FunNature & AnimalWhat impacts does tree monoculture have?

What impacts does tree monoculture have?

Contrary to the false perception that a forest is simply a place with many trees, in reality they are complex ecosystems that, in addition to a well-developed tree canopy, present other layers of vegetation, fauna and associated microbiota, a structure and a network of complex relationships They are the product of a long process of succession.

A land planted with a large number of trees, which is maintained artificially, in order to exploit its resources —wood, food or any other product—, is not a forest, but a crop. And many of those tree crops, which occur in regular, defined rows, are made up of a single species. They are forest monocultures .

effects on water

A direct effect of monoculture, when compared to a wild community, is the use of water resources , which is significantly higher, although it depends on the planted species.

In general, a wild environment has different plant layers, which means that the water evapotranspired by the lower layers is retained by the ecosystem itself and returns to precipitate in the form of condensation and dew. Only the water evapotranspired by the tree cover is lost to the atmosphere. However, in a monoculture of trees without lower plant layers, all the water absorbed by the plants is evapotranspired by the tree cover, an event that favors the loss of water from the soil.

If the monoculture is of a tree that is particularly efficient at absorbing water from the substrate, the environmental impact can be overwhelming . This is the case of eucalyptus, an invasive species widely used as a monoculture in Spain and in many other countries, which has traditionally been used to drain wetlands.

Effects on Nutrients

Furthermore, tree monoculture, compared to mature forests, oversimplifies the carbon cycle. In a natural forest, the carbon taken up by plants accumulates in the soil very efficiently when the plants die or lose their leaves. Although in general forests are inefficient as

carbon sinks, they are exceptional as stores, capable of retaining carbon for a long time . However, monoculture plantations not only have low carbon sequestration efficiency, but also store much less compared to unmanaged primary forest.

On the other hand, the mature forest usually has a complex and highly specialized microbiota in its soil, with multiple types of bacteria and fungi associated with the ecosystem, including nitrogen-fixing bacteria and mycorrhizae . Nitrogen-fixing bacteria obtain atmospheric nitrogen and include it in the nutrient cycle. Mycorrhizae, for their part, are symbiotic associations between certain species of fungi and the roots of some plants, which play a major role in the absorption of nutrients such as calcium, magnesium, phosphorus or potassium.

Monocultures, on the other hand, tend to have a poor and unspecialized microbiome , so nutrient cycles are, in general, much less efficient. This makes tree monocultures very unproductive in terms of nutrients and, far from enriching the soil, often impoverishes it.

Biodiversity issues

Perhaps the most relevant impact of monocultures is the effect they have on biodiversity. A natural ecosystem generally has a great wealth of species that are distributed in complex food webs. Even when a forest is dominated by a single tree species, it maintains a rich understory and abundant associated fauna.

However, in a monoculture of trees, the complexity of the ecosystem is reduced to an extreme. Plant layers and primary productivity are lost, and with this, a large part of the fauna is forced to move or disappear and food webs are simplified.

The species richness, not only of plants, but also of insects, reptiles, mammals and birds in a monoculture is minimal compared to a forest. Its composition is severely affected by changes in the structure of the plant community.

These impacts can be partially mitigated with adequate management and good management of the space . It would be desirable to avoid the use of invasive alien species and to conserve patches of the original primary forest , embedded in the plantations, and of a sufficient size to serve as a refuge for biodiversity. Mixed crops , which include several species of trees and not just one, also tend to have better results.

But these strategies only mitigate the impact, they do not solve the problem . If we really want to preserve the biodiversity of natural environments, it will be necessary to propose a much more drastic change , which will have repercussions on the socioeconomic system, from its base.

References:

Cannell, M. G. R. 1999. Environmental impacts of forest monocultures: water use, acidification, wildlife conservation, and carbon storage. En J. R. Boyle et al. (Eds.), Planted Forests: Contributions to the Quest for Sustainable Societies (pp. 239-262). Springer Netherlands. DOI: 10.1007/978-94-017-2689-4_17

Iezzi, M. E. et al. 2018. Tree monocultures in a biodiversity hotspot: Impact of pine plantations on mammal and bird assemblages in the Atlantic Forest. Forest Ecology and Management, 424, 216-227. DOI: 10.1016/j.foreco.2018.04.049

Marron, N. et al. 2019. Are mixed-tree plantations including a nitrogen-fixing species more productive than monocultures? Forest Ecology and Management, 441, 242-252. DOI: 10.1016/j.foreco.2019.03.052

Rożek, K. et al. 2020. How do monocultures of fourteen forest tree species affect arbuscular mycorrhizal fungi abundance and species richness and composition in soil? Forest Ecology and Management, 465, 118091. DOI: 10.1016/j.foreco.2020.118091

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