sentences:  Birch may also grow in pure stands near the beech forests, in the mountainous areas on siliceous bedrock; these areas are typically of only small extent and generally quite patchy with sessile oak ("Quercus petraea") and trees of the genus "Sorbus".The silver fir ("Abies alba") is found on the cool, deep-soiled slopes of the flanks of the Pyrenees, from Navarre to Montseny, forming pure fir woods or, more often, mixed forests with beech trees.The most important areas are in Lleida (Lérida), with 170 square kilometres.