The flora of Scotland is an assemblage of native plant species including over 1,600 vascular plants, more than 1,500 lichens and nearly 1,000 bryophytes. The total number of vascular species is low by world standards but lichens and bryophytes are abundant and the latter form a population of global importance. Various populations of rare fern exist, although the impact of 19th-century collectors threatened the existence of several species. The flora is generally typical of the north-west European part of the Palearctic realm and prominent features of the Scottish flora include boreal Caledonian forest (much reduced from its natural extent), heather moorland and coastal machair. In addition to the native varieties of vascular plants there are numerous non-native introductions, now believed to make up some 43% of the species in the country. There are a variety of important trees species and specimens; a Grand Fir in Argyll is the tallest tree in the United Kingdom and the Fortingall Yew may be the oldest tree in Europe. The Arran Whitebeams, Shetland Mouse-ear and Scottish Primrose are endemic flowering plants and there are a variety of endemic mosses and lichens. Conservation of the natural environment is well developed and various organisations play an important role in the stewardship of the country's flora. Numerous references to the country's flora appear in folklore, song and poetry. Habitats Scotland enjoys a diversity of temperate ecologies, incorporating both deciduous and coniferous woodlands, and moorland, montane, estuarine, freshwater, oceanic, and tundra landscapes. Approximately 18.5% of Scotland is wooded, 14.5% of which is non-native forestry plantation, but prior to human clearing there would have been much larger areas of boreal Caledonian and broad-leaved forest. Although reduced significantly (some estimates are as high as 98%), significant remnants of the native Scots Pine woodlands can be found in places. 17% of Scotland is covered by heather moorland and peatland. Caithness and Sutherland have some of the largest and most intact areas of blanket bog in the world, supporting a distinctive wildlife community. 75% percent of Scotland's land is classed as agricultural (including some moorland) with urban areas accounting for around 3% of the total. The number of islands with terrestrial vegetation is nearly 800, about 600 of them lying off the west coast. Scotland has more than 90% of the volume and 70% of the total surface area of fresh water in the United Kingdom. There are more than 30,000 freshwater