Calcareous screes
Calcareous mudstone scree with gentians on the Matiri Range, western Nelson (Peter Williams)
Calcareous rocks are limestones and marble. Screes are the accumulation of course debris, ranging from gravel (2-64 mm) and cobbles (64-256 mm) to patches of boulders (>256 mm) mantling slopes (often in association with protruding in situ bedrock). Slopes are generally somewhat below the natural angle of repose of about 32 degrees. Screes are usually armoured on the surface with large fragments overlying finer particles that often hold water. Screes have also been referred to as ‘shingle slides’ (Fisher 1969). Screes can form under a forest canopy in some situations, but this category concerns mostly open land above treeline. Calcareous screes are only sparsely vegetated, often with grasses such as Poa spp. and Elymus spp., and herbs, including several gentians.
Synonyms
Calcareous scree and loose rock.
Where do they occur?
In the South Island, calcareous screes are abundant near or above treeline on the Arthur Range and Matiri Range in western Nelson, and on the Chalk Range in southern Marlborough. In the North Island, all areas of calcareous scree are less than 100 m2.
Notable flora and fauna
Threatened plants include the nationally critical Castle Hill forget-me-not (Myosotis colensoi), Castle Hill buttercup (Ranunculus paucifolius) and Myosotis angustata, the nationally endangered Enys aniseed (Gingidia enysii), the naturally uncommon Poa sudicola, Arnold forget-me-not (Myosotis arnoldii), Poa xenica, Ward Beach gentian (Gentianella astonii subsp. arduana), and Gentianella filipes.
Threatened fauna include the naturally uncommon black-eyed gecko (Mokopirirakau kahutarae), which is rare on the Arthur Range. Nationally vulnerable rock wrens (Xenicus gilviventris) are characteristic of these and other scree and boulder fields throughout the South Island.
Threat status
Vulnerable (Holdaway et al. 2012)
Threats
Invasion exotic herbs include thistles (Cirsium spp.) and grasses, especially high-polyploid grasses, Chewing’s fescue (Festuca rubra), meadow grass (Poa pratensis) and cocksfoot (Dactylis glomerata). At lower altitudes calcareous screes are very vulnerable to vines such as old man’s beard (Clematis vitalba) and banana passion vine (Passiflora spp.). Red deer, chamois, hares, and possums are widespread and goats can be locally abundant and very damaging. Mining may pose a threat, but only at lower altitudes in locations where screes are vulnerable to being quarried for lime.
Further reading
Dawson JW 1988. Scree Plants in Forest Vines to Snow Tussocks: The Story of New Zealand Plants. Victoria University Press, Wellington.
Druce AP, Williams PA, Heine JC 1987. Vegetation and flora of Tertiary calcareous rocks in the mountains of Western Nelson, New Zealand. New Zealand Journal of Botany 25: 41-78.
Fisher FJF 1969. Rock, shingle-slide and riverbed. In: Knox GA ed. The natural history of Canterbury. Wellington, AH and AW Reed. Pp. 106-116.
Rogers GM, Courtney SP, Heenan, PB 2018. The calcicolous vascular flora of New Zealand: life forms, taxonomy, biogeography and conservation status. Science for Conservation 331. Wellington, Department of Conservation.
Wardle P 1991. Vegetation of New Zealand. Cambridge University Press. 672 p.