Deciduous forests have a variety of dominant species (sessile oak, silver birch, aspen poplar, etc.) depending on altitude and geographical location. These forests typically lie between 1 000 and 1 600 m on shady slopes, with the trees reaching 10 to 15 m in height. They are floristically rich and have a high tree cover.
Predominantly Quercus rotundifolia forests occur at an average altitude of 1 400 m on sunny slopes. These trees never exceed 10 m in height. The understorey is made up mainly of box (Buxus sempervirens). This species of oak is ecologically very adaptable and will also grow on poor soils.
Predominantly Quercus humilis forests are typical of the sub-Mediterranean montane zone lying between 1 000 and 2 000 m with an annual rainfall of about 800 mm. They cover large areas, and the trees can reach 15 m in height. Quercus humilis is generally accompanied by Montpellier maple (Acer monspessulanum), Quercus rotundifolia, Acer opalus and Scots pine. The undergrowth is made up of box, Rosa sp., juneberry (Amelanchier ovalis), etc., but also contains such Mediterranean shrubs as common privet (Ligustrum vulgare) and Crataegus monogyna. The herbaceous layer contains Fragaria vesca and Anemone hepatica. These forests have suffered from extensive human pressure.
Andorra's conifer forests account for the major part of the country's forested area and are made up of three main species: Scots pine, silver fir and black pine. Each species has its ecological preference: Scots pine is found between 1 200 and 1 800 m, silver fir between 1 500 and 2 000 m on shady slopes, and black pine above 1 900 m.
The dominant storey in conifer forests reaches a height of 20 m in optimum conditions, but tends to average about 15 m. The structure of these forests varies considerably, depending on exposure, human interference and altitude. Tree cover becomes lower with increasing altitude, because of the stronger winds and sunlight, poorer soil, etc., in subalpine zones. Pine and fir forests generally have two or three storeys.
Conifer forests are affected by two kinds of human pressure - alpine ski trails and motorways - both of which cause a fragmentation of forest habitats, thus leading to serious degradation of the forest environment.
Silver fir reaches a height of 30 m. Situated in the high montane zone at between 1 400 and 1 800 m, it is typically found on moist, cool, shady slopes, and needs a minimum annual rainfall of 800 mm. These stands cover only a small percentage of the country. Silver fir is dotted around the Soldeu, Ordino, Setúria and Madriu districts.
Scots pine, reaching heights of 25 or 30 m, is the commonest species in the country and is not fussy about the type of substratum. It forms large stands between 1 200 and 1 700 m with an annual rainfall of 600 to 900 mm. Thus it occupies medium-altitude mountains as well as the lower part of the subalpine level, and can be found up to 2 000 m on sunny slopes. Scots pine forests have in general replaced other plant communities (particularly Quercus humilis stands) after overexploitation. Box is the commonest shrub in their understorey.
Black pine is predominant in the subalpine zone at between 1 700 and 2 200 m and is the commonest species in Andorra after the Scots pine. It requires less moisture than the latter and is found with an annual rainfall of about 1 000 mm, where it can reach a maximum height of 25 m.