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Publication - Consultation paper

Beavers in Scotland: consultation on the strategic environmental assessment

Published: 12 Dec 2017

A consultation on the policy to reintroduce beavers to Scotland and the strategic environmental assessment of this policy.

Contents
Beavers in Scotland: consultation on the strategic environmental assessment
4.4 Beavers and Terrestrial vascular plants

4.4 Beavers and Terrestrial vascular plants

4.4.1 How beaver activity affects terrestrial vascular plants

There are two main mechanisms through which beavers affect vascular plants: directly by being eaten and indirectly through successional habitat change (tree- felling, changes in water levels and changes in wave action). Habitat change is specifically addressed in this report in sections 4.2 impacts upon tree species and sections 4.3/ 4.4 impacts upon freshwater plant species.

Compared with the information available on indirect impacts caused by habitat change, there is relatively little information on direct impacts by beavers on vascular plants. Despite 60-80% of the North American beaver diet being reported as aquatic vegetation, much of the literature on beaver impacts on vascular plants is in connection with tree species. At Knapdale, it was noted that the proportion of the beaver diet comprising plants other than trees is unknown, but is likely to be higher during the summer due to greater availability and nutritional quality of plant material.

4.4.1.1 The terrestrial vascular plants at greatest risk from direct impacts will tend to be species which occur in habitats close to waterbodies and watercourses.

In Norway, Eurasian beavers have been found to be strongly associated with deciduous trees. It has been shown that the abundance of deciduous trees within 40 m of the river bank was a key determinant of beaver presence (or absence) in Norway. Vascular plant species associated with woody shrubs and trees are therefore available for beavers to eat.

The importance of terrestrial open land for foraging is not clear. Land outside woodland has been recorded as part of the territory of Eurasian beavers in both the Netherlands and Norway. Activity is generally constrained to within 50 m of a watercourse, with the majority much closer. In the Netherlands Eurasian beavers were found to forage mainly within 6 m of the water's edge. Vascular plants in open areas are therefore potentially available for beavers to eat, but foraging might be predicted to be within a few metres of the water's edge.

4.4.1.2 The proportion of non-woody plants in beavers' diets varies according to the habitat in which the beavers live and the time of year.

Beavers have been considered to be opportunistic feeders, eating what is available. However, they do appear to be selective as regards their diet. One study found that Eurasian beavers mainly ate woody food in all seasons. Bark and a small amount of roots of monocotyledonous plants were eaten in the winter. In the spring, woody food was eaten with a few herbs and roots. The summer diet was similar to the spring diet, but with more bark. The conclusion was that beavers select food according to the nutrients it provides. Where nutrients are lacking, beavers may target certain plant species in order to obtain sufficient quantities of essential nutrients. Yellow water lily Nuphar lutea, a relatively scarce plant in Scotland and eaten by beavers, is rich in sodium and phosphorus. In the Netherlands the large size of Eurasian beaver territories may be because beavers require sufficient sources of minerals during gestation.

Plant defence mechanisms are also important and might explain why captive North American beavers have been recorded eating more North American white water lily Nuphar odorata than expected. Plant defences might also explain why, at some locations, beavers avoid non- woody plants. Therefore, beavers will tend to feed on both woody and non-woody plants, targeting those species which are most nutritious and avoiding species with natural defences.

4.4.1.3 Habitat change influenced by beavers is a consequence of increased water inundation and herbivory.

Flooding has significant impacts upon riparian vegetation as terrestrial habitat is converted to aquatic, lentic habitat. Initially, flooding will kill many tree species that become submerged. However, the shallow edges, characteristic of beaver ponds, encourage emergent vegetation. The hydrological gradient associated with the edge of beaver ponds increases vascular plant diversity and provides habitat characterised by saturated soils with an open canopy.

Plant biodiversity within beaver meadows is no greater than adjacent riparian communities. However, the community composition of these meadows is fundamentally different from other riparian ecosystems. Hence, the presence of beavers results in an increase in habitat heterogeneity, which may ultimately increase herbaceous plant species richness. One North American study recorded species richness increasing by 33% in the riparian zone at the landscape scale as a result of beaver activity.

A summary (see Table 4.4.1) of the potential interactions between beavers and terrestrial vascular plants is presented below; where possible these have been attributed to a neutral, positive or negative effect.

Table 4.4.1: Summary of potential interactions between beavers and terrestrial vascular plants.

Activity

Mechanism

Positive effects

Negative effects

Notes

Felling

Change in riparian woodland: Opening of woodland canopy and increased patchiness

  • Potential overall increased diversity at landscape scale due to increase in habitat heterogeneity
  • Increased localised diversity of species associated with an open canopy, e.g. grassland species
  • Theoretical localised decrease in or loss of species which require lower light levels

Very little information regarding species impacts. See Annex 1 Table 4.2.1 for effects on woody species

Felling

Change in riparian woodland: Change in relative abundance of different tree species

  • Increased localised diversity of species associated with an open canopy, e.g. grassland species
  • Theoretical localised decrease in or loss of species which require lower light levels

Very little information on species impacts. See Annex 1 Table 4.2.1 for effects on woody species

Feeding

Feeding on specific terrestrial herbaceous and aquatic plant species

  • Potential localised decrease in or loss of palatable species

Direct impacts recorded for a very small number of species. Some species on the Scottish Biodiversity List could be adversely affected at local levels. See Annex 1 Table 3.7 for effects on aquatic species

Dams/pond creation

Change from lotic to lentic habitat

  • Potential localised decrease in or loss of riparian species, although opportunities for new riparian edge to be colonised

Indirect loss through water inundation not recorded, but theoretical. Loss might be balanced by displacement. See Annex 1 Table 3.7 for effects on aquatic species

Dams/pond creation

Change in hydrological processes on riparian and downstream habitat

  • Species of wetland habitats likely to benefit at local levels
  • Species which may be sensitive to wetter conditions may decrease or be lost at local levels

This might be positive/negative or neutral, depending on the area and species concerned

Dams/pond creation

Longer term successional changes after dam abandonment, e.g. beaver meadows

  • Increased diversity of species associated with increased habitat heterogeneity

Indirect habitat creation/restoration initiatives as result of beaver presence

Beaver used to promote opportunities for riparian and freshwater habitat creation/restoration

  • Any riparian woodland and/or wetland restoration programme is likely to benefit many flowering plant species in the medium to long term. There will be increased diversity of species associated with increased habitat heterogeneity

4.4.2 Distribution of terrestrial vascular plants in the beaver policy area

The following section concentrates on those terrestrial vascular plant species of conservation importance that are likely to overlap with core beaver woodland and as such may be positively or negatively affected by beaver activity.

4.4.2.1 Terrestrial vascular plant species of conservation importance

To determine whether the activity of beavers on (native) terrestrial vascular plant species is significant in the context of this Strategic Environmental Assessment, the assessment of impacts (positive and negative) has focussed on those species for which beaver activity may affect directly or indirectly (as discussed above), which are considered as having conservation importance and as such are afforded national protection wherever they occur. N.B unlike other receptors discussed in this SEA, there are no species of European importance.

Table 4.4.2 below therefore identifies those terrestrial vascular plant species of conservation importance that utilise 'potential core beaver woodland' (as described in section 4.1 of this report) and are found within the beaver policy areas. Most of the sites have a vascular plant assemblage. Only those species within the assemblage that overlap with beaver core habitat have been screened in. Some of the sites are also notified for a single individual vascular plant species. The assessment in section 4.4.3 deals with each individual vascular plant species in turn.

Table 4.4.2: Summary of terrestrial vascular plant species of conservation importance within the beaver policy area that overlap with potential core beaver woodland

SSSI

SSSI feature

Species that overlaps with beaver core woodland

Craighall Gorge SSSI

Vascular plant assemblage

Lesser hairy brome Bromopsis benekenii
Shady horsetail Equisetum pratense
Whorled Solomon's-seal Polygonatum verticillatum

Den of Airlie SSSI

Whorled Solomon's-seal Polygonatum verticillatum

Whorled Solomon's-seal Polygonatum verticillatum

Den of Riechip SSSI

Whorled Solomon's-seal Polygonatum verticillatum

Whorled Solomon's-seal Polygonatum verticillatum

Eastern Cairngorms SSSI

Vascular plant assemblage

Twinflower Linnaea borealis

Hare Myre, Monk Myre and Stormont Loch SSSI

Vascular plant assemblage

Twinflower Linnaea borealis
Creeping Lady's-tresses Goodera repens

Keltneyburn SSSI

Vascular plant assemblage

Shady horsetail Equisetum pratense
Lesser hairy brome Bromopsis benekenii
Small cow-wheat Melampyrum sylvaticum
Whorled Solomon's-seal Polygonatum verticillatum
Wintergreen (Orthilia secunda)

Milton Wood SSSI

Whorled Solomon's-seal Polygonatum verticillatum

Whorled Solomon's-seal Polygonatum verticillatum

Rescobie and Balgavies Lochs SSSI

Vascular plant assemblage

Coralroot Orchid Corallorhiza trifida

Romadie Woods SSSI

Whorled Solomon's-seal Polygonatum verticillatum

Whorled Solomon's-seal Polygonatum verticillatum

Tulach Hill SSSI

Vascular plant assemblage

Shady horsetail Equisetum pratense

4.4.3 Assessment of likely effects on terrestrial vascular plant species of conservation importance in the beaver policy area

Each of the vascular plant species identified in Table 4.2.2 above are discussed in turn below in the context of those effects (positive or negative) that have been identified as a result of beaver activity. Assessment of has been made in the context knowledge of the species ecology as well as the individual sites and their condition. Where mitigation or monitoring maybe appropriate, this has been identified in the narrative. Further discussion relating to the management of beavers including mitigation and monitoring options is provided in section 5 and 7 respectively.

For species and habitats of conservation interest in the wider countryside there will be an ongoing need to assess data derived from general surveillance and monitoring activities that are already in place, and intervene with management if and when necessary. This will be informed by a more strategic approach to management being developed in due course.

Beaver opportunities

As summarised above, beaver activity has the potential to create many positive effects. More than that, any riparian woodland and/or wetland restoration programme is likely to benefit many flowering plant species in the medium to long term. There will be increased diversity of species associated with increased habitat heterogeneity.

4.4.3.1 Consideration of potential positive effects on terrestrial vascular plant species of conservation importance

The impact of beaver activity on those vascular plant species discussed below is considered to have a positive or neutral effect. A summary is provided outlining positive effects in general terms, following by individual species assessment at the end of this section.

Some terrestrial plant species might be expected to benefit in riparian habitat, whilst shade-loving species might decline. Terrestrial species which are associated with a high water table are expected to benefit from habitat creation by beavers.

Based on the experience in North America, and at Knapdale, interactions between beavers and other grazing and browsing animals will be important. It is likely that at both Knapdale and Tayside impacts caused by beavers will be influenced by local grazing pressures.

There is limited scientific information on the impacts of beavers on vascular plants (other than tree or shrub species), so it is possible to provide only a tentative prediction of possible future impacts.

Impacts through herbivory are most likely to affect terrestrial species within the foraging range of beavers, alongside ponds and streams. Some species currently growing in areas where beavers might change the habitat might be displaced. Other species will benefit from the creation of such habitat change.

The species most likely to be affected, either positively (or negatively, see below), by beavers are those which are already restricted in distribution and/or abundance, and which occur in potential beaver habitat close to waterbodies.

The positive effect of beaver interaction with terrestrial vascular plant species can be summarised as:

  • Changes in relative abundance of different tree species likely to see increased localised diversity of species associated with an open canopy, e.g. grassland species
  • Species of wetland habitats likely to benefit at local levels (see section 4.9)
  • Successional changes after dam abandonment likely to see increased diversity of species associated with increased habitat heterogeneity
  • Potential overall increased diversity at landscape scale due to increase in habitat heterogeneity

Individual species accounts follow below.

The main mechanisms by which beavers could impact this species is either through felling of trees in the riparian zone leading to the opening up of the canopy which could lead to change in light levels, especially shading, or through directly herbivory. Beavers are strictly herbivores; they have a very varied diet with strict seasonality and have been recorded eating around 80 different types of tree species and nearly 150 others plant species including aquatic macrophytes and herbaceous plants. Diet selection appears to be based on nutrient requirements and not necessarily related to local abundance.

Shady Horsetail (Equisetum Pratense)

This is an evergreen herb, typically found on sloping sites where the substrate is derived from calcareous alluvial silts or sands, especially lightly wooded stream banks in the lower parts of upland valleys. It can also extend onto open moorland, and is found on grassy slopes beneath base-rich upland cliffs.

Knapdale

There are no sites in the Knapdale beaver policy area designated for shady horsetail.

Tayside

  • Craighall George SSSI
  • Keltneyburn SSSI
  • Tulach Hill SSSI

SSSI Assessment

As described above, the general habitat requirements of shady horsetail are such that they could overlap with beaver core habitat. Although some of the sites listed above may have populations of shady horsetail that are located beyond the reach of beavers due to local topography, as beavers don't generally utilise steeply sloping banks.

The shady horsetail generally prefers a light canopy so any felling and subsequent changes to the woodland structure are likely to be generally positive. There is no scientific evidence available to determine whether beavers would preferentially select this national scarce species when foraging.

Therefore, while there are natural heritage interests of national importance on these sites, they are unlikely be adversely affected by beaver activity. Monitoring would add to the knowledge base and help clarify whether the potential benefits indicated above would contribute to the provision of improved habitat conditions for this species.

Mitigation

If beaver colonise these sites, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.

Twinflower Linnaea Borealis

This is a creeping perennial, woody at base, of both native and planted Scot pine pinus sylvestris woodland, where it occurs in slight to moderate shade, on barish ground or leaf litter, sometimes with an acidic healthy herb flora. It spread vegetatively and by seed, though seedling establishment seems largely restricted to disturbed ground.

Knapdale

There are no sites in the Knapdale beaver policy area designated for twinflower.

Tayside

  • Eastern Cairngorm SSSI
  • Hare Myre, Monk Myre and Stormont Loch SSSI

SSSI Assessment

As beavers generally avoid pine (see section 4.2), the overlap between this nationally scarce species is expected to be minimal at both sites. With respect to the Eastern Cairngorms SSSI, previous surveys of twinflower populations across the Cairngorms National Park have revealed that tall and dense growth of sub-shrubs e.g. Ling Heather Calluna vulgaris, which cast heavy shade on plants below are limiting vegetative spread and flowering of Twinflower. At Hare Myre, Monk Myre and Stormont Loch SSSI, it's the old growth pine woodland surrounding Stormont Loch and Hare Myre that provides suitable habitat for this plant.

Therefore, while there are natural heritage interests of national importance on these sites, they are unlikely be adversely affected by beaver activity.

Mitigation

No mitigation has been identified. See section 7 for details on the approach to SCM and beavers.

Creeping Lady's-Tresses (Goodera Repens)

This is a creeping, evergreen perennial herb of semi-natural and planted coniferous woodland, usually of Scots Pine Pinus sylvestris, where it grows in slight to moderate shade in moist layers of moss and pine needles.

Knapdale

There are no sites in the Knapdale beaver policy area designated for creeping lady's-trees.

Tayside

  • Hare Myre, Monk Myre and Stormont Loch SSSI

SSSI Assessment

The old growth pine woodland surrounding Stormont Loch and Hare Myre provides habitat for the nationally scarce creeping lady's-tresses Goodyera repens. As beavers generally avoid pine (see section 4.2), the overlap between them is expected to be minimal.

Therefore, while there are natural heritage interests of national importance on this site, they are unlikely be adversely affected by beaver activity.

Mitigation

No mitigation has been identified. See section 7 for details on the approach to SCM and beavers.

Wintergreen (Orthilia Secunda)

A rhizomatous, mycorrhizal, evergreen perennial herb of damp Calluna heather and Vaccinium (cranberry, cowberry and bilberry) dominated plant communities, mostly in pine and birch woodland but also on open moorland. It also grows in lefts and on ledges in rocky gullies and on rocky stream banks.

Knapdale

There are no sites in the Knapdale beaver policy area designated for wintergreen.

Tayside

  • Keltneyburn SSSI

SSSI Assessment

While this species appears to occupy a number of different micro-habitats (as referred to above), some of which may overlap with beaver core woodland, it appears to like more open less shady habitats. Suggesting that any beaver felling and subsequent changes to the woodland structure are likely to be generally positive or neutral.

Therefore, while there are natural heritage interests of national importance on this site, they are unlikely be adversely affected by beaver activity. Monitoring would add to the knowledge base and help clarify whether the potential benefits indicated above would contribute to the provision of improved habitat conditions for this species.

Mitigation

If beaver colonise these sites, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.

4.4.3.2 Consideration of potential negative effects on terrestrial vascular plant species of conservation importance

The impact of beaver activity on the vascular plant species discussed below is considered to have a negative effect or have to the potential for a negative effect.

The main mechanisms by which beavers could impact this species is either through felling of trees in the riparian zone leading to the opening up of the canopy which could lead to change in light levels, especially shading, or through directly herbivory. Beavers are strictly herbivores; they have a very varied diet with strict seasonality and have been recorded eating around 80 different types of tree species and nearly 150 others plant species including aquatic macrophytes and herbaceous plants. Diet selection appears to be based on nutrient requirements and not necessarily related to local abundance.

Whorled Solomon's-Seal (Polygonatum Verticillatum)

This is a rhizomatous perennial herb usually found on moist, nutrient-rich, usually basic, soils in wooded gorges and on a wooded river bank. Plants reproduce vegetatively, by rhizomatous spread but fruiting is generally poor with recruitment from seed apparently infrequent. Flowering seems to be restricted by excessive shading.

Knapdale

There are no sites in the Knapdale beaver policy area designated for whorled Solomon's-seal.

Tayside

  • Craighall George SSSI
  • Den of Airlie SSSI
  • Den of Riechip SSSI
  • Keltneyburn SSSI
  • Milton wood SSSI
  • Romadie Woods SSSI

SSSI Assessment

In Great Britain, on the western fringe of its range, it is confined to a comparatively small area of East-Central Scotland, where it is known from twelve sites, all in wooded ravines in Perthshire, of which five overlap with core beaver woodland. It's generally located in steep gullies and wooded ravine; the population located in the Den of Airlie SSSI, in particular, is considered to be on the edge of habitat likely to be accessible to beavers.

Whorled Solomon's-seal appears to tolerate a lighter open canopy so any felling and subsequently changes to the woodland structure are likely to be generally positive, particularly for flowering. There is no scientific evidence available to determine whether beavers would preferentially select this species when foraging. However, its distribution is severely restricted and it is considered nationally rare.

Therefore, there are natural heritage interests of national importance on these sites, which could potential be adversely affected by beaver activity.

Mitigation

If beaver colonise these sites, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.

Lesser Hairy Brome (Bromopsis Benekenii)

This is a tufted perennial herb of lightly shaded habitats on moist, moderately base-rich soils, including woodlands especially, upland ash woodland, beech also scrub and hedgerows; it occasionally persists on sites of former woodland. Some bare soil is necessary for successful establishment from seed. Almost entirely lowland.

Knapdale

There are no sites in the Knapdale beaver policy area designated for lesser hairy brome.

Tayside

  • Craighall George SSSI
  • Keltneyburn SSSI

SSSI Assessment

The lesser hairy brome generally prefers a lightly shaded canopy so any felling and subsequently changes to the woodland structure are likely to be generally positive overall. There is no scientific evidence available to determine whether beavers would preferentially select this species when foraging. However, this is a rare plant which is deemed nationally scarce.

Therefore, there are natural heritage interests of national importance on these sites, which could potential be adversely affected by beaver activity.

Mitigation

If beaver colonise these sites, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.

Small Cow-Wheat (Melampyrum Sylvaticum)

Once widespread in Britain and Ireland small cow-wheat, an annual hemiparasite (and therefore gains additional water, nutrients and organic compounds from the roots of host plants), is now restricted to only 19 sites, mostly in Scotland north of the Highland Boundary Fault. Of these, only five sites support more than 500 plants and seven sites support populations of 100 individuals or fewer.

These small populations typically persist in isolated remnants or small fragments of upland woodland along river gullies, in steep-sided ravines or high up on rock ledges. At lower altitudes this species occupies high humidity sites - close to water, north-facing and under a closed canopy. At higher altitudes the climate is cool enough to maintain adequate moisture levels without a dense canopy, although the shorter growing season constrains plant size.

Knapdale

There are no sites in the Knapdale beaver policy area designated for small cow-wheat.

Tayside

  • Keltneyburn SSSI

SSSI Assessment

The overlap with core beaver woodland is likely to encompass the humid, damp shady conditions referred to above. Therefore any beaver felling that opens up the woodland canopy and reduces this micro-habitat is potentially unlikely to be beneficial to this nationally scare plant.

Therefore, there are natural heritage interest of national importance on this site, which could potential be adversely affected by beaver activity.

Mitigation

If beaver colonise this site, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.

Coralroot Orchid (Corallorhiza Trifida)

A saprophytic herb usually found in shaded damp, alder Alnus and willow Salix carr on raised mires and lake margins, but also occurs in dune-slacks with creeping willow salix repens. More rarely, it grows in tall-herb fen in birch Betula and pine Pinus woods (amongst sphagnum) and on moorland. It may colonise secondary habitats including plantations and quarries.

Knapdale

There are no sites in the Knapdale beaver policy area designated for coralroot orchid.

Tayside

  • Rescobie and Balgavies Lochs SSSI

SSSI Assessment

There is potential for some overlap in the distribution of this species and beavers, particularly to the west of Balgavies Lochs. Given its relatively wide ecological niche as described above, any felling activity by beavers at this site which opens up the woodland canopy and reduces the shaded nature of the woodland may not be beneficial to this nationally scarce orchid, although this remains uncertain.

Therefore, there are natural heritage interest of national importance on this site, which could potential be adversely affected by beaver activity.

Mitigation

If beaver colonise these sites, impacts should be monitored through SCM and appropriate mitigation put in place. See section 5 for beaver management techniques used to mitigate the impact of beaver foraging activity. See section 7 for details on the approach to SCM and beavers.


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