2.3.1 Variation in root systems
The radicle (embryonic root) of a seedling often continues to grow downwards as a primary or tap root, but in most trees the main root system consists of secondary lateral roots which grow sideways away from the stem, usually no more than 1-2m below the soil surface but with some deeper roots. Different species have different rooting patterns and in this respect three common European conifers differ greatly in their ability to adapt to varied soil conditions. Young silver fir Abies alba has the least branched roots, and the adult has a deep root system with a dominant tap that does not adapt well to shallow soils. In its first year Scots pine Pinus sylvestris has the longest primary and the largest number of spreading secondary and tertiary roots. Its many small roots enable it to flourish in dry barren soils, yet the great plasticity of the main root system, which can be reliant on long laterals but whose tap root can penetrate very deeply in suitable soils, allows this tree to adapt to diverse habitats from dry sand to wet peat. The third species, Norway spruce Picea abies has a mature root system composed of shallow laterals and its primary root stops growing after 5 years; consequently this tree is commonly uprooted by wind (Fig. 2.5) but with its widespread root system it does well on a wide range of soils. The rooting patterns of different species will vary depending upon what other trees are growing near: for example a number of studies in Germany have shown that when Norway spruce or ash Fraxinus excelsior are growing with beech Fagus sylvatica, the spruce and ash tend to produce their fine roots more shallowly than when grown alone, and the beech produce fine roots more deeply.
In most forests, the canopy of each tree is discrete from surrounding trees. This 'canopy shyness', first mentioned in Chapter 1, is caused primarily by physical interference of approaching branches as they knock into each other when blowing in the wind. In the relative stillness below ground, however, roots are not contained in the same way and continue to grow past each other. In temperate trees the spread of roots away from the trunk is around 2-3 times the width of the canopy, and up to four times on dry sandy soil. This wide spreading of the roots (with a radius of 30 m or more in the case of an oak) is what allows the tree to absorb enough water and nutrients to keep its huge structure functioning. However, the strategy for optimizing uptake of these essential components varies considerably between different trees. The ash has very long branched lateral roots that exploit a large volume of soil. Trees like this use their long coarse roots to extend great distances into large volumes of soil, and have extensive root
systems. These root patterns work well in winter-rain regions where tree roots, unlike those of grasses, can draw water from great depths in dry summers; they are also very effective in stony soils whose water is not uniformly distributed. Trees with intensive root systems rely on the very effective use of the water in a much smaller volume of soil; as exemplified by the beech Fagus sylvatica with its shorter laterals with numerous short and extremely fine terminals. Though both these systems function well under normal conditions, beech suffers more than most trees in drought and in southern England its stands were badly affected by the very dry summer of 1976. This suggests that having absorbed all the water near it, the tree does not extend its laterals toward remaining damp soil quickly enough. Beech does have a couple of insurance policies. On easily penetrated rocks, such as fissured chalk, roots can grow many metres down and extract water from nearer the water table. Also beech can grow internal roots within damp, decayed and hollow trunks allowing extra water absorption. A very large internal root system was present in a tree that blew down in the Wyre Forest in central England some years ago. These are examples of adventitious roots, roots which arise directly from pre-existing stems. Internal roots are also found in a variety of other temperate trees such as elms, yews and large trees of yellow birch Betula lutea in humid North
America but especially in tropical trees. Undoubtedly these roots give the trees a competitive advantage by allowing extra water uptake and recycling of nutrients from the otherwise not very useful body of dead heartwood.
Adventitious roots above ground have several other uses in forest trees. A number of rain-forest trees, both tropical and temperate, produce canopy roots from the trunk and branches which grow down the outside of the tree and exploit humus pockets collected by the numerous epiphytic plants. These roots can also reabsorb nutrients that have been leached out of foliage higher up and washed down the tree. More widespread around the world, some trees and shrubs develop adventitious roots on stems touching the soil and this can result in vegetative reproduction. This layering often gives rise to plants that eventually become functionally - and often physically - independent of the parent. An extreme case of this is seen in the layered beech at Arley Arboreum, Worcestershire, England where an erect tree planted at least two centuries ago has now completely disappeared, but its layered remains form a contorted series of vigorous trunks and branches that now cover around a quarter of an acre (0.1 ha). Roots above soil level are, of course, used by a variety of epiphytes (see Section 5.5.1). Aerial roots of epiphytic orchids possess an outer covering (velamen) of dead cells that absorbs water from the atmosphere; these roots can also carry out photosynthesis. Northern rata Metrosideros robusta trees in North Island, New Zealand, start their life as epiphytes, grow to the ground, and eventually outlive and replace the host trees.
A consequence of tree roots spreading so far is that in a forest they inevitably intermingle. For example, in the mixed hardwoods of Harvard Forest, Massachusetts, Lyford and Wilson (1964) found the roots of 4-7 trees below the same square metre of ground and mostly close together. Where these roots touch, they are likely to fuse together. These root grafts commonly weld the lateral roots of an individual tree into a rigid root plate which helps hold the tree up. But equally, such grafts can join together the below-ground systems of adjacent trees of the same species and, more rarely, different species. These are sometimes responsible for the spread of disease (such as Dutch elm disease -see Section 5.4.5), while the remaining connected tree will sometimes take over the root system of a neighbour whose trunk has been cut down. A thinned conifer plantation south of Exeter in south-west England came to contain a healed stump devoid of leaves, which contined to live and to increase slightly in girth. Such stumps are clearly receiving photosynthate from neighbours to which their roots are joined. Cases like this also occur in Douglar fir plantations in New Zealand. Such stumps have been kept alive via root grafts for several decades. It is interesting here that rather than competition allowing the stronger individual to take from the weakened stump, the exchange is from the dominant individuals to the underdog, but probably not enough to disrupt the overall dominance pattern. Cut-down hardwoods would of course recover by coppice growth but may be equally helped by surrounding uncut trees.
Adding all these factors together can result in impressive interactions between trees and soil. Lateral pressures exerted by hurricanes and other strong winds on tall trees are very great and so their roots have to be correspondingly strong. Although the coastal redwood Sequoia sempervirens can reach a height of 115m and the giant sequoia (big tree) Sequoiadendron giganteum 95 m, these closely related species native to California have shallow root systems that rarely go deeper than 2 m from the surface. Despite this, available soil resources are effectively exploited by roots which often spread tens of metres from the trunk. An undisturbed layer of thick damp mulch on the forest floor is beneficial to the health of both these trees that take their mineral nutrients and 85-90% of their water requirement from the soil, the remaining water being absorbed directly from the atmosphere by the leaves.
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