Solar radiation is scattered by cloud cover, aerosols and other particles in the atmosphere, all of which are affected by global changes. Furthermore, the diffuse fraction of solar radiation is increased by more frequent forest fires and likewise would be if climate interventions such as stratospheric aerosol injection were adopted. Forest ecosystem studies predict that an increase in diffuse radiation would result in higher productivity, but ecophysiological data are required to identify the processes responsible within the forest canopy. In our study, the response of a boreal forest to direct, diffuse and heterogeneous solar radiation conditions was examined during the daytime in the growing season to determine how carbon uptake is affected by radiation conditions at different scales. A 10‐year data set of ecosystem, shoot and forest floor vegetation carbon and water‐flux data was examined. Ecosystem‐level carbon assimilation was higher under diffuse radiation conditions in comparison with direct radiation conditions at equivalent total photosynthetically active radiation (PAR). This was driven by both an increase in shoot and forest floor vegetation photosynthetic rate. Most notably, ecosystem‐scale productivity was strongly related to the absolute amount of diffuse PAR, since it integrates both changes in total PAR and diffuse fraction. This finding provides a gateway to explore the processes by which absolute diffuse PAR enhances productivity, and the long‐term persistence of this effect under scenarios of higher global diffuse radiation.

Cutaway peatland is a marginal land, which without further management is an unfavourable environment for plant growth due to low bearing capacity, high acidity and unbalanced nutrient composition of the soil. After wood-ash application, the soil becomes enriched with P and K, creating better conditions for tree growth. In addition to being economically viable, tree plantations ensure long-term carbon storage and promote habitat restoration. In a three-year term, we studied how distance from a drainage ditch and three different doses of wood-ash—5, 10, and 15 tons per hectare—affect the diversity of vascular plants in a tree plantation on a cutaway peatland. Plant species richness, vegetation cover and composition were positively affected by the distance from the drainage ditch and application with fertiliser, but in most cases, fertiliser dose had no significant effect. Both cover and species diversity were not affected by the planted tree species. In a tree plantation, herbaceous plants provide soil fertility by decay and recycling, and reduce mineral leaching in the long term. Since vascular plants play an important role in both the development of habitats and tree growth, it is important to know how multiple factors influence the development of vegetation in tree plantations.

Management of cutaway peatlands is a key issue in many countries. Whilst afforestation has been considered as a suitable after use option for cutaway peatlands, growing conditions after peat harvest are often adverse. To increase soil fertility and neutralise soil acidity, wood ash, an energy production by-product, can be used. In this study, we examined whether wood ash fertiliser affects growth (survival, height, root collar diameter), leaf morphological (mass, specific leaf area, leaf water content) and physiological traits (chlorophyll concentration, fluorescence parameters, photosynthetic and transpiration rate) of planted deciduous trees in a cutaway peatland in the central part of Latvia. At our study site in Latvia, we examined Alnus glutinosa, Betula pendula and Populus v. Vesten tree species and tested 0, 5, 10 and 15 t ha-1 wood ash doses in three replications. Tree species used in the study exhibited different reactions to the wood ash fertilisation. A. glutinosa traits were the least affected, whereas for Populus v. Vesten and B. pendula wood ash fertilisation was beneficial. In most cases we detected significant differences between unfertilised and fertilised plots, but rarely saw an effect of increased wood ash dose. Due to the lack of significant benefits and for environmental concerns the lowest wood ash dose is recommended. Overall, the fertilisation improved the success of afforestation and, by increasing the amount of leaf litter, is likely to indirectly alter understorey conditions.

Fertilization is a method to enhance tree growth and timber production. Ammonium nitrate and wood ash are commonly used fertilizers, which can be applied at the same time to increase levels of both nitrogen and other macro- and micronutrients. We studied how ammonium nitrate and wood ash fertilization affects photosynthetic activity and transpiration at leaf level in a deciduous tree plantation in former agricultural land with mineral soil, located in the central part of Latvia (Keipene parish). Additionally, we performed foliar and soil nutrient analyses. Our results support the notion that nitrogen fertilization may not result in increased photosynthetic activity. It is possible that the photosynthetic activity has increased at canopy scale along with increasing leaf area, not at leaf scale. Wood ash addition seems to have resulted in higher photosynthetic activity for hybrid alder, although it could not be explained with phosphorus availability. Although closely related to photosynthesis, in most cases transpiration was not positively affected by fertilization. Environmental factors, such as humidity, temperature and wind speed may have a greater effect on this process.

Afforestation is one of the widely used methods of reclamation in cutaway peatlands where spontaneous revegetation is not feasible, but fertilization or liming is usually required to encourage the development of vegetation cover. Wood ash application increases planted tree biomass, but the role of spontaneously incoming species in early‐stage biomass accumulation is lesser known. The amount of below‐and aboveground biomass accumulation was evaluated in relation to the drainage ditch position (2 or 9 m from the ditch edge) and the amount of wood ash applied (0, 5, 10, or 15 megagrams ha⁻¹) three years after fertilization. The biomass accumulation, species richness, and plant carbon and nitrogen ratio were positively impacted by wood ash treatment, but biomass allocation was species dependent: for all tree and herbaceous species with individual root systems, the proportion of biomass accumulated in roots was higher in the unfertilized group, but for Phragmites australis with a clonal root system, it was contrariwise. The application of 10 Mg ha⁻¹ wood ash was the most productive, with an average of 485.0 g/m² total plant dry biomass accumulation next to and 301.3 g/m² further from the drainage ditch, compared to the unfertilized 36.8 g/m² (2 m from ditch) and 13.9 g/m² (9 m from ditch). During the initial vegetation establishment, herbaceous species comprised the majority of the biomass, primarily graminoids.