Environmentalists trying to help rainforests recover from human destruction or natural disaster may have been barking up the wrong tree. Instead of helping the rest of the forest regenerate, as had been assumed, nitrogen-fixing trees impede the growth of other species, at least in Costa Rica.
Plants cannot grow without nitrogen. but most can’t make use of the molecular nitrogen in the atmosphere, so they depend on others to “fix” it into a form they can incorporate. Consequently, nitrogen-fixers are essential to enriching and restoring ecosystems, as seen in the use of peas to restore depleted soils for wheat or oats.
“Symbiotic nitrogen-fixing trees are thought to provide much of the nitrogen required to fuel tropical secondary regrowth and therefore to drive the rate of forest regeneration,” Benton Taylor, a PhD student at Columbia University, writes in the Proceedings of the National Academy of Sciences. However, when Taylor tried to measure these trees' benefits he found the exact opposite.
Nitrogen fixers make up about 10 percent of the trees in Central and South America, even though they are just 1 percent in Canada and the northern USA. Taylor studied a data set, collected over 17 consecutive years, looking at eight patches of high rainfall rainforests in Costa Rica, both old growth and areas at various stages of regeneration. This provided measures of the impact of nitrogen fixers across 1-hectare (2.5 acres) plots, and on a much smaller scale.
Both locally and across entire plots, areas with more nitrogen-fixers experienced slower overall growth. The pattern held in both new and old forests. Where nitrogen-fixers took up 35 percent of the area, they were able to shut non-fixer growth down almost entirely. Taylor and his co-authors attribute the effect to the greater demand nitrogen-fixers have for water and other soil nutrients, and the shade they throw on other plants.
Technically, no tree actually fixes nitrogen itself. Rather the trees dubbed "nitrogen-fixers" form symbiotic relationships with microorganisms, usually rhizobia bacteria, that fix the nitrogen for them. Nevertheless, the effect is the same: trees that have this capacity enriching the soil around them with nitrogen, where it can be taken up by other plants.
Something so unexpected is always interesting, but the findings also have very important implications. So much rainforest has recently been destroyed by human hands that getting the planet back to health will require extensive restoration. For both a habitable climate, and the preservation of biological diversity, this will mean planting pioneer trees that can allow healthy forests to grow around them. We could easily assume nitrogen-fixers should be the first stage, but Taylor’s work indicates this should be done with great care, and other ecosystems need to be checked in similar ways.