Eryri today can be said to be mainly grassland, this is as a result of the removal of the natural woodland for timber, fuel, and charcoal and most important of all, agricultural land - be it pasture or agrarian cultivation. The clearance of the natural forest began over 4,000 years go in Neolithic times. In the uplands this clearance developed the areas of heath we see today as well as precipitating the formation of blanket peat-land.

In Eryri the climatic climax vegetation would have been broad-leaved woodland, mainly of sessile oak, with birch forests on the higher ground. The tree line of this forest would have been around 500m on most slopes; the bracken seen on the hills today give a rough clue as to the presence of the woodland. Very little of this 'natural' woodland remains, one of the few upland woods that remain is Coed-y-Rhygen, near Trasfynydd; while Coed Padarn on the hillside around the Vivian Quarry in Llanberis is a good example of a sessile oak wood.

At various points in Eryri, a clue as to the variety of plant species possible can be found in the areas fenced off to stop sheep from preventing re-growth by grazing; one of the islands on Llyn Idwal - moated by the lake waters and safe from the sheep also sports luxurious growth.

Sheep are therefore the most important factor affecting plant regeneration; apparently in the Middle Ages the pressure of 0.15 sheep per hectare was enough to retain/maintain large areas of grassland. Today, with around 3 sheep per hectare the pressure is obviously greater and species regeneration is affected. In woodland open to grazing one may find mature trees and shrub growth but few, if any, forest seedlings managing to overcome grazing pressure to replenish the forest trees.

A number of grasses can be found in the hill-slopes of Eryri. Mat-grass is usually the main species found on the very poor acid soils of the upper slopes; it is an aggressive plant apt to overgrown other areas of grassland if they are overgrazed. Sheep will only eat the mat-grass if nothing else is available.
In wetter areas one finds purple moor-grass in large swathes, while on the lower slopes Sheep's Fescue is dominant. In some areas the teaching of plant nutrients down slope (catena) allows meadow grasses and bents to grow. Sheep are drawn to these as they offer improved grazing, their droppings increase soil fertility and in these areas white clover can be found, helping to produce reasonable pasture.