Understorey plants are by definition those living below the tree canopy. Unless the canopy is very open, as in birch woodlands, light will be in short supply for most of the growing season (see Section 1.3.2 above), hence it is referred to as the dark phase of the year. Deciduous canopies will go through the light phase in the non-growing season when the leaves have fallen but, of course, most understorey plants will not then be growing. Evergreen trees have a year-round dark phase.
In the English Midlands the light phase usually lasts at least 5 months and is referred to by Rackham (1975) as the bare half-year, the dark phase corresponding to the leafy half-year. It is no accident that temperate deciduous woods are floristically at their best early in the year; many 'pre-vernal' plants gain extra light by growing in early spring before the trees unfurl their leaves -see Box 3.1 and Fig. 3.9 which show the spring-time phenology of other species.
Lack of light is not the only problem for understorey plants. Water can also be in short supply; beech forests are often bare below as much from intense water competition as from the dense shade of the canopy. Pollination can also be problematic. Wind speeds are drastically reduced below the woodland canopy, especially when in leaf. In British woodlands the most prominent wind-pollinated herb is the summergreen dog's mercury Mercurialis perennis which flowers early in February and March when wind speeds are relatively high. Most woodland herbs are insect pollinated and face the problem of being found by insects in a dark woodland. The solutions are to flower before the canopy closes (the commonest option), produce pale or white flowers to show up in the gloom (such as wood sanicle Sanicula europaea) or fragrant flowers (such as wood sage Teucrium scorodonia).
The bryophytes of the ground layer are remarkably tolerant of dry, dark conditions and grow as and when conditions are suitable. Their main problem is being swamped by falling leaves in the autumn which, under trees with hard acidic foliage such as beech, sycamore and other maples, may effectively put them in the dark for much of the year. In such a woodland, the bryophytes are largely restricted to rocks, raised roots and dead wood from which the leaf litter readily slips or is blown.
Herbivory can be a distinct problem for wintergreen and evergreen plants since in the winter they are a scarce source of food within reach of hungry animals. In such situations, investment in chemical and physical defences is advantageous, in line with Grubb's scarcity-accessibility hypothesis (Grubb, 1992). This states that such defences will be grown where the plant is either scarce or readily accessible to herbivores and the cost of investment in defences is less than the cost of regrowth or if death is likely. This may explain why the European holly, a typical evergreen understorey tree, invests in leaf prickles to deter deer in winter (most hollies around the world are deciduous and prickle-less), and why the slow-growing evergreen yew invests in potent chemical defences. It also helps explain why holly loses its prickles above around 3 m, above the height to which native herbivores can reach.
Having outlined these numerous problems, we are left with the question of just why so many plants do grow under trees. The simple answer is that they often grow in woodlands because they cannot grow elsewhere. Woodland plants can cope with strong shade and the other limitations imposed by a tree canopy, but would not survive the intense competition which they would meet in the denser field layers of other vegetation types.
Vegetation mosaics of different scales exist in all forests and woodlands. These develop as a consequence of gap formation and the revegetation of former gaps, whose size varies from that created by the death of a single tree up to landscape scale openings created by agents such as hurricanes and forest fires. There is also horizontal variation in light and water caused by the mosaic of tree canopies. Thus oaks can crowd together with small gaps between them while ponderosa pines in dry areas of western North America are widely spaced due to root competition for water. Variations in soil nutrients, pH, moisture and aeration, together with herbivore grazing patterns (from insects killing particular trees or shrubs to deer creating large grazed lawns) also have an effect. Even apparently uniform plantations contain small-scale variations in conditions leading to patches of understorey plants and insects. It should not be surprising, therefore, that on the scale that we experience when walking through woodlands, some are seemingly uniform while others are variable over very short distances.
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