Black Firs & Cranberry Bog SSSI - BLACK FIRS WOODLAND (008)
Staff member responsible: PAUL CANDLIN
Unit Id: 1022697
Unit area (ha): 4.4918
Unit Status: Live Gridref: SJ 747 501
Condition (click for history): Unfavourable - No change Assessed by: HOLLAND, (TOM)
Last assessed: 16/03/2018 Last field visit: 17/07/2017
ISA Survey: View Surveys
Date of site check: 17/07/2017 Last CSM assessment: 16/03/2018

Basin fen –

  • Scored against the latest (2017) FCTs, the Cranberry Bog basin fen (Schwingmoor and its lagg within units 5, 6 and 7) fails to hit i) habitat extent targets; ii) scrub cover targets and iii) bryophyte composition targets. It would therefore be assessed as unfavourable. This is despite this being a really nice bit of habitat and apparently in similar or better condition to that present at notification.
  • As I understand it the fundamental reason for adverse condition seems to be hydrological. The basin fen is drained and suffers from eutrophication. Enriched water was (and probably still is) brought into the basin from agricultural and domestic sources from the surrounding surface water catchment. (Drainage and lowered water levels means the habitat occupies a smaller area than it would if a more natural hydrological regime was restored – hence its failure to hit the extent targets.)
  • Ironically the perimeter ditch (that surrounds and partially drains the schwingmoor and cuts through the lagg), might be protecting the remnant schwingmoor and some of remnant lagg from eutrophication (by intercepting the catchment’s surface water run-off).
  • There is a suspicion that water levels in the basin have risen in recent years (probably because of a collapse in the ditch that is partially blocking the Cranberry Bog outfall – stop 23 on annotated aerial photo below, along with other parts of the ditch network becoming choked with vegetation). This seems to have allowed some recovery of eutrophic wetland habitats to the peaty lagg surrounding the perimeter ditch (both inside and outside of the SSSI). Most of this habitat is of moderate quality and eutrophic – for example MG10 rush pasture on both the western side of the basin (stop 21 and 22) and the south-eastern sides (stop 27 and 28) and a recently formed area of open-water at SJ750501. However it does contain some interest – cowbane spreading out of the ditch around stop 26 and waders and wildfowl on the newly formed pool (including 40 lapwing, two oystercatchers and 20 greylags).
  • As yet, the raised waters levels don’t seem to have had an adverse eutrophication impact on the schwingmoor and lagg habitats inside the perimeter ditch. Although Phalaris arundinacea (an indicator or enrichment/eutrophication) is a frequent dominant choking the perimeter ditch, it and other eutraphents, such as Juncus effusus, are only scarce inside the perimeter ditch, within the lagg woodland, and they are almost absent from the open schwingmoor. It is suspected that the schwingmoor floats above a pool of, what might be eutrophic water, and is mainly or entirely rain-fed (ombrotrophic). (The pool below the schwingmoor is thought to be partially groundwater-fed.
  • We recorded Carex rostrata on the open schwingmoor, occurring as a 5-10 wide border between the lagg woodland and the centre of the schwingmoor. This gives the schwingmoor’s vegetation within this border some similarity to M4 Carex-Sphagnum mire. MATCH suggests a 40% similarity to M4 for the vegetation in stops 2, 7, 10 and 13. This suggests this part of the schwingmoor is supplied, in part, by minerotrophic water, and is not entirely ombrotrophic. I don’t think a M4 Carex rostrata fringe to the schwingmoor has been recorded by previous surveyors (e.g. Walker in 2001 or Wheeler in 199?). If this is a new development does it suggest that minerotrophic water has become a more important component of the schwingmoor’s water supply – that it is has become less ombrotrophic? If so, why? Is it another consequence of raised water levels? If so does it suggest the minerotrophic component is reasonably clean (otherwise we might expect Typha latifolia rather than Carex rotrata)? If it is reasonably clean does it suggest it is sourced from groundwater rather than surface water (or is the surface water supply now cleaner than it was, as a consequence of the HLS agreement)? I think the catchment is still receiving diffuse water pollution from both agriculture and septic tanks, so I would be surprised if the surface water catchment was clean enough. We can see how the M4 develops – whether it spreads or remains stable. Perhaps it has been revealed and spread as a results of scrub management (and has nothing to do with changes in hydrology). We’ve got GPS grid references for its present limits (SJ74915 50433 on the northern edge and SJ74927 50401 on the southern edge) and SWT are going to do some transects to monitor vegetation change across the schwingmoor.
  • The vegetation of the ombrotrophic part of the tiny schwingmoor is very good. Although it fails the Sphagnum target (because it does not contain two peat-building species at the required frequency), it does seem to have a high cover and frequency of Sphagnum capillifolium as well as Sphagnum recurvum, as well as frequent Drosera and Vaccinium oxycoccus. A comparison is probably unfair and pointless, because of the size differences, but in some ways Cranberry bog looks in better condition than Chartley Moss, which has lower covers and frequencies of peat building Sphagna and higher covers of Aulacomnium palustre, (indicative of drier conditions).

Dystrophic pool –

  • It fails because of its lack of any characteristic vegetation zones. It might also fail to hit the water quality targets, but we don’t have any recent data to make an assessment. I think there is was an agreement for EA to take water samples, but access is difficult and it fell through. It’s a pity the schwingmoor doesn’t spread across the pool’s surface. I think I have read that the pool is too deep for succession to occur. Could it be encouraged artificially? Aerial photos show no movement between 2005 and 2015.

Wet woodland –

The wet woodland on the lagg of Cranberry Bog seems ok. It is perhaps a bit too similar to W6 (suggesting some eutrophication), but negative indicators (e.g. nutrient-loving and dry-loving species) are scarce, except in the perimeter ditch (where Phalaris is often a dominant).

The wet woodland and wetland habitat in Black Firs is almost non-existent. It is limited to the ditch banks and bottoms. Most of the wood is closer to W10 oak wood than any wetland habitat. As such it fails its ground flora, composition and extent targets

Number of adverse condition reasons: 1
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