Researchers have shown that forest biodiversity is healthier than monoculture. Find out what this means for future reforestation plans.
The Toronto International Film Festival (TIFF) begins today. I’m not exactly a cinephile, but I do love IMAX movies. One of my favourites is called Tropical Rainforest. It walks viewers through the 400 million-year history of rain forests. We get a taste of the bewildering variety of species who live in a rainforest environment, whether on the forest floor or in the forest canopy. The key takeaway from the film is that a stable environment promotes diversity and diversity reinforces a stable environment. It’s a virtuous circle.
Unfortunately, diversity is something we are losing throughout the global biosphere. Many scientists tell us that we are going through a sixth mass extinction today; one that is worse than when the dinosaurs disappeared 65 million years ago. Ecologists tell us that 99% of the earth’s species are at risk. Readers will know that climate change is harming diversity but it’s not the main culprit. There are five causes for our present mass extinction. They are, in order of importance, land and sea use, direct exploitation of organisms, climate change, pollution and invasive alien species. In other words, the problem is people.
Films like Tropical Rainforest and another IMAX blockbuster called Coral Reef Adventure, tend to focus on ecosystems with very high levels of diversity. This is appropriate. Not only are these habitats visually stunning, but it makes sense to raise awareness about species lost in the areas that have the most to lose. We also need to bear in mind, though, that less diverse habitats are more vulnerable than those with a rich variety of plants and animals. This includes the Carolinian and Boreal forests in this part of Canada, for example.
The trees are all in rows, all the same species
As I’m driving through the countryside around here, I quite often pass reforestation projects. These are areas where new trees have been planted on what was vacant land. One can tell instantly when a wooded area is one of these projects for two reasons. The trees are all arranged in neat symmetrical rows, and they are all the same species, often white pines. That was how these projects were managed when they began 50 years ago. This is what scientists call a monospecific forest.
Which leads us to a study that has just come out in the journal Global Change Biology. Researchers at the University of Freiberg have revisited the world’s oldest field trial on biodiversity. They looked at the stability and productivity of forests with varying levels of diversity. The field trial is called the Sandanilla Biodiversity Experiment. Located it Panama, it focuses on biogeochemistry and plant health.
The Sandanilla site was originally a forest that was clear cut in 1952-1953. It was used to grow crops for a couple of years and then as pasture. It consists of 9 hectares of land divided into 32 plots. There are 24 plots in the main plantation and 8 more in the high diversity plantation. The main plantation was planted in 2001 and the high diversity plantation was planted in 2003. The founders of the site started with 6 species of fast-growing trees that were locally useful, both economically and environmentally.
High quality background data
The trouble with studying biodiversity in most forests is that it’s hard to separate the effects of biodiversity from other factors. Researchers can look at forests planted by humans, but many of these are not very old. That means it’s too early to capture the long-term effects. That’s the advantage of looking at the Sandanilla plantation. The plots in the main plantation have either 1, 2, 3 or 5 native tree species. Careful records have been kept of the size and height of the trees, which provides high quality background data.
The study found that where 2 and 3 species were planted instead of just 1, the productivity of those plots was 25-30% higher. The plots where 5 species were planted together were 50% more productive than those with 1 species. The researchers also looked at years where there was a strong El Nino effect. They found that, during the dry spells caused by El Nino, the difference between the more diverse plots and the monoculture plots was even greater.
Researchers concluded that forests with a range of tree species are not only more productive, they are also more stable and resilient. With the climate emergency, we want to be able to store more C02 by planting millions of trees. These findings tell us that we can store more C02 using less space if we plant mixed-species forests instead of the kinds of reforestation projects I go past in my car.
Nature is inherently diverse
Nature is inherently diverse. Ecology shows this again and again. The biosphere is a complex network of interrelated species. No living thing exists in isolation. They can only thrive in the context of a healthy and diverse ecosystem. Even their non-living, or abiotic, surroundings are factors in the well-being of the web of life. Humans need to understand that these interrelated systems are beyond our comprehension. We need to focus on studying them rather than exploiting them.
We always have more to learn if we dare to know.