The forest varieties found along the Park’s valleys are the result of the combined action of two fundamental ecological factors: the region’s distinctive climatic regime and the geological substratum. High rainfall, relatively mild temperatures and a fairly limited temperature range contribute to an oceanic climatic regime which favours the growth of lush and diversified vegetation, classifiable as follows:

• CASTANETUM phytoclimatic zone with populations of broad-leaved mesophylls, thermophyllic beech groves and pine forests;
• FAGETUM zone with pure and mixed beech groves, mixed pine forests and sporadic ash groves;
• PICCETUM zone with microthermal beech groves, mixed clusters of silver fir, spruce and beech, larch groves, mountain pine and alder wood, and grassland.

On the sunnier slopes, characterised by quite poor, permeable and shallow soil, grow mixed clusters of European hornbeam and flowering ash, and sometimes also sycamore and beech.
In terms of silviculture, in general these vegetation growths constitute woods of reduced economic value, with their main production being firewood. At higher altitudes and on more evolved soil, pure or mixed beech groves grow, which in quantitative terms represent the most important vegetation for the Park, with the possibility of providing both timber and firewood. The fir tree woods comprise mainly spruce, but can also include beech or, in certain conditions, silver fir. These woods are used to provide mostly timber. Above 1,400 m, there are the occasional larch groves and widespread mountain pines which have a marked protective and colonizing behaviour. Even higher up, the forest vegetation gives way to high altitude pasture. All state-owned woods are included in the Forest Management Plans, which represent a necessary instrument for the proper management and use of forest resources.