Negative Density Dependence and Forestry

The diversity of perspectives and backgrounds in this course is astounding. Every person in the course, either as a participant or as an instructor, has such varying interests and these become quite obvious during group hikes and discussions. What people notice, what people pick up on, is interesting enough on its own, not even considering the sheer volume of information we collectively possess as a group.

A few questions have come up for me, since we’ve covered such a range of topics; sometimes it’s difficult to keep track of it all. Something that has come up frequently is negative density dependence in plants. This appears to take place in both temperate and tropical climates; it is where seedlings of a particular species, X, have a lesser chance of surviving if they are surrounded by other individuals of their own species (i.e. there is low dispersal distance from the mother plant for its seedlings). Host parasites may use the mother plant, or high density of individuals from species X as a cue to parasitize the organism. Once this occurs, these individuals are quite vulnerable, especially given their close proximity to others of the same species.

This observation has significant ecological implications, in that it can explain the species diversity observed in a particular area of forest, and why forests aren’t simply dominated by a single species. Negative density dependence has significant implications for the forestry sector as well. As a former tree planter, I know that reforestation companies re-plant tree species that are a) economically valuable, b) fast growing, and c) often a single species. These, sometimes vast, monoculture plots are therefore highly vulnerable to pests. This explains why the seedlings are frequently sprayed with herbicides and insecticides prior to planting. Monocultures are obviously poor business models for a number of reasons, but perhaps an attempt to diversifying the trees planted could help reduce yield losses and make the stands more resilient to pests. How likely this is to occur, however, is questionable at best. Another approach would be to increase the distance between trees planted, if they are the same species, to reduce negative density dependence. This would, again, be logistically quite difficult, given that tree planters are paid by the number of trees they plant and increasing the distance between trees planted would dramatically reduce their (already sparse) pay.

I don’t really know of any answers to this question, apart from the obvious support for selective cutting and planting regimes, as opposed to clear cutting approaches in forestry. There have been many cases of small-scale sustainable forestry management, where trees are selectively harvested and planted to maintain a relatively diverse and stable ecosystem. How this may be expanded to the large-scale forestry companies, and if there is interest in doing so, is a much greater question.



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