| | Plants [TTj Decomposers
| | Plants [TTj Decomposers
Increasing resistance to herbivory
Figure 5.1 Scheme predicting the changes in abundance of trophic groups along a gradient between low to high productivity (a-c). The area of each section corresponds to its abundance. (Redrawn from Grime, 2001. Plant Strategies, Vegetation Processes and Ecosystem Properties. John Wiley & Sons.)
affect how many and what types of herbivorous animals are able to live. Of course, this is not a one-way process: the herbivores also affect the vegetation. Some effects are adverse for the plant - grazing and trampling - while others are positive - pollination, dispersal of fruits and seeds, and even more significantly, promoting nutrient cycling (see Section 8.3). These mutual influences result in an evolutionary stalemate where plants evolve defences which prevent decimation by herbivores, and the herbivores evolve ways of breaching these defences.
An interesting question to ask is whether these herbivore-plant interactions are sufficient to regulate the numbers of herbivores: does the quantity or quality of available food limit herbivore reproduction or survival? Or, are herbivore populations regulated by parasites and/or carnivores (predator-prey interactions). Such questions, whether control is bottom-up or top-down, have greatly exercised ecologists over the last century and, as yet, no real consensus exists (see Lindstrom et al., 2001). However, of clear practical importance is what happens when the balance between plants and herbivores is disturbed by unusual conditions such as dry years or a sudden genetic change in one party (such as the
Dutch elm disease fungus) or the introduction of an exotic species that has left its controlling factors behind: these are discussed later in the chapter.
Defoliation by insects and other animals has complex side effects; it increases light penetration through the canopy, reduces competition for abiotic resources such as water and nutrients, leads to alterations in plant species composition (discussed later), increases the rate of nutrient leaching from foliage through damage, and accelerates the rate of fall of nutrient-rich litter into the decomposition subsystem. The cycling of nutrients (abiotic flux) is enhanced because defoliation stimulates redistribution of nutrients from boles and branches to components with high turnover rates such as leaves and buds. It can thus be argued that defoliation, paradoxically, promotes more consistent plant production in the long term than would be expected.
It is clear that defoliation affects not just the forest but also the plant itself. A number of deciduous trees show a burst of new lammas growth in late summer (named after Lammas day, the first day of August when the festival of first fruits was formerly celebrated). This is usually ascribed to the need to replace leaves destroyed by insects, and allow extra growth once the insects have pupated. Whether subject to insect attack or not, deciduous trees form new leaves throughout the summer, either continuously as in ash, or in the
Figure 5.2 Mean diameters of young trees of pedunculate oak Quercus robur measured at the ends of the years 1980-85. They were grown in the open at a spacing sufficient to avoid competition but were subject to artificial defoliation. Trees (b), (c) and (d) were defoliated in the years 1981-3 (shown hatched) but not subsequently. (a), control, not defoliated. (b), one-third defoliated. (c), two-thirds defoliated. (d), totally defoliated. (From Hilton et al., 1987. New Phytologist 107. Blackwell Publishing.)
distinct determinate flushes found in oaks, whose second flush of yellowish leaves is usually produced towards the end of June, and may be followed by a third at the beginning of August. Hilton et al. (1987) investigated the effect of defoliating oak seedlings at three different levels in mid-June for 3 successive years when the leaves had fully expanded. Defoliated plants differed from the controls in four major respects having: (1) earlier production of the lammas growth, together with the formation of more lateral branches which were susceptible to winter frost damage; (2) production of smaller but more abundant leaves; (3) reduced diameters of main stems (Fig. 5.2), from which were calculated relative growth rates (RGR) which varied both with defoliation treatment and growing conditions over the year and which returned to normal values as soon as defoliation ceased; (4) formation of wood with many vessels and a lower than normal proportion of xylem fibres, similar to the wood normally produced in spring.
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