synthesis of publications 11 2.4 Integration of results 11 3. RESULTS 12 3.1 Scope of the evidence 12 3.2 Results of the searches and selection of publications for detailed review 13 3.3 Sub-Question 1: What is the typical thickness of the fen-peat layer in a lowland raised bog? 14 3.3.1 Background 14 3.3.2 Categorisation and assessment 14 3.3.3 Synthesis 14 3.4 Sub-Question 2: Does residual peat depth influence surface-water chemistry and bog restoration? 16 3.4.1 Background 16 3.4.2 Categorisation and assessment 18 3.4.3 Synthesis 20 3.5 Sub-Question 3: Does residual peat depth influence the hydrology of bog restoration? 23 3.5.1 Background 23 3.5.2 Categorisation and assessment 26 3.5.3 Synthesis 35 3.6 Sub-Question 4: Does residual peat depth influence the vegetation achieved during bog restoration? 38 3.6.1 Background 38 3.6.2 Categorisation and assessment 39 3.6.3 Synthesis 46 3.7 Sub-Question 5: What is the timescale of transition from poor-fen conditions to ombrotrophic bog in natural or managed peatland succession? 47 3.7.1 Background 47 3.7.2 Categorisation and assessment 47 3.7.3 Synthesis 49 iii 3.8 Sub-Question 6: What are the potential effects on a bog restoration programme of using a residual peat depth having an average minimum depth of 0.5 m? 50 3.8.1 Background 50 3.8.2 Categorisation and assessment 50 3.8.3 Synthesis 53 4. DISCUSSION 54 4.1 The source of the ‘average minimum depth of 0.5 m of peat’ 54 4.1.1 Early guidance about residual peat depth 54 4.1.2 Later UK guidance about residual peat depth 54 4.1.3 Recent international recommendations 55 4.2 Justification for the ‘average minimum depth of 0.5 m of peat’ 56 5.