The biodiversity of young forests vs. mature forests

A myth perpetuated in the 70s, and later, had it that old growth forests are more biologically diverse than new growth forests. The truth is actually the opposite. The more mature a forest becomes, the less diversity of plants and animals that are supported. It really isn’t hard to see why this is the case.

Defining bio-diversity

The meaning of bio-diversity can be confusing to many people. It isn’t hard to figure out that it refers to the number of plant and animals found, however it refers to the number of species rather than to the number of individuals. This distinction is important in understanding why aged forests aren’t as diverse as new forests. They may contain a large number of individuals, yet may have very few different species, and thus are not very diverse.

Forest evolution-new growth

Most forests go through a predictable sequence. Young forests are normally those with fast-growing trees and bushes that flourish in places that get abundant sunshine. Aspen, cottonwoods and willows might be among the first plants to grow, along with a huge number of species of bushes, grasses, annuals and perennials. These are usually plants that love sunshine. 

Animal life is as varied as the plants at this point. The insects, other invertebrates, reptiles and amphibians that make the early forest their home, multiply. The number of rodent and other mammal species is also large, because there are so many different kinds of plants available to eat. Predator species are abundant, because of the number of available prey.

Forest evolution-old growth

As forests get older, conifers and hardwoods take over from the fast growing softwood trees. These trees produce great amounts of shade and don’t allow the sun loving bushes adequate sunshine to be able to grow abundantly. The number of species dwindles to those that can live in limited sunlight. This limits the number of kinds of plants that can survive, cutting down on the number of species that are found.

Likewise, the number of animals, having fewer kinds of plants to eat, become represented by fewer species. If they can’t eat, they can’t survive. For the same reason, predators drop in number and species, because there are fewer prey to feed upon. 

This progresses as the forest becomes even older. According to Steve T., retired from the US Forest Service, “We see the lack of diversity in older forests all the time. Though we don’t like seeing a major forest fire that burns everything to the ground, it is helpful. It turns back the clock so new growth has a chance to start again, which means more plant and animal species in the area.”

The US Forest Service, in fact, lets fires burn in many places, partly for this reason. The fires don’t just remove the debris from the forest floor, they allow a new growth forest to form, which encourages diversity.

Again according to Steve T., an old growth forest is likely to have as much as 90% fewer plants and animals that a new growth forest has. People often see this outside forested national parks. In the park, the plant-life is rigorously protected. There is a greater likelihood of seeing more plant and animal species outside of the park, where the same regulations aren’t observed. 

This all adds up to older forests being less bio-diverse than newer forests. A forest ages naturally. However, it is a mistake to think that old growth forests harbor more animal or plant species than younger forests, since this has been disproved.

Additional resources:

US Forest Service

Oregon State University, Biology department