Post from Graeme Flett on November 4, 2016 at 9:26PM
Monocultures are natural
From New Zealand Tree grower August 2016
Some environmentalists have criticised plantations because they are almost always monocultures. Monocultures are claimed to be unnatural. Are they? Is there evidence that monocultures do not occur in nature.
This question was discussed at length in the excellent publication by Piers Maclaren in Environmental Effects of Planted Forest 1996 – FRI Bulletin 198. There are many examples throughout the world of naturally occurring monocultures. Our own beech forests, for example, tend to be natural occurring monocultures.
It is relevant to recall the work of Jones in the mid-1940s. E W Jones, a lecturer in silviculture at the Oxford School of Forestry in the 1950s and 1960s, was a passionate supporter of selection system of forest management – a mixture of species and age classes as practised in Switzerland and France. In some ways this management system is the equivalent to our current continuous cover forestry.
In both management systems there is no clear felling. In the 1940s Jones evaluated the few European forests that had not been ‘devastated or changed by man’. Because there were so few examples of untouched European forests, Jones extended his study to include North America. In the northern, central and southern USA there were few virgin forests left but there were still large areas left in the west. Jones, much to his surprise, observed that ‘...all aged forests with irregular canopy answering to the forester’s picture of selection forest type appear to be rare’.
Jones records that most of the ‘virgin’ forests were even-aged monocultures or near monocultures. Jones reasoned that the prime cause of the resultant forest structure was fire caused by lightning strikes. My interpretation of these observations by Jones is that virgin forests ultimately end in catastrophe, such as by fire, insect outbreak, hurricanes or volcanic eruptions. Jones concluded that the concept of climax vegetation is ‘... a concept only, never existing in practice...’.
Although Jones was my lecturer in silviculture at Oxford I do not recall him including these research findings in his lectures. It was much later that I became aware of his earlier publication. Jones’s findings are probably applicable to temperate and boreal forests and not tropical forests.
Using resources better
The Piers Maclaren bulletin evaluates the question – Are monocultures at greater risk from catastrophic epidemics than mixed species plantings? Piers cites many examples, both in New Zealand and overseas, that ‘stands of mixed species do not necessarily provide protection to individual species within those mixtures’. He also makes two very appropriate comments –
- ...researchers can concentrate scarce resources on addressing fewer species of pathogens
- ... any major threat to the radiata pine industry would be counted by the full resources of the forest sector. If New Zealand’s commercial forest estate was fragmented into many species, the response to a pest or disease to one type of tree would be less than total and therefore be less effective.
Another reason why a single species plantation might be healthier than mixtures is the likelihood that the stands managed for maximum productivity would impose minimum stress on the individual, therefore ensuring that natural immunity is high. The ability of individual trees to resist disease is promoted by some management practices, for example, site preparation, fertilisation, thinning and pruning.
Plantation mixtures of many tree species were trialled in the initial plantings by the Forestry Division of the Lands Department – the forerunner of the Forest Service.Almost all mixtures were complete failures – one species came to dominate the plantation.The best example of one species eventually dominating is the redwood stand at Whakarewarewa, perhaps the most photographed forest stand in New Zealand.
The stand was originally planted in larch at four feet by four feet.Then, every fourth larch tree in every fourth row was replaced with a redwood. Soon after planting the larch probably grew faster than the redwoods only to be overtaken. Now almost none of the initial larch plantings remain.This also explains why the redwoods that still exists are at a minimum of 16-foot spacing, about five metres.
Monocultures are natural forests. Mixtures rarely stay as mixtures as one species soon comes to dominate. Monocultures are at no greater risk than mixtures. Indeed, they may be at less risk.
Disclaimer: Personal views expressed in this blog are those of the writers and do not necessarily represent those of the NZ Farm Forestry Association.