Monoculture, species diversification, and disease hazards in forestry.

Authors: C. K. S. Chou
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 1981, Issue N.Z.J.For. 26(1) 1981, pp 20-36, May 1981
Publisher: New Zealand Institute of Forestry

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Abstract:
The large-scale establishment of even-aged pure stands, commonly known as monocultures, often arouses serious objections on grounds of increased disease and pest risks. A critical survey of literature showed little to support such a view; serious diseases have occurred in mixed and exploitable natural forest, while there is no clear evidence that outbreaks of diseases in pure stands can be ascribed to lack of species diversity. The theoretical basis of such a view is also questionable. There are, first of all, no definitions that adequately characterise typical epidemics, mixtures, or pure stands, so it is hardly possible to make broad generalisations about the effect of stand composition on disease. The very concept of disease occurrence viewed, say, as a "triangle" of host, pathogen, and environment, precludes overemphasis on any one factor in isolation from others.
An undisturbed natural forest or ecosystem may be stable within a certain period. The principles governing such stability are still little known, but it is certainly reasonable to assume that an understanding of such principles is highly important to our forestry practice. It can be questioned, though, whether they are all applicable to man-made systems. At present there is little reason to assume that any departure from "nature" must increase disease hazards. Besides, the use of monocultures need not entail sacrificing the tree-to-tree genetic variation that provides some protection from major disease losses.
There is a tendency to exaggerate the advantages of diversifying into a range of major production species to the point of ignoring its disadvantages, difficulties, and its sheer practical unrealities i" some countries. For example, increased importation of planting material for diversification may increase disease risks by introducing pathogens. We have witnessed this over the last two decades. Moreover, with a greater number of tree species and therefore of potential disease problems, one must either increase the cost of
*Forest Research Institute, Rotorua.
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MONOCULTURES

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plant protection or give less attention to any one species. The role of diversification in pathology can be very negative if the ultimate goal of pathology is to allow us to grow a good healthy crop where and when we want it.

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