Annamari Laurén

Wood biochar amendment can be used to improve soil properties, support tree growth, and increase soil carbon stocks. The long‐term (> 5 years) effects of different biochar types and doses on established stand growth are unknown, as most studies are short‐term (≤ 5 years) pot or field trials with small seedlings. We applied 0, 5, and 10 Mg ha ⁻¹ wood biochar pyrolyzed at 500°C and 650°C in young established boreal Scots pine ( Pinus sylvestris L.) stands in Finland and studied tree growth response during the first 10 years after amendment. Biochar did not affect the 10‐year cumulative growth, but annual growth response analysis indicated that biochar application tends to increase growth at least in the short‐term. Growth response increased with biochar dose, but not consistently with pyrolysis temperature. The 10 Mg ha ⁻¹ dose of biochar pyrolyzed at 650°C increased the annual diameter growth of dominant trees by 0.25 mm (7.2% higher than control). Large‐scale use of biochar to enhance forest growth in practical forestry may not be economically feasible because of its high cost, but biochar application remains a possible strategy for increasing long‐term soil carbon stocks. However, more long‐term field studies from boreal forests are still needed to confirm these perceptions about growth response duration after biochar application.

Peatland drainage aims at enhancing tree growth by improving soil aeration and nutrient availability through the lowering of water table, but it also accelerates peat decomposition and increases CO ₂ emissions. CO ₂ emission estimates of drained peatlands are typically based on surface‐level measurements, whereas studies on peat decomposition have focused on substrate quality, microbiological processes, or external environmental drivers. However, there is limited understanding of how peat physical characteristics control decomposition and CO ₂ release across different soil depths. To improve long‐term predictions of carbon (C) balances in boreal drained peatlands, more evidence is needed on depth‐wise relationships between decomposition and peat physical properties. We collected undisturbed peat columns from two distinct drained forested peatlands and investigated the effects of peat physical properties on CO ₂ efflux under laboratory conditions using continuous‐flow gas flux measurements with chamber‐enclosed peat samples. In the top 15 cm soil, mass‐based CO ₂ efflux increased with soil air‐filled porosity and decreased with higher density and water content. In deeper layer, effluxes increased near water saturation, suggesting contributions from accumulated CO ₂ or anaerobic respiration. Our results show that the relationship between physical properties and CO ₂ emissions share consistent patterns in peats with distinct CO ₂ production potentials and that these relationships shift across the soil layers in boreal peatland forests, providing new insight into the decomposition dynamics in peat layers exposed to drainage.

Ash fertilization substantially increases stand growth in drained boreal forested peatlands. Growth response lasts for several decades. Ash fertilization is economically viable management practice and it also can increase ecosystem carbon sink. The long-lasting growth response is a combined result of increased supply of growth limiting phosphorus and potassium, higher soil pH, enhanced microbial activity and nutrient release in soil. An indirect growth response emerges as a feedback from higher foliage mass, increased evapotranspiration and subsequent lower water table and increased organic matter decomposition and nutrient release. Previous studies indicate that growth response with respect to unfertilized control is greatest when water table is high. Growth response decreases when water table lowers. In this study, we use Peatland simulator SUSI to search for water management where synergetic benefits from stand growth, ecosystem and soil carbon balance, and nitrogen export to watercourses is achieved. We simulated the combined effects of ash fertilization and water management in Scots pine (Pinus sylvestris L.) stands in different site fertility classes in Southern-Finland, Central-Finland and Northern-Finland. Results indicate that the growth response and effects on carbon balance and nitrogen export are best when water table is maintained high. In intensively drained areas the growth response is small and ash fertilization increases carbon emissions and nitrogen export to water courses.

Drainage in peatland forests has onset a succession, where a mor humus has formed on the top of the peat. Our latest study revealed that trees on transformed drained peatlands grow better when the water table (WT) is rather close to the soil surface. This is due to the hydrological and biogeochemical function of the mor layer. Mor layer is a substantial nutrient storage and source, it contains most of the roots and includes macropores that enable oxygen supply for roots even when WT is elevated. This suggests that in future peatland forestry the improved growth under higher WT would be synergetic to several other ecosystem services (ES): Growing forests with higher WT improves climate change mitigation via reduced peat carbon (C) emissions and enhanced stand and ecosystem C sequestration, improved resiliency due to reduced drought risks and smaller nutrient export to water courses. The importance of mor layer has been unrecognized, and its characteristics, function and consequences for ES provision remain virtually unexplored. We outline how mor layer affects the function of transformed drained peatlands, and apply the understanding to define sustainable water and forest management strategies taking into account ES across site fertility range under current and changing climate. This is achieved through application of process-based Peatland simulator SUSI. Understanding the mor layer function supports renewal of forest regeneration, planning of water and forest management and adaptation to climate change. Wood production with higher WT is an intermediate form between rewetting and current water management. The new peatland management has potential to cause immediate climate cooling effects through enhanced forest growth and decreased soil C emissions whilst decreasing nutrient loading to water courses.

Drained peatland forests cover a substantial fraction of boreal landscapes and are increasingly exposed to drought under a warming climate. After drainage, a mor humus layer has gradually developed on top of the peat—these sites are called transformed drained peatlands. This transformation fundamentally alters soil hydraulic properties and may increase drought sensitivity by weakening hydraulic connectivity between soil layers. However, the hydrological role of the mor layer and its impact on rootzone moisture availability remains poorly represented in current peatland ecohydrological models.This study investigates how the mor layer regulates soil moisture and water table dynamics during dry periods. We compare the soil moisture and water table dynamics of a mor–peat profile using three different hydrological modelling approaches: i) a hydrostatic equilibrium model for the whole profile, ii) a mor-layer bucket model coupled to peat in hydrostatic equilibrium, and iii) a full 1D numerical solution of the Richards equation.Preliminary results indicate that the mor layer can substantially modify near-surface soil moisture during drying events by limiting upward capillary flow from deeper peat layers. This hydraulic decoupling leads to faster topsoil drying and altered soil moisture profiles compared to simulations with an assumption of hydrostatic equilibrium. On the other hand, the drying mor layer rapidly starts to restrict evapotranspiration and protects water storage in the underlying peat. These effects are most pronounced during prolonged summer droughts, when evapotranspiration demand is high and capillary upflux becomes critical for sustaining root-zone moisture.The proposed improvements to the description of hydrological interactions within the mor–peat profile advance process-based simulation of drought responses in transformed peatland forests. The results contribute to a better mechanistic understanding of soil moisture dynamics under meteorological extremes and provide a foundation for assessing drought risk and adaptive water management strategies in peatland forestry.

Nitrogen (N) fertilization increases carbon sequestration and wood production in boreal upland forests and thus supports climate change mitigation. Excess N may, however, alter the cover, biomass and biodiversity of the ground vegetation. But little is known about how the species composition and cover are affected by the different intensities of N fertilization. Here, we evaluated the short-term impacts of N fertilization on the cover of vascular plant and bryophyte species in two Scots pine (Pinus sylvestris L.) -dominated and two Norway spruce (Picea abies (L.) H. Karst.) -dominated stands in Eastern Finland. The fertilization was done using a forwarder in the Scots pine stands and a helicopter in the Norway spruce stands, which are typical vehicles for this work in practical forestry. Funnel traps were used to measure the applied N doses at the plot level. Each study site also included unfertilized control plots. The percentage cover of vascular plant and bryophyte species was estimated at the beginning of the experiment, and then twice within 2–4 years after the fertilization. Overall, N fertilization had no significant impact on the cover of any vascular plant or bryophyte species. The cover of many species also decreased over the years, both in the fertilized and unfertilized plots. The exceptionally warm and dry summer of 2021 may have contributed to this result. However, further studies are needed to evaluate long-term responses in the ground cover and species composition to the N fertilizer doses used in practical forestry.

Boreal peatland forests have been extensively drained to increase timber production, but the maintenance of shallowed ditches has been questioned due to increased greenhouse gas (GHG) emissions and negative impacts on water quality. Ditch network maintenance (DNM) lowers water table, which typically increases tree growth, but also increases rate of peat decomposition and consequently CO2 emissions. Multi-objective forest planning balances between the conflicting economic gains and adverse environmental impacts. We used a process-based Peatland simulator SUSI to simulate three management scenarios for 20 forest stands, covering the variety of growing conditions in Finland. We studied how DNM with a reduced ditch depth (60 cm) and a conventional ditch depth (90 cm) affected stand growth, GHG balance, and nitrogen and phosphorus export. Over a 20-year simulation period, annual volume growth response was on average 0.8 m3 ha–1 when ditch depth was changed from 30 cm to 60 cm and 1.0 m3 ha–1 when ditch depth was changed from 30 cm to 90 cm. In southern Finland, DNM decreased stand growth in fertile sites. Soil GHG emissions increased on average by 49% and 119% in 60 cm and 90 cm ditch depths, respectively, compared to 30 cm ditch depth. The cost of reducing GHG emissions ranged from 0–22 € per ton of CO2 in our study sites and scenarios. Our results support the idea that omitting DNM or reducing ditch depth may lead to acceptable compromises, as the marginal cost of soil GHG emissions considerably increases with increasing ditch depth.