The hills that crown the Holy Mountain of Crea, which is scarcely populated because of the excessive steepness and easy erosion of the slopes, have conserved from time immemorial, pieces of the natural forest that have dominated the countryside in this area since the IX century (high Medieval). The natural systems have been altered very little, while in other parts of Monferrato they have been drastically reduced because of extended clearing and expansion of vine and cereal cultivation.
The natural woods that cover these hills can be put into two well differentiated typologies: the mesoxerophilous which covers the crest and higher slopes in the warm, shaded areas, and the mesophilous woods at the bottom of the valley and lower slopes.
The mesoxerophilous woods are made up of oak (Quercus pubescens) and Fraxinus ornus mixed with other arboreous species, one of which is the rare Turkey oak (Quercus cerris) and the now very rare Sylvester Pine (Pinus sylvestris).
These are sparse woods where abundant light filters because of the exposition, allowing for the development of a thick thermophile herbaceous layer along with the nature of the terrain create the micro-climatic conditions where the effect of the winter temperatures are strongly moderated. In the shrub layer the Viburnum lantana, Cytisus sessilifolius, Coronilla emerus, the beautiful Campanula medium which is found exclusively in Province and on the Ligurian-Piedmont Apennines, Asperula purpurea, Inula bifrons, Spartium junceum and various orchids like Epipactis atropurpurea vegetate.
The mesoxerophilous woods on the crest contrast with the mesophilous species at the bottom of the valley and with the northern expositions made up of Quercus robur and Carpinus betulus. These are thick shady woods with a thin shrub layer often substituted with chestnut copse or locust tree woods. There is a rich variety of early spring blooms in the herbaceous layer that take advantage of the first rays of sun before the trees are covered in foliage impeding the rays of light in reaching the ground. Among these the most well-known are the Pulmonaria officinalis, Primula vulgaris, some violets (Viola riviniana, V. reichembachiana, etc.), Hepatica nobilis and Erythronium dens-canis.
The survival of natural woods is threatened by the spreading of locust (Robinia pseudoacacia). Highly competitive allochthonous species after being cut down tend to take over the autochthonous species creating naturalistically impoverished woods.