Root competition and specialist adaptations

The roots of individual plants compete with those of the same or other species when foraging for water and essential nutrients. The stratification shown by the aerial parts of forest plants is paralleled by the vertical zonation of their roots as described in Chapter 1, those of the largest plants usually penetrating to the greatest depths. Vascular plants use their roots to forage for water and mineral nutrients and also to anchor them in the ground. Trees tend to split this function: the shallower feeder roots absorb most of their mineral nutrients, often assisted by mycorrhizal associations (see Chapter 5), while the deeper roots take up water and anchor the trunks. This vertical layering is made use of in agroforestry where trees and crops are grown together with the assumption that the crops are using water near the soil surface and the trees are using deeper water, maybe 20-30 cm down. However, this assumption has never been fully proved and inevitably there is still some competition between the crops and trees; crop yield is normally lower near the trees. Although root layering may diminish competition between herbs and trees it certainly does not eliminate it, so competition can be very important in the success of establishing young trees. Bare areas in many young beechwoods result from the permeation of the ground by their roots as well as the heavy shading of an almost unbroken canopy: seedlings face stiff competition for resources both above and below ground. The mosaic of plants on the forest floor is strongly influenced by the availability of light in space and time, but variations in humus and nutrient contents, soil pH, aeration and moisture also influence the distribution of shrubs, herbs and bryophytes, which can often be used as indicators of environmental conditions.

Modern studies have placed an increasing emphasis on the importance of below-ground competition. Working with seven herbaceous understorey species from an unproductive old field in Michigan, Rajaniemi et al. (2003) found that increased productivity resulting from fertilizer application was associated with a loss in plant diversity. These changes were caused almost exclusively by root competition; above-ground competition from more vigorous growth had small effects on the community but did not contribute to changes in diversity. This has important implications for forests; indeed in Britain NPK fertilizer drift from fields caused by the wind has had an adverse effect on many woodland margins, whose soils now support larger populations of stinging nettle Urtica dioica, a strongly competitive species, and have rather lower biodiversity, than they did a few decades ago.

An important aspect of root competition is the ability of a root to determine whether another root is from the plant to which itself belongs: self/non-self root discrimination as Falik et al. (2003) put it. They used experimental pea Pisum sativum plants that had been pruned to leave just two roots. These were planted in small pots such that a pot contained two roots either of the same plant or of two different plants. When such a plant was put in a pot by itself, root development was significantly less than when the two roots within a pot came from two physiologically different plants. These results point to an ability to respond to competition from roots of another plant (by growing more vigorously towards the offending roots in preferentially exploiting the soil) while avoiding it between its own roots; and, since the roots respond to each other without actually touching, to the production of a signal of some kind that allows this discrimination. Evidence suggests that this also happens in a range of other plants and it will be interesting to see just how widespread this self recognition is in forest plants.

Gersani et al. (2001) developed a game-theoretic model that considered the effects of intra- and inter-plant competition (i.e. within one plant and between plants) on root proliferation and reproductive yield. They predicted that if soil space and resources per individual were held constant, then plants would produce more roots and give less reproductive yield (fewer seeds) per individual as the number of plants sharing the combined space increased. This theory turned out to be compatible with the results obtained when tested using containers of soybean Glycine max plants grown both in isolation and together under glasshouse conditions. Even though plants growing together each had the same volume of soil as when growing alone, sharing resulted in more roots being grown. Sharing plants grew 85% more root mass (weight) but since overall plant mass did not change this was at the expense of above-ground growth. Sharing plants also produced 30% less mass of seeds. The implications of different types of root behaviour are important; these authors suggest that the plants appear to invest their resources effectively and make greater root growth when there is competition; different roots and parts of a plant assess and respond to opportunities in a way that maximizes the good of the whole plant. If a plant is operating as a co-ordinated whole it should first produce roots in unoccupied soils, then in soil occupied by a competitor (as found by Falik et al, 2003), and lastly in soil where its own roots are already present.

Narrowly endemic species (i.e. those found only in a particular geographical region) are often restricted to distinctive edaphic (soil) environments, as in the case of two species of hakea (Hakea oldfieldii and H. tuberculata) which occur in winter-wet shrublands growing on skeletal soils 0-20 cm deep overlying massive ironstone rock in Mediterranean south-west Australia. This area is the major centre of diversity for this Proteacean genus, which consists of woody perennials ranging in size from shrubs to small trees. Poot and Lambers (2003) studied these together with five other species of Hakea found on more common soils (including two from eucalypt woodland) growing their seedlings in pots 40 cm deep and making harvests at 62,125 and 188 days. Initially the ironstone endemics allocated a significantly greater proportion of dry mass to their roots, although this difference evened out later. At the last harvest the two rare ironstone endemics had an average of 64% of their roots in the bottom 10 cm of the pots, as opposed to a mean of 35% for the other five species. These two species had considerably greater root lengths for a given plant mass because their average root diameter was less; only in these two species did the main root axis continue to grow at the same rate after reaching the bottom of the pot. Specializations shown by the two ironstone endemics undoubtedly increase the chances of obtaining access to water before the onset of severe summer drought in their native habitats. In more common soils these same traits, which compromise competitive ability both above and below ground, are likely to reduce the chance of survival.

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