New clue explains high growth rates of lodgepole pine

Canadian Lodgepole pine (Pinus contorta) has become a popular feature of forestry in Northern Sweden, because of its higher growth rate compared to native Scots pine. This growth difference is particularly mysterious because Lodgepole pine does not grow as well in its native environment (Canada) compared to when it is brought to Sweden. A new study led by a research team at the Swedish University of Agricultural Sciences provides some new clues that may help solve this mystery. The team collected roots from Pinus contorta forests in the USA and Canada (where it is native)*, as well as numerous countries where it has been introduced, including Sweden, Finland, Scotland, Chile, and New Zealand. They used the roots to extract and sequence fungal DNA, which allowed them to identify which species of fungi the tree associated with in different environments. Fungi growing inside tree roots are mostly beneficial, but also can be parasitic depending on which species they are. The study showed that introduced Lodgepole pine always associated with different communities of fungi in environments they are introduced to compared to their home environments, regardless of which type of environment to which they are introduced. At European sites, especially those in Sweden, Lodgepole pine roots were heavily colonized by a specific species of fungi (Piloderma) known to be beneficial to trees (referred to as an ectomyccorhizal fungi), whereas this fungal species was very uncommon at North American sites. The results of the study builds on previous research by the research team, which showed that Lodgepole pine grows better in Swedish soil compared to Canadian soils. Lead author of the study, Associate Professor Michael Gundale says, "Now that we know that soil is important for the growth success of Pinus contorta in Sweden, and also know which soil organisms might be responsible for this, we can plan the next phase of experiments to understand how this specific fungal species differently affects Lodgepole pine and Scots pine." Gundale further says, "There is a great diversity of fungi in the soil, and knowledge of how the different fungal species affect a variety of tree species would be key knowledge that could ultimately be used to help native species achieve the same high productivity of some exotic trees species."

Another key finding of the study was that Lodgepole pine does not appear to encounter beneficial fungi everywhere it is introduced. Lodgepole pine has been introduced extensively in the southern hemisphere, where no closely related tree species exist. Their study showed that the beneficial fungi (called mycorrhizae) harbored by native trees in the southern hemisphere (such as in New Zealand and Chile) are not a good match for Lodgepole pine, and that Lodgepole pine has to bring the fungi it needs with it in order to be successful in those environments. Gundale said, "Lodgepole pine appears to be very lucky in Sweden, because all the fungi it needs to grow well are already here, which is clearly not the case everywhere it is introduced." Once Lodgepole pine finds the fungi from its home range that it needs, it becomes very invasive in the southern hemisphere. Gundale says, "The high degree of invasiveness of Lodgepole pine in the southern hemisphere suggests it should be monitored and studied very closely here in Sweden to prevent similar invasions from occurring here."

The study was published August 17 in the Journal of Ecology.