Nitrogen Deposition :: Inland Rock & Scree

Effects and implications

• On P limited sites direct effects of N accumulation can affect lichens and mosses leading to species loss and a reduction in species richness.

Overview: evidence, processes and main impacts

Limestone pavements

Limestone pavements have not been studied specifically with respect to N deposition but aspects of responses of communities on calcareous grasslands and neutral/acid grasslands may be informative. Critical loads for these cover a wide range, illustrating a range of susceptibility depending on whether the system is N limited or P limited (see critical loads section below). Nitrogen limited sites would be the most sensitive. Mosses and lichens are expected to be the most sensitive components of these systems, with N deposition leading to species changes and loss. Rough grass species can increase, leading to shading out of herb species, from overtopping and increased litter production.

Scree

Inland rock outcrop and scree habitats are widespread in upland areas of the UK, with more limited occurrence in the lowlands. This habitat covers a wide range of rock types, varying from acidic to highly calcareous, occurring throughout the uplands, at high altitudes, but also at low altitudes notably in northern Scotland.  Acidic rock and scree are especially widespread, whereas calcareous communities are restricted by the underlying geology, and good stands of tall-herb vegetation also tend to be restricted by heavy grazing. Screes are typically dominated by Cryptogramma crispa and other ferns, lichens and bryophytes. Ferns also thrive in the deep humid crevices. On cliff ledges, tall herbs such as Sedum rosea and Angelica sylvestris are generally abundant. Chasmophytic vegetation (in rock crevices) is usually dominated by ferns such as Asplenium viride and small herbs such as Thymus polytrichus and Saxifraga spp. Bryophytes and lichens also occur in crevices but are able to flourish on the open rock surfaces where there is a lack of competition from vascular plants.

Many rock habitats, especially cliff faces, rock ledges, gorges and boulder fields are inaccessible to grazing animals and are unmanaged. Others are more accessible, such as fine screes and gently sloping rock outcrops, where accessible grazing may keep the vegetation in check. Burning can affect the more heather-rich rock faces with fires spreading up on to rocky slopes from muirburn below. The inaccessibility of rock habitats to grazing animals, especially of rock ledges, provides a refuge for many vascular plants that are sensitive to grazing, including numerous local and rare species. Notable species of upland rock and scree habitats include Athyrium distentifolium, Woodsia ilvensis, Carex rupestris, Cicerbita alpina, Artemisia norvegica, Hieracium sect. Alpestria, Salix lanata, Saxifraga cespitosa and S. cernua.

These species may be susceptible to N, as nutrient availability is generally low, but there have been no experimental studies. The extreme conditions may help limit new species moving in and capitalising on the increased N supply. Likewise N stimulated competition effects may not be a problem given the very distinct niches. Mixed herb communities growing amongst scree may respond in a similar fashion to montane and sub-alpine grassland.

Pollutant deposition type and risk areas

These sites are usually too far from NH₃ or NOx sources for these gases to present a threat.

Indicators of N enrichment

[see acid / calcareous grasslands]

• Loss of key species, forbs, legumes, bryophytes and lichens
• Increased foliar % N
• Lower soil CN