北密毛柳

The willows are a large family, ranging from tall, stately trees, providing a useful source of timber, to ground-hugging shrubs found in some of the coldest regions of the north. Woolly willow is one of the latter types, and although it can reach a height of well over a metre, the plant's characteristics reflect the harshness of its environment. Willows, in fact, have adapted themselves more than any other tree type – even the pines – to the cold northern latitudes. Like all willows, this species is 'dioecious', occurring as either male or female trees. Woolly willows' catkins appear during the month of June in Scotland. Female catkins are insect-pollinated, unlike nearly all other native catkin-bearing trees, such as the hazel, beech and oak and even the related poplars, which are wind-pollinated. They provide an important source of food for insects, especially bumblebees, at that time of year.

The woolly willow is listed as a priority species in the UK Biodiversity Action Plans. Many of the sites where it survives in Scotland have statutory protection, and the action plan specifies a number of conservation measures to further improve its chances of survival. The two agencies responsible for the woolly willow in Scotland, Scottish Natural Heritage and The National Trust for Scotland, are currently implementing these measures. On some of the sites, grazing pressure is being reduced, especially that from red deer, by fencing. It has also been recommended that where plants exist in isolation, new individuals, propagated from wild seed, should be planted close to the existing specimen. This would improve the chances of successful insect pollination.

The woolly willow is aptly named as the silvery-green, oval leaves have a thick coat of hairs, and grow to five centimetres in length. The male catkins are a golden yellow colour, measuring up to a centimetre thick and five centimetres long. The female catkins can grow to nearly eight centimetres.

This species is found at high altitudes, above the normal tree line, from 500 to 1000 metres above sea level, and in Scotland it grows in areas that cannot be reached by grazing animals. It requires a calcareous or lime-rich soil.

Woolly willow is found in the mountainous circumpolar regions of northern Europe and Asia. In Scotland, the species has been found growing in fifteen 10km squares, and probably represents a relic population from just after the Ice Age. Elsewhere in its range, the woolly willow is restricted mostly to arctic or sub-arctic habitats.

Classified as Vulnerable in the UK, and protected under the Wildlife and Countryside Act, 1981 as amended

Most of the populations of woolly willow in Scotland are very small, comprising fewer than one hundred plants. Four of the sites consist of a single individual, and this makes the plant vulnerable, especially as it is a dioecious species. Individual trees therefore have no close source of cross-pollination for them to reproduce successfully. The increase in grazing over the willows' habitat has reduced the populations to those that cannot be reached by sheep, cattle or deer. Here, they are also vulnerable to rock-falls, avalanches and general erosion.