Due to the limiting factors of light intensity, changing photoperiod (day length), temperature and rainfall a seasonal succession occurs in the field and ground layers.
Canopy in winter: Until late spring there is very little restriction to light entering the woodland. The flowers which begin to grow on the woodland floor (ground layer) will be small like the violet.
The same canopy in early summer: The canopy is the last leafy layer to expand and restrict light falling on to the ground and field layers of the woodland. At this time they will be at their peak and most diverse. After June the reduction in light intensity caused by the darkening of the leaves will restrict these layers to the hardiest species.
Over the year a cycle of appearance and disappearance of both leaves and flowers may occur. Those that grow first to avoid the shade when the canopy closes over must have food in reserve that was manufactured the previous year. Perennating organs, like rhizomes and bulbs are used to store this food. This will induce rapid growth and an early production of a flower. This must be tempered with the possible shortage of pollinating insects. Violets and Wood Sorrel produce additional cleistagamic flowers (able to self-fertilise).
After flowering, leaves may remain as long as possible to ensure maximum food production for the next year. Wood Anemone leaves are short lived and this limits the outgrowths from the rhizome each spring: it dies at the previous year’s end, extending at the other. As spring advances so the succession of plants continue, favouring the larger ones until a sub-layering occurs. Eventually by June and July the canopy may be so dense. particularly in beech, that the wood becomes very dark.
Yellow Bird’s-nest: a saprophytic species, feeding on dead leaves in a beech wood.
Beech has well adapted leaves which absorb all but a few percent of the available light. Hence, it is virtually impossible for photosynthesis to take place in the ground and field layers. Plants begin to grow which use other methods of obtaining food. Here, saprophytic plants may grow (including orchids) which lack leaves but have a fungal association with the roots enabling them to utilise organic matter in the soil. Other species able to survive here will be parasitic species.
Eventually the shade is too great and the slower growing shade plants such as the nightshade, sedges and ferns take over. By the autumn this too will have died back with a substitution by fungi which consume the “rain” of dead organic matter. Fungi are present through much of the year but only become obvious with their fruiting bodies during the autumn.
An oak-beech woodland in late winter. Note the large amounts of dead organic matter on the floor.
By autumn the change in light and temperature cause few species of plants to grow and the woodland floor becomes the domain of fungi. These are largely saprophytes, feeding off dead matter and at this time there is plenty of leaves falling from the canopy.