Poplar can outperform both spruce and pine on forest land

Swedish forests are dominated by spruce and pine, which also form the foundation of the forest industry. However, increasing damage from storms, drought, and insects, especially in spruce stands, has created a need for complementary tree species. Ideally, a broadleaved species that can both secure production and enhance biodiversity.

Poplar – a good complement

Tree species that have attracted increasing attention as alternatives include poplar and hybrid aspen. These species are successfully cultivated on agricultural land today, but that creates competition with food production. The best option would be to grow poplar on other types of land, such as forest land.

The problem is that forest soils are often acidic and nutrient-poor, making it difficult for poplar to thrive. When soil pH drops below 5, aluminum becomes soluble and damages the roots, preventing water and nutrient uptake.

Outperforming spruce and pine

To address these challenges, SLU doctoral student Luca Muraro conducted field experiments, genetic studies, and a systematic literature review.

In large-scale field trials, poplar was planted alongside birch, aspen, hybrid aspen, spruce, and pine. Two types of sites were compared: long-term forest land and former farmland already afforested with spruce.

The results were clear. Poplar struggles to grow on acidic soils—unless the soil is treated with ash. Ash recycling not only provides nutrients but also raises the soil’s pH. On forested former farmland, poplar often outperformed both spruce and pine.

“What surprised me most were the striking results. We did not expect the addition of ash to be so effective. We observed a doubling of growth,” says Luca Muraro.

New clones tolerate acidic soils better

Muraro’s research also shows that different poplar clones vary in their sensitivity to acidic soils. Through selection and breeding, aluminum-tolerant clones can be developed to cope with the low pH conditions typical of forest soils.

To make poplar cultivation on forest land feasible, both ash recycling and less acid-sensitive plant material may be needed.

“We need smart combinations of the right genetic material and the right soil treatment. In practice, that could mean planting tolerant clones on difficult sites, while applying ash treatments where it is practical,” says Luca Muraro.

The research behind the thesis has been funded in part by the Swedish Energy Agency.

You can read the thesis here: Fast-growing broadleaved alternatives for Swedish forestry.

Luca Muraro will defend his thesis on 7 November in Alnarp, Skåne.