The National Park vegetation is made up of distinct plant communities from the Central System. An important part of them are Guadarrama endemism. The most representative are the Scots pine or Valsaín pine (Pinus sylvestris var. Iberian), the high mountain thickets of broom and creeping juniper, and the xerophyte pastures living on the summits of the Sierra.
In the summits and slopes environment form complex mosaics where alternate wet grass, peat bogs, ponds and lakes, xerophyte pastures, cracks and rocks ledges vegetation communities, etc. This is the growing habitat for the greatest diversity of plant communities and the most unique flora of the territory grow.
In the National Park are represented eight natural systems referred to in the National Parks Law 30/2014.
In relation to the Spanish natural terrestrial systems the parks should represent, there are eight in the territory that encompasses the Sierra de Guadarrama National Park: high mountain brush, high mountain pastures, woody steppes of height and pebble grounds; natural system formations and unique mountain and high mountain natural systems reliefs; unique glacial and periglacial area; oak woods, beech and birch woods; Portuguese oak and Pyrenean oak forests; pine forests, groves and juniper woods; high mountain wetlands and lakes; water courses and riparian forests.
Within the biogeographic unit that forms the Central System, the Sierra de Guadarrama is a well-defined territory, characterized by a greater degree of continentality than the adjacent mountain ranges, both to the East and toward the West. Mainly due to this factor, the plant communities that inhabit the Sierra are characteristics of these central Spain mountains. These communities species change with the different slopes altitudes, under varying environmental conditions, mainly temperature and precipitation.
The main vegetation formations are the holm oak, the Pyrenean oak, the Scots pine, the thickets of broom and creeping juniper, and the xerophyte pastures.
The Holm oak (Quercus ilex subsp. Ballota) develop in the Sierra foothills area, so it can only be found in a timely manner in the National Park. One of the most representative species is the prickly juniper (Juniperus oxycedrus).
The Quercus pyrenaica woods occupy an important extension on the mountains, although, since its optimal environment does not exceed 1,600 m height, the greater part of the masses are outside the boundaries of the park. In some ravines and enclaves grow some specimens having considerable size, witnesses of some more mature forests that once covered these slopes. In permanently hydromorphic soils, and in the domain of the holm and the Pyrenean oak, develop the ash (Fraxinus angustifolia) and Pyrenean forests, which for the most part are full of pastures by now.
The Scots pine (Pinus sylvestris var. Iberian) are the Park most representative forests and covering he greater extension. The altitudinal limit of the forest, with some stands or scattered copies that rise above the 2,100 m. In the lower parts, the flower cortege accompanying is dominated by the common fern (Pteridium aquilinum) and the white broom (Genista florida). In these areas, mixed forests of pine and Pyrenean oak are frequent, the presence of holly (Ilex aquifolium) is common. In the upper area, it will appear with other bushes as the broom (Cytisus oromediterraneus) and the common juniper (Juniperus communis subsp. Hemisphaerica and J. communis subsp. Alpine).
Within the forest soil, it is necessary to emphasize the existence of numerous examples of groves and relict stands, formations that were present in the past and at the present time are relegated to certain valleys and ravines and in certain directions. Among them are the birch Betula celtiberica and yew (Taxus baccata).
Above the pine forests develop extensive high mountain thickets of high mountain, brooms (Cytisus oromediterraneus) and creeping juniper (Juniperus communis subsp. Alpine).
Xerophyte pastures grow in the areas close to the summits or in the juniper meadows, where the scrubland cannot keep continuity. It is a community dominated by herbaceous perennials and padding, such as the Festuca curvifolia grass. It is diverse and many of the species are Iberian endemism and optimal eurosiberian plants that reach in these mountains its southern limit of distribution (Hieracium vahlii subsp. Myriademum, Armoury caespitosa, Jasione crispa subsp. Centralis, Silene ciliata subsp. Elegans, etc.).
In addition, in these areas, and in some massifs, the glaciers has modeled the relief resulting in a landscape of steep peaks, rocky ledges and buckets. This morphology has helped to create new and unique habitats for vegetation, where develop communities of wet grasslands (matgrass, helophytic turf communities, ponds and lakes vegetation communities) and other rupicolous communities that colonize the National Park rocks and scree (Saxifraga pentadactylis subsp. Willkommiana communities, Senecio pyrenaicus and Digitalis purpurea subsp. Carpetana communities, etc.).