Ants in Sweden avoid alien Lodgepole pine

In the 1970s, the Canadian Lodgepole pine was widely planted in Sweden in efforts to increase timber production. Although there are now more than 600,000 hectares of Lodgepole pine in Sweden, we still have little knowledge of its ecological effects.

In a large-scale field experiment with 15 paired stands of Lodgepole pine (Pinus contorta) and Swedish Scots pine (Pinus sylvestris), researchers have now investigated the impact of Lodgepole pine plantations on forest-dwelling ants.

"Our study shows that red wood ants thrive less well in Lodgepole pine plantations than in native Swedish pine plantations," says Therese Löfroth, researcher at the Swedish University of Agricultural Sciences, SLU, who led the study.

More than 40 000 ants identified

In the study, more than 40,000 ants were captured using pitfall traps, which are buried in the ground. The ants collected were identified to species and the researchers then compared species and numbers in the different forest stands. In all stands, ant hills were also inventoried, and observations have been made of ants foraging up and down tree trunks.

"We found that there were fewer ant hills in Lodgepole pine forests than in ordinary native pine forests. There were also fewer foraging trails of ants on the trunks of Lodgepole pine trees than on our native pine, indicating that ants do not find as many aphids to "milk" in Lodgepole pine trees, says Therese Löfroth.

As the aphids suck in plant sap from the canopy, they also produce sugar-rich faeces - honeydew - which the ants suck directly from the aphids.

Even the horse ant opts out of the Lodgepole pine forest

But what about other kinds of ants than the common red wood ants, do they thrive in Lodgepole pine forests?

No, other ant species such as horse ants and red ants don't seem to thrive there either, which surprised the researchers.

"Since red wood ants are aggressive and defend their territories against all other ants, we thought that if red wood ants don't thrive in Lodgepole pine forests, other ant species might do better in the absence of these aggressive red wood ants. This turned out not to be the case, and we saw that all species of ants thrived better in ordinary native pine forests,' says Therese Löfroth.

In summary, the researchers conclude that our native pine forest is a more favourable habitat for ant species. This is probably because Lodgepole pine stands are shadier and cooler, there are fewer aphids to feed on honeydew, and the long needles of Lodgepole pine is not good building material for ant hills.

"To benefit biodiversity, and in this case the ants, the species they depend on and on which yet other species depend on them, we recommend mixing up Lodgepole pine plantations with other tree species, or not planting large areas with Lodgepole pine, says Therese Löfroth.