Role Of Insects In Forest Succession

Although trees can live for thousands of years (e.g., giant sequoia, Sequoiadendrongigantea), natural forests are dynamic plant communities that can change very slowly over thousands of years or very quickly over a few days to a few years. Fires, volcanic eruptions, strong winds, and snow and ice often have dramatic effects on forest succession. Insects can also have this effect during outbreaks (referred to as epidemics or gradations). For example, bark beetles, together with their associated bluestain fungi, are known to kill millions of trees over thousands of hectares. The forest shown in Fig. 1 may change from an old

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FIGURE 1 Mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Scolytidae), killed lodgepole pine, Pinus contorta var. ldtifolia, in 1982 in Glacier National Park, Montana. [Photograph by Mark D. McGregor, www.forestryimages.org.]

growth (ca. 80 years), even-aged, predominantly lodgepole pine forest to a younger forest of lodgepole pine if a fire occurs subsequent to the mountain pine beetle infestation. Lodgepole pine cones often remain closed for many years and thus can store a large seed crop on the tree. Following a fire, which opens the cones, an almost pure forest of lodgepole pine is established over very large areas. Thus, the stage is set for another mountain pine beetle outbreak in 80 to 100 years.

Insects and fungi are the most important biotic agents that affect forest succession. In general, these organisms influence the number and growth rate of trees through space and time. Insects that feed on seeds influence reproduction of trees, shoot-feeding and defoliating insects influence growth rates of trees, and insects that infest the main stem of trees accelerate the mortality of trees in a forest. All of these effects occur through interactions with other biotic and abiotic agents as part of the complex forest cycle that involves gaps, building, and mature and degenerate phases. Insects therefore impact forest succession by influencing the forest cycle. The changes in these parameters affect the distribution and abundance of trees at a given moment in time and can be projected into the future using mathematical modeling techniques.

FIGURE 2 Diagrammatic representation of the plant life cycle, illustrating a tree as a series of modular units (the shoots). There are four basic components in the life cycle: (I) the seed bank, (II) the recruitment and establishment of individuals of the population from the seed bank, (III) the growth of individuals, and, (IV) reproduction and dispersal. (Reproduced from Coulson and Witter, "Forest Entomology: Ecology and Management. Copyright 1984. This material is used by permission from John Wiley & Sons, Inc.)

FIGURE 2 Diagrammatic representation of the plant life cycle, illustrating a tree as a series of modular units (the shoots). There are four basic components in the life cycle: (I) the seed bank, (II) the recruitment and establishment of individuals of the population from the seed bank, (III) the growth of individuals, and, (IV) reproduction and dispersal. (Reproduced from Coulson and Witter, "Forest Entomology: Ecology and Management. Copyright 1984. This material is used by permission from John Wiley & Sons, Inc.)

Insects affect the growth rate and mortality of trees by feeding on various parts of the tree. Thus they affect the life cycle of trees (Fig. 2) by influencing the size of the seed bank, the amount of recruitment, the growth of individuals in height and volume, and the reproduction and dispersal of the tree species. Insects consume seeds within fruiting structures or on the ground, thus reducing the size of the seed bank. They consume young seedlings, thus decreasing the recruitment of new trees into the population. They kill tips and shoots, suck plant fluids from the phloem and xylem, and consume the foliage, thus reducing the photosynthetic capacity of the tree and, as a result, reduce the growth of individuals. Insects also kill trees by reducing their growth rate so they cannot compete with other individuals of the same or different species for light, space, water, and nutrients. Trees are killed quickly, often in a few weeks, following severe defoliation by moths or sawflies and by bark beetles that introduce pathogenic fungi into the phloem and xylem. Direct and indirect interactions occur between insect species and between insects and forest pathogens. Root disease pathogens may indirectly affect bark beetle populations by weakening trees and thus predisposing them to attack by bark beetles. Direct interactions with pathogens may occur when, for example, insects transmit the pathogen, as with the pitch canker pathogen, Fusarium circinatum, in California and the Dutch elm disease pathogen, Ophiostoma novo-ulmi, in Europe and North America.

The examples drawn upon in this article emphasize those insect taxa that affect the survival of trees and in turn influence successional patterns of forests. This treatment thus includes phytophagous species and their natural enemies. Some authors refer to this interaction between insects and plants as a predaceous or parasitic relationship. The predator kills the host but the parasite does not kill the host directly. Insects may also form commensalistic and mutualistic associations with trees. In a commensalistic interaction, the tree is not affected, for example, when it is used as a resting place, whereas the tree benefits in a mutualistic interaction, such as with a pollinator. Our objective then is to emphasize the negative interactions between insects and their host trees, in which only the insect benefits and the tree is debilitated or killed. The tree does, however, benefit in the long term at the population level, at which succession allows nutrient turnover and reproduction allows adaptation of the tree species to environmental changes.

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