Wood ash doubles poplar growth on productive forest land

Poplars have traditionally been planted on arable land in Sweden and are known to be nutrient and water demanding while a soil pH greater than 5 is also required for optimal growth. Less optimal pH levels have been hindering the use of poplars on forested and forested arable land. In this study PhD student Luca Muraro and his research colleagues Henrik Böhlenius and Anneli Adler have investigated how the pH-increasing application of wood ash, lime and biochar affected the survival and growth of poplar seedlings.

“What surprises me in the results is the magnitude. We did hypothesize that wood ash would increase early growth for poplars, but we did not expect that mixing wood ash in the soil would double the height in forest land and almost double it in forested arable land”, says Luca Muraro.

The poplar seedlings were one year old at the time of the planting and all seedlings were provenance hybrids within Populus trichocarpa. The experiments were established between 2019 and 2020 in southern Sweden at two side types: on forest land sites with a continuation of forest coverage for more than 100 years and former agricultural land planted with one rotation of Norway spruce which had grown for 40 to 70 years, referred to as forested arable land.

Treatments affected growth

The early growth of the seedlings increased equally independent on whether wood ash or lime was mixed with the soil or applied on one square meter of the soil surface, while applying the wood ash or lime in a cylinder hole of 0,3 meter in diameter on the planting spot had a marginal effect on growth. The higher growth of the seedlings treated with wood ash compared to lime-treated seedlings is suggested to be caused by the positive effects of wood ash on nutrient availability, nutrient supplementation and reduction of toxic metal ions.

Application of biochar did not influence growth performance. The researchers suggest that since biochar has a longer residence time in soil compared to wood ash or lime its direct impact does not alter soil conditions quickly enough to benefit poplar seedlings, but its soil-improving effects may instead be more long-term.

Plant survival

Plants treated with wood ash, lime, or biochar had a higher survival rate than untreated plants on forest land. On forested arable land only the lime-treated plants displayed higher survival rates compared to untreated plants.

“Before planting we scarified the soil to expose the mineral soil, which aids in seedling establishment and eliminates competing vegetation. Over the three years of the experiment, no additional tending measures were taken, meaning the seedlings faced competition for resources, especially at the more fertile forested arable land sites. This setup mirrors a real-world scenario where soil scarification is done before planting, but no further action is taken until the first pre-commercial thinning”, says Luca Muraro.

Sustainability aspects and future focus

While wood ash offers both economic and environmental benefits due to being an inexpensive and abundant by-product of biomass combustion and doesn’t require any additional input to produce, lime production brings more sustainability challenges.

“Sourcing lime involves costly processes like pulverizing limestone or chalk, and it presents financial challenges due to rising extraction, transportation, and application costs. Moreover, lime mining contributes significantly to atmospheric carbon dioxide emissions, estimated at up to 0.75 metric tons of CO2 per ton of lime produced. Given these drawbacks, wood ash stands out as an attractive alternative to lime”, Luca Muraro argues.

His further research will be focused on the largely unexplored issue of finding poplar plant material that is tolerant to high aluminium levels, which is common in low soil pH sites. The long-term effects of the experiments will be measured and followed up.

“Combining wood ash application with these tolerant clones could be highly effective and further expand the potential areas where poplars can be established. Our research has the potential to enable poplar cultivation in a wider range of sites, helping to meet the growing demand for wood products while also easing the strain on already limited agricultural land”, Luca Muraro concludes.

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The study is financed by:

• The Swedish Energy Agency (Snabbväxande lövträdsplanteringar - framtidens råvara till Biodrivmedel)
• Rappe- von Schmiterlöwska stiftelsen
• Hildur & Sven Wingquists stiftelse för skogsvetenskaplig forskning
• Bengt Hanssons Skogs-och Lantbruksstiftelse
• Stiftelsen Petersson-Grebbe