Boreal peatlands harbour large stores of carbon as peat below their surfaces. Climate change is expected to cause drying in northern peatlands, which will in turn impact the carbon balance of these ecosystems that is maintained by high water tables and the hydrologically sensitive plants growing there. This study aims to quantify how vegetation will be structured (I) and photosynthesize (II, III) in a future climate as emulated by long-term water level drawdown (WLD). To do this, changes in the vegetation and its photosynthesis after WLD are linked, and the response of Sphagnum mosses to periodic drought is investigated.
Field measurements were done at a long-term WLD field experiment that contained a rich (mesotrophic) fen, a poor (oligotrophic) fen and a bog (ombrotrophic) site. Measurements included vegetation surveys from existing permanent sample plots and leaf-level carbon dioxide exchange measurements. For an experiment in controlled conditions peatland surface cores from this field experiment were transported to a greenhouse where the photosynthesis of lawn Sphagna during and after an experimental periodic drought was measured.
The field study revealed that the response of peatland vegetation to WLD depend on peatland type. The species composition in the rich fen was the most impacted by WLD, while the bog vegetation demonstrated stability. Similarly, large increases in photosynthesis occurred following WLD on the vascular plant-covered rich fen, while changes were negligible on the Sphagnum-carpeted bog. The vegetation on the two fens shifted from an open sedge-, or sedge and Sphagnum-dominated ecosystem, to a tree-dominated ecosystem. Canopy development following WLD further accelerated vegetation changes by shading and sheltering the understorey vegetation. Vascular plants were the most likely to increase productivity from WLD as they are best suited to utilize the nutrients made available by peat mineralization, while Sphagnum moss photosynthesis was impacted little. The greenhouse study revealed that lawn Sphagnum mosses exposed to long-term WLD were more vulnerable to drought compared to those from wet sites. Large capitula typical to fen Sphagnum species appeared to be beneficial for surviving periodic drought.
This work demonstrated that climate change as emulated by long-term WLD will have a large impact on the vegetation composition of northern peatlands and increase photosynthetic function of these ecosystems, fens in particular. To better predict climate feedbacks from these changes, carbon dynamics including peatland vegetation dynamics should be updated in global process models. Future research to better understand the tipping point of different peatland types after WLD in different climatic regions will help us to predict changes in these diverse and globally important systems.
Boreal peatland forests are an important source of timber. Recently, timber harvesting has been extended to warmer months, resulting in machinery traffic over unfrozen soils, and leading to higher levels of soil disturbance, such as deeper ruts. Despite this, our knowledge of the impact of soil disturbance on peat physical properties and soil biochemistry is still limited. To address this gap, I conducted a study to examine the effects of soil disturbance caused by harvesting machinery during thinning operations on the soil physical, chemical, and biological properties and vegetation of drained boreal peatland forests. To assess the rate of recovery, I sampled six sites that formed a chronosequence covering 15 years since thinning. The results showed that soil disturbance caused an increase in the bulk density and field capacity of peat, along with a decrease in total porosity. In the vegetation, moss biomass and root production were reduced, but sedge cover increased. Furthermore, recently disturbed areas exhibited greater soil CO2 production potential, as well as higher soil CO2 and CH4 concentrations compared to control areas. However, CO2 and CH4 emissions, microbial communities, and cellulose decomposition rate were not impacted. Although the rate of recovery varied, all studied properties impacted by disturbance were fully recovered within 15 years. As the water retention characteristic (WRC) describes soil structure and its alterations, it a useful for disturbance assessment. Thus, I propose how WRC can be predicted using artificial neural networks. Overall, the study demonstrated that while drained boreal peatlands are sensitive to disturbance, they are also resilient to mechanical soil disturbance caused by thinnings.
Below-ground carbon (C) allocation studies in boreal forests are scarce and have high levels of uncertainty in ecological and modelling studies. The uncertainty of fine root turnover and the heterogeneity of fine root distribution are the main barriers to quantifying the below-ground C allocation. Unravelling the below-ground C litter inputs of boreal forests, including fine roots and ectomycorrhizal (EcM) mycelia, could provide fundamental information for quantifying biogeochemical cycles. This thesis evaluated the below- and above-ground litter C inputs along a site type gradient of Scots pine (Pinus sylvestris) sites in southern Finland, and a distinct silver birch (Betula pendula) site in northern Finland. Furthermore, the Scots pine pioneer/fibrous root growth phenology was observed and compared with the modelled growth of the above-ground organs (predicted by the dynamic CASSIA model) in southern Finland in 2018, when there was an unusual summer drought. Fine root turnover was observed by minirhizotrons (MR) and the root growth phenology was observed by flat-bed scanners, both of which direct methods are known to provide reliable results in root research.
Based on the daily root growth monitoring experiments, we found that the timing of intensive root growth lagged behind the growth of above-ground organs (shoots, secondary xylem, buds, and needles). Interestingly, we found a clear root growth increase while the needle growth decreased, which may have been caused by a shift of non-structural carbohydrates (NSC) from above-ground to below-ground. The low temperature and summer droughts may have constrained the fibrous root growth, but not influenced the pioneer root growth, which indicates that pioneer roots could be more tolerant to severe climate variations.
Increasing nutrient availability could clearly increase the above-ground C allocation but not the below-ground allocation. Our study sites CT, VT, MT were named after Cajander’s Finnish site type theory in the order of increasing nutrient availability. Our study found that the nutrient-poor site CT tends to have significantly higher fine root longevity and biomass than the relatively nutrient-rich sites VT and MT. Fine roots could allocate more biomass below the ground and survive longer in nutrient-poor conditions. The distal tips of tree roots reflect the forest foraging ability, as shown by the fact that EcM root tips per basal area and fine root biomass per basal area both increased gradually from nutrient-rich to nutrient-poor sites and from low to high latitudes. Overall, we found that below-ground litter accounts for 21-58% of total litter inputs in boreal forests. This finding indicates that the C allocation pattern could be a specific effect of species and latitudes. The Scots pine in the southern sites allocated up to one third of total litter inputs below the ground but the northern silver birch allocated over half of total litter inputs below the ground.
In conclusion, we suggest that the growth phenology and litter inputs of below- and above-ground organs should always be observed and quantified together. Understory species contributed significantly to litter C inputs which should not be neglected in boreal forests.
Moreover, future studies should be focused on the shifting of below- and above-ground C allocation response to extreme climate and also on the need to include EcM mycelia and root exudates in the accounting of below-ground litter pools.
Thermal modification (TM) has been widely used to improve the dimensional stability and durability of wood. However, the performance of thermally modified wood (TMW) in condi-tions where it must endure continuous changes in ambient moisture content are not entirely clear. This thesis investigated the chemical components, cellular structure, and physical prop-erties of thermally modified Scots pine, Norway spruce, and European ash wood exposed to long-term water contact condition, different temperature and relative humidity condition and natural weather condition.
The results showed that increase in TM intensity reduced the equilibrium moisture content (EMC) and improved the dimensional stability of wood mostly in a tangential direction. TM did not affect Brinell hardness, while increase in EMC decreased wood hardness. Prolonged exposure to water mainly changed hemicelluloses and cellulose and increases the hygrosco-picity of both modified and unmodified wood. In addition, the initial higher acidity of TMW tends to promote the degradation of the cell-wall compounds, resulting in faster degradation in TMW than in unmodified wood during water contact exposure.
Degradation of lignin and leaching of the degradation products during the weathering ex-posure leaves wood with a grey hue and surface with higher relative cellulose and hemicellu-lose content. TMW presented less changes in lignin structure and color due to its condensed lignin structure and lower hygroscopicity compared to unmodified wood. The lower EMC and fiber saturation point (FSP) value of TMW compared to unmodified wood indicates that TM can limit water absorption during weathering. Therefore, TMW showed less cupping than unmodified wood in wet conditions. Brinell hardness was slightly decreased in all specimens due to cell wall degradation and increase in EMC. Additionally, increase in the TM intensity improved weathering performance of wood by reducing the surface chemical changes, water accessibility and cell wall porosity.
Passive recovery or active restoration approaches may be used in the repair of degraded ecosystems. The effects of such measures on ecosystem patterns and processes, including boreal forest soils and vegetation, are poorly understood. This thesis examines the impacts of both active and passive restoration approaches on soil organic matter (SOM) and vegetation in the boreal forests of eastern Finland.
The study sites were located in managed and protected boreal forests in the same region in Finnish North Karelia. In the study sites, I measured soil and vegetation patterns, and the environmental controls on SOM decomposition in relation to the proximity of decaying logs.
In actively restored sites, the burned, partly harvested site had lower humus SOM stocks and displayed vegetation biomass and cover patterns that suggested stronger disturbance than the other sites. Burning decreased and homogenized vegetation diversity through spatially-uniform extinctions and limited colonization 10 years after fire. Green tree retention partially alleviated the impacts of disturbance on vegetation biodiversity. Proximity of dead wood (but only of non-charred logs) enhanced conditions for SOM decomposition. Charred logs did not exhibit this effect, which suggests a previously unknown linking of forest fires to soil processes via charred wood.
In the passive recovery sites, legacies of slash-and-burn regimes have persisted in the forests for more than a century. The disturbed forests had a higher volume of large birch trees and lower SOM stocks. In boreal conditions, passive restoration may take more than a century before ecosystem properties return to their pre-disturbance state. Soil properties may be more challenging to restore than above-ground tree structures.
My results indicate that active and passive restoration approaches may produce quite different pathways and outcomes. In general, the active restoration approach with low severity fires that is currently applied appeared to not harm forest soils; in particular it left the deeper mineral layers intact, and may provide a more rapid way to restore ecosystem properties. However, there is an urgent need to cover a longer successional time series to reveal the exact differences between active and passive restoration trajectories. The inherent differences between the focus of the passive restoration approach (to recover ecosystem naturalness in a more holistic sense) and the active restoration approach (targeting specific species, habitats, structures and processes in the ecosystem) should be duly acknowledged.
National forest inventory (NFI) data are commonly used in national and regional scenario analyses on forest production and utilization possibilities. There is an increased demand for similar analyses at the sub-regional level, and further, to incorporate spatially explicit data into the analyses. However, the fairly sparse network of NFI sample plots allows analyses only for large areas. The present dissertation explored whether satellite imagery, NFI sample plot data and the k nearest neighbour estimation method can be employed in generating spatial forest data for scenario analyses at the local level. The method was first applied in the area of two villages in Eastern Finland to quantify the effects of administrative land use and technical land-form constraints on timber production. Secondly, the impacts of three alternative regional felling strategies on suitable habitat for the Siberian flying squirrel (Pteromys volans) were assessed.
As a scenario analysis tool, the Finnish forestry dynamics model MELA was used. Management units for simulations of forest development and management activities were delineated by means of image segmentation and digital maps on restriction areas, and new weights for NFI sample plots, that is, the representativeness in these units, were estimated by means of satellite image data. The performance of different segmentation methods and different spectral features in the estimation were examined. Image segments corresponding to forest stands enabled the use of patch- and landscape-level models in the prediction of suitable habitat.
Satellite image-based estimation of new NFI sample plot weights was found to be a feasible method for generating forest data for scenario analyses in areas smaller than is possible with the plot data only, for example, for municipalities. Satellite imagery with large geographic coverage and continuous NFI field measurements provide cost-efficient data sources for versatile impact and scenario analyses at the local level.
The peatlands in Southeast Asia have been impacted and turned to vast carbon dioxide sources (CO2) by land management often involving drainage and deforestation. The amount of released CO2 in decomposition is related to management intensity likely resulting from altered conditions for decomposition. However, the link between decomposition processes and land-use change are poorly understood.
To provide insight to the effects of land-use change intensities to decomposition processes in Central Kalimantan, Indonesia, we examined physical (dry bulk density, total pore space, particle size) and chemical properties (pH, loss-on-ignition; total concentrations of N, P, K, C, Ca, Mg, Mn, Zn, Al, Fe, S, Si, DOC and DON; organic matter quality characterized by infrared spectroscopy and on compound level), which together were used to determine the decomposition stage and decomposability (i.e., substrate quality) of peat. The peat biological properties (microbial biomass and enzyme activity) were used to provide insight to the decomposition activity at various land-use types and as a response to known peat properties. The study sites were: near-pristine swamp (i) and drained (ii) forest, deforested and drained degraded (iii), agricultural (iv) and reforested (v) sites.
At the most intensively altered deforested sites the peat was denser, finer and enriched with recalcitrant compounds. The highest enzyme activity and microbial biomass were in the surface peat of swamp forest, where the amount of labile carbohydrates was highest. The six years ago reforested site did not yet show signs of recovery in peat properties, which was likely due the limited litter production capacity of the young plantation and microbial activity limited by chemical weeding. The main conclusion is that the litter input, or rather the lack of it after land-use change, and intensive management practices forms the main factors affecting to decomposition processes and leading to poorer substrate quality and reduced biological activity.