In a study spanning 192 sites and four continents, researchers have discovered that conifers planted in the Southern Hemisphere grow up to four times faster and are more drought-resistant than those growing in their native ranges.
This expedited growth and resilience could make conifers a strong candidate for carbon storage, but there may also be drawbacks to planting trees outside their home turf.
University of Mississippi ecologist Stephen Brewer was among more than two dozen researchers across multiple continents who studied the growth rate of pines, spruces, firs, cedars and other conifers in their native and nonnative habitats. The group published its findings in Nature .
"That result is surprising because ecological theory predicts that there should be a tradeoff between how fast a plant can grow and how tolerant it would be of stressful conditions," said Brewer, Ole Miss professor of biology. "This is interesting to us because with the elevated carbon dioxide concentrations in the atmosphere contributing to climate change , there's a desire or push for afforestation, or planting trees in areas where there weren't any before, as carbon sinks.
"For that to be effective, you want the trees to grow quickly and not die in droughts."
While the exact cause for the trees increased resilience needs more study, the researchers found that Northern Hemisphere conifers planted in the Southern Hemisphere had higher concentrations of carbohydrates, such as sugars and starches, that could be used to store energy during stressful seasons.
"This phenomenon of skipping the growth-stress tolerance trade-off is something crucial that this study is reporting," said Alex Fajardo, researcher at the Institute of Ecology and Biodiversity and professor at the University of Talca . "It is also valuable that we pursued finding the physiological mechanism responsible for this: an increase in carbon reserves needed for drought tolerance."
Although the findings position conifers as well-suited for afforestation efforts, the researchers cautioned that more factors are involved.
"The easy answer to this question is to advocate for plantations with nonnative conifers that can both grow (quickly) and tolerate drought," Fajardo said. "However, this decision has some serious risks, as nonnative conifer plantations, particularly in the Southern Hemisphere, have come with high biodiversity and social costs."
Pines and Douglas firs, two of trees researchers studied, do not naturally grow in the Southern Hemisphere, but they have been planted in countries including Chile, Brazil and South Africa as a part of the lumber industry for decades.
"The problem is, these pines are not necessarily staying in their plantations; they are actually expanding into natural, native habitats," Brewer said.
"In Brazil, it's escaping into what they call their Cerrado, which is a savanna, and it grows much faster there and just basically eliminates all the native vegetation. It's actually become a big problem."
The problem of escaping or encroaching pines has also affected Mississippi, he said. Local lumber industries began replacing longleaf pines with slash pines in the 1930s because of their faster growth rate .
Both species are native to Mississippi, but longleaf pines are more fire-resistant. The threat of fire wiping out entire stands has prompted many foresters and nonprofit agencies to return to the more resilient longleaf .
"I've seen wildfires completely eliminate entire stands of slash pines, whether that's in Mississippi or in the Southern Hemisphere," Brewer said. "The Cerrado in Brazil gets comparable rainfall amounts to what we get along the Mississippi Gulf Coast, and it's a savanna for the same reason that we have longleaf pine savannas on our coast: fire.
"Fire is what maintains those savannas, and if you plant slash pines in really high densities, there could be an actual elevated fire risk, too."
The researchers also found that native Southern Hemisphere trees in the genus Nothofagus, known as southern beeches, performed relatively well in growth and drought-tolerance measures.
"In our study, we also found that native angiosperms of the genus Nothofagus did a decent job growing and tolerating drought," Fajardo said. "They may be a more potentially sustainable alternative."
