Once a person realizes that plant diversity drops as a forest matures, it becomes easier to understand how selective logging can alleviate the problem and lead to increased plant diversity. Much of this has to do with how selective logging is conducted and what happens as a result.
Selective logging overview
Selective logging is the forest harvest practice of removing only certain trees in a forest, while leaving others there. Sometimes this means that the forest is thinned out, allowing the trees that are remaining to become larger than they would if they remained in a cramped state. Other times, selective logging might be to select certain trees that are good-sized, while not removing the small trees or those that are fully mature and producing seeds. In either case, only certain trees are removed, leaving the others to continue growing.
Causes of diversity decline
Though there is more than one reason that the diversity in a forest declines as it gets older, a common and major cause of the reduction in the number of plant species the forest supports results from the amount of sunlight that is received on the forest floor. As trees grow larger, they tend to shade out the ground. Plants that normally require large amounts of sunlight can no longer grow if the shade becomes too dense. This can greatly reduce the number of plant species found, and the problem gets worse as the trees continue to get larger. Eventually, only shade-loving plants, like ferns and some grasses, are in evidence.
Additionally, as the trees grow larger, they also normally drop more leaves or needles yearly. At first, the leaf litter is small enough that it normally decomposes from one year to the next. However, as the trees become larger and the amount of yearly leaf litter grows, it tends to build up, layer upon layer, making it more difficult for seeds to reach the soil and germinate.
Opening the forest
In the process of removing trees selectively, two things often occur. First, though care is usually taken not to cause more damage than necessary, the falling of the timber and necessary action to remove it usually disturbs the leaf litter, creating patches of bare soil here and there where the leaves and needles have been moved. Because of this, seeds that didn't previously have a chance to work through the dense vegetative mat are suddenly able to reach the ground, where they can germinate and take root.
As importantly, the removal of select trees creates openings in the forest canopy. This means that more sunlight is able to reach the forest floor. Plants that are tolerant of limited shady conditions are able to grow and flourish again. This is seen in tropical forests at an accelerated rate, when a tree falls because of rot or decay. Within a short time, the forest often takes advantage of the additional light, and plants that need the sunlight have a chance to dominate, at least in a small area.
A combination of the increased ability for a wide range of plant species to germinate and grow, along with the increase in sunlight, means that the number of species that can and usually will grow, increases. Plant diversity climbs greatly in a relatively short amount of time.
As related by a retired silvaculturist for the U.S. Forest Service, named Steve, selective logging is a method of choice in many forests, primarily because there is less maintenance involved in replanting and clean-up. Though there are some people who are opposed to this harvest method, selective logging can save the forest service money and time. At the same time, he acknowledged that, within a couple years of selective logging a given forest, the number of plant species that began growing increased enormously.
Selective logging is a good method that attempts to balance harvest with growth. In the process, it opens the forest up to sunlight and disturbs the forest floor so that a greater number and variety of plants can grow. This is good all the way around, and more people are becoming aware of it.