European High Mountain (Alpine) Vegetation and its Suitability for Indicating Climate Change Impacts

Abstract

High mountain (alpine) vegetation in Europe occurs above the climatic treeline or substitute vegetation from north of the Arctic Circle to the Mediterranean. As bearing the least modified ecosystems, high mountains offer an opportunity to use their plant and animal species for studying climate change impacts. However, the indicator value of the different vegetation types varies. Treelines, often used in reconstructing past climate by palaeo-scientists, are, in most cases, suppressed by past or present land use and, as a result, their changes need careful interpreting. Glacier forefields are the theatre of primary succession and vegetation changes there have an innate temporal dimension that needs to taken into account. Changes in snowbeds can occur over a relatively short time and are readily interpretable, as long as potential confounding impacts by herbivores are excluded. In the long-term, remote alpine summits with long-established vegetation (but less so those in the sub-nival zone, where primary succession is underway) are likely to yield useful and interpretable information beyond the shortto medium-term impacts of the vagaries of mountain weather. L?szl? Nagy (e-mail: lazlonagy? btinternet.com) EcoScience Scotland, 2/1, 27 Glencairn Drive, Glasgow G41 4QP, Scotland. INTRODUCTION High mountains have traditionally been described as those with glacial features and periglacial processes (e.g. Gerrard 1990). Relatively few mountain ranges (Scades, Alps, Pyrenees, Caucasus) possess active glacial characteristics in Europe today. However the number of those that retain glacial landforms resulting from former glacial activity is large. These glacial landforms largely determine vegetation and periglacial processes (e.g. cryoturbation, solifluction) are important habitat factors through the specific conditions they create for plant growth. High mountains occur in Europe from north of the Arctic Circle to as far south as 35 degrees latitude. In this paper the term high mountain is applied to those mountains which have a potential altitudinal treeline, the upper limit of which is determined by low temperatures (Fig. 1). Reflecting climate, glaciation history and vegetation similarities and differences along a latitude axis, high mountains in a European context have been classified as northern boreo arctic type (sometimes called alpine tundra), middle latitude (alpic system, sensu Ozenda 1985), and Mediterranean type (e.g. Ozenda 1994; 2002). Within each of these latitude classes, elevation zones or belts have been identified, generating a number of systems from Scandinavia (e.g. P?hlsonn 1994), via the Alps (e.g. Ozenda 1985; Grabherr 1997) to the Mediterranean (e.g. Ozenda 1975; Qu?zel 1979; Rivas-Mart?nez 1981). The altitude classification of vegetation is based on recognising distinct assemblages along a gradient from the lowlands to the line of permanent snow (where applicable). Such classification in the mid-latitude high mountains (Alps, Carpathians) includes the following zones: colline (lower mountain slopes with warmth-loving trees as natural vegetation), montane (usually naturally tall-forested mountain slopes up to the timberline), subalpine (an ill defined zone of open low-stature forest including the so called krummholz, with its gnarled dwarfed trees interspersed with natural or man-made grasslands), the alpine (with its closed 'grasslands' or sedge heaths) and the uppermost sub-nival zone (scant patchy vegetation, largely dominated by bryophytes and lichens). There are analogous altitude belt or zone classes for the Scandina vian and Mediterranean mountains. For example, the Apennines, being a Mediterranean mountain range, bear a [cryo/oro-]Mediterranean (mountain) vegetation at and above the treeline. The high mountains along the Pyrenees ? Corsica ? Central Apennines Mt Olympus line are characterised by the existence of two diff?rent types of vegetation: one on north facing slopes (resembling the vegetation of more northerly mountains) and the other on south facing ones where a Mediterranean type, thorny cushion vegetation dominates. The altitude zones, or vegetation belts, are climate driven and appear to have been rather stable Biology and Environment: Proceedings of the Royal Irish Academy, Vol. 106B, No. 3, 335-341 (2006). ? Royal Irish Academy 335 This content downloaded from 207.46.13.113 on Thu, 06 Oct 2016 04:09:47 UTC All use subject to http://about.jstor.org/terms Biology and Environment Fig. 1 ? Treeline-forming tree species and replacement scrub vegetation in the high mountains of Europe after Grabherr et al. (2003). The map was compiled using public data provided by the US National Geophysical Data Center (Global Ecosystems Database and The Global Land One-kilometer Base Elevation (GLOBE) Digital Elevation Model). The numbers refer to the following species and mountain areas: 1. Betula pubescens ssp. tortuosa (Iceland, Scandes, Chibiny Mts of the Kola Peninsula, eastern Urals) 2. Betula pubescens, Pinus sylvestris (Scottish Highlands) 3. Picea abies ssp obovata, Larix sibirica (western Urals) 4. Pinus uncinata (Pyrenees) 5. Larix decidua, Picea abies, Pinus cembra, P. mugo, Alnus viridis (Alps, Carpathians) 6. Picea abies, Pinus mugo (Rila) 7. Alnus viridis ssp suaveolens (Corsican mountains) 8. Fagus sylvatica, Pinus mugo (central Apennines) 9. Pinus leucodermis, Juniperus communis ssp hemisphaerica (southern Apennines) 10. Genista aetnensis [Abies nebrodensis] (Mt Etna) 11. Pinus heldreichii, P. mugo (northern Dinarids) 12. Pinus peuce, P. heldreichii (southern Dinarids) 13. Pinus heldreichii (Hellenids) 14. Cupressus sempervirens (Crete) 15. Juniperus communis ssp hemisphaerica, J. sabina, Genista baetica (Sierra Nevada) 16. Picea orient?lis, Pinus kochiana (dry slopes); Betula litwinowii, Acer trautvetteri, Sorbus caucasigena (humid); Quercus macranthera, Q. pontica, Fagus orient?lis (Caucasus) in the Alps for the past c. 8000 years (Amman 1995), as opposed to north-west European mountains, where a large reduction in the alpine zone during the last 10,000-15,000 years (Birks 1986) points to a long ongoing change in environmental conditions. The purpose of this paper is to provide a brief overview of vegetation types at and above the treeline and to examine the suitability of the different vegetation types for indicating climate induced changes. THE VEGETATION OF EUROPEAN HIGH MOUNTAINS The vegetation of high mountains comprises a large number of different types, mostly reflecting the geomorphic diversity of the landscape. The types found in the European mountains, in general, fall into the following broad categories: treeline ecotone scrub, dwarf-shrub heath, grass, sedge and moss heaths, snowbeds, open cryptogamic communities, tall-herb meadows, mires, scree and rock vegetation (for regional accounts and references in European alpine areas see chapters 3.1-3.10 in Nagy et al. (2003). Bohn et al. (2000) have listed the above vegetation types for the Potential Vegetation Map of Europe under (1) subarctic, boreal and nemoral-montane open woodlands, as well as subalpine and oro Mediterranean vegetation (47 regional formations); (2) mountain tundras and sparse mountain vegetation (6 regional formations); (3) alpine vegetation in the boreal, nemoral and Medi terranean zone (20 regional formations). Winter snow cover is one of the main factors in shaping vegetation types and species distributions. The glacial landscape of high mountains provides extremes of snow cover in the winter, ranging from exposed ridges with little or no snow to sheltered areas where wind-blown snow may accumulate to depths of several metres. The implications of unequal snow distribution and duration for plant diversity patterns in European alpine regions have been demonstrated by Virtanen et al. (2002; 2003a). A typical sequence along a slope from ridge to slope bottom consists of: (1) wind-clipped lichen-rich dwarf-shrub or moss heath, (2) grass-sedge heath, (3) herb-rich meadow, (4) snowbed, and (5) high altitude bog (see Makarov et al. 2003 for an example from the orthern Caucasus). Late-lying snow in depressions with deep, fine soil is responsible for the existence of snowbed vegetation types, ranging from the so-called early snowbeds, which are dominated by