Soil moisture conditions control nutrient accumulation, carbon storage and tree growth in boreal forest landscapes

Forest and soil properties change across landscapes due to the complex interactions between various environmental factors. In many landscapes, topography exerts a major influence on the variation in soil moisture conditions, which in turn largely affects soil properties and processes. This thesis synthesises the results from four studies (papers I-IV), with the underlying aim to increase the understanding of how environmental factors, in particular, soil moisture, control the variation of nutrient accumulation, carbon storage, and tree growth within boreal landscapes. The four studies were all based on an extensive survey of a 68 km2 boreal forest landscape in northern Sweden. In Paper I, soil moisture conditions were predicted using multiple terrain indices. The results emphasised within-study validation and how digital elevation model resolution together with user-defined thresholds influence prediction accuracy. Paper II focused on how multiple environmental drivers influence the variation in soil carbon-to-nitrogen (C/N) ratios, with a noteworthy result that the ratio decreases as soil moisture conditions increase. Paper III presented how, soil moisture conditions significantly controls the distribution and partitioning of carbon stocks, with large increases in total carbon stock observed as soil moisture conditions increases, which was observed at both plot and landscape scale. The results in paper IV showed that, estimates of forest site quality decrease in response to increased soil moisture conditions. In conclusion, the research discussed in this thesis emphasises the importance of studying forest ecosystems on a landscape scale, an approach that can provide key insights into the factors that influence variation of different attributes of boreal forest ecosystems

Rumslig och tidsmässig variation av DOC export och vikten av områdesspecifika flödesmätningar

The export of dissolved organic carbon (DOC) is an important component in both water quality and carbon cycling within boreal catchments. DOC exports are mainly controlled by discharge. Therefore, the resolution and quality of discharge data is essential when quantifying DOC exports as well as for the understanding of its spatial and temporal variation. Although there are many studies quantifying DOC exports, many fail to consider the variability of discharge between different catchments within a landscape, instead often assuming uniform specific discharge. In this study, spatial and temporal variation in DOC concentrations and exports was studied for 13 nested subcatchments of a boreal catchment located in northern Sweden. The subcatchments were studied during a nine-year observation period (2009-2017). Discharge was monitored at each site and exports were calculated using daily DOC concentrations interpolated from frequent water sampling. Positive correlations between discharge and DOC concentrations were observed for most of the sites. Higher concentrations correlated with increased wetland coverage. Seasonal variations in DOC concentrations were found between subcatchments dominated by forest compared wetlands. The annual mean DOC export ranged between 26 and 108 kg ha-1 yr-1 and correlated positively with wetland coverage (R2 = 0.67, p < 0.001). Spatial variation on both annual and seasonal timescales, could to a large extent be explained by the catchment characteristics using principal component analysis. Forest and wetland coverage, as well as the size and elevation of each subcatchment explained most of the spatial variation of DOC exports. The importance of these catchment characteristics varied with season, where the two-month long spring season contributed between 32 to 52% of the annual DOC export. For annual exports, the proportion exported during spring season was larger for forest-dominated catchments compared to wetland catchments. Comparing export calculated adopting variable discharge to exports adopting uniform specific discharge revealed an average absolute difference of 18% ranging between 5 to 32% for individual subcatchments. DOC exports from wetland-dominated catchments were underestimated while forest-dominated catchments were overestimated. This study clearly demonstrates that adopting uniform specific discharge provides erroneous quantifications of DOC exports by not being able to capture the spatial variability within the system. This highlights the importance of discharge data with high resolution and quality when estimating element fluxes.Exporten av löst organiskt kol (DOC) är en viktig komponent för både vattenkvalitet och kolcykeln i boreala avrinningsområden. Exporten av DOC är till största delen kontrollerad av flöde. Därför är flödesdata av hög kvalitet och upplösning essentiell vid kvantifiering av DOC export och dess tidsmässiga och rumsliga variation. Även om det finns många studier som kvantifierar exporten av DOC, är det få som tar hänsyn till den specifika avrinningens variation inom landskapet genom antagandet av likformig specifik avrinning. I denna studie, studeras den rumsliga och tidsmässiga variationen av koncentrationer och exporter av DOC. Studien omfattar 13 delavrinningsområden i ett borealt landskap i norra Sverige under en nioårsperiod (2009–2017). Flödesmätningar utfördes vid varje avrinningsområde. Exporter beräknades med dagliga koncentrationer framtagna genom linjär interpolering mellan frekventa vattenprovtagningar. DOC koncentrationer varierade med av flöde. Höga koncentrationer korrelerade med ökad andel våtmark. Årstidsvariation av koncentrationer skiljde sig mellan delavrinningsområden dominerade av våtmark jämfört med skog. Den genomsnittliga årliga exporten av DOC var mellan 26-108 kg ha-1 år-1 och korrelerade positivt med andelen vårmark (R2 = 0.67, p < 0.001). Rumslig variation för exporter under år och årstider kunde till stor del förklaras av avrinningsområdens karaktär. Andelen skogsmark och våtmark, samt storlek och höjd över havet korrelerade med den rumsliga variationen av DOC export. Betydelsen av landskapsvariablerna varierade med årstiderna. Den två månader långa vårsäsongen bidrog mellan 32% och 52% av den årliga DOC exporten. Andelen av den årliga exporten under våren var större för avrinningsområden med hög andel skogsmark jämfört med våtmark. Export beräknad med flödesdata från varje område jämfört med att använda samma flödesdata för samtliga områden resulterade i en absolut skillnad på 18%. För enskilda avrinningsområden varierade skillnaden mellan 5% och 32%. DOC exporten från våtmarker underskattades medan export från skogsmarker överskattades vid användandet av samma flödesdata för alla områden. Denna studie demonstrerar tydligt att användandet av samma flödesdata för samtliga avrinningsområden är problematiskt vid beräknande av DOC export. Metoden gör det inte möjligt att fullt ut fånga den rumsliga variationen i landskapet. Denna studie belyser därmed vikten av flödesdata med hög upplösning och kvalitet vid beräknande av export

Predicting soil moisture conditions across a heterogeneous boreal catchment using terrain indices

Soil moisture has important implications for drought and flooding forecasting, forest fire prediction and water supply management. However, mapping soil moisture has remained a scientific challenge due to forest canopy cover and small-scale variations in soil moisture conditions. When accurately scaled, terrain indices constitute a good candidate for modelling the spatial variation of soil moisture conditions in many landscapes. In this study, we evaluated seven different terrain indices at varying digital elevation model (DEM) resolutions and user-defined thresholds as well as two available soil moisture maps, using an extensive field dataset (398 plots) of soil moisture conditions registered in five classes from a survey covering a (68 km2) boreal landscape. We found that the variation in soil moisture conditions could be explained by terrain indices, and the best predictors within the studied landscape were the depth to water index (DTW) and a machine-learning-generated map. Furthermore, this study showed a large difference between terrain indices in the effects of changing DEM resolution and user-defined thresholds, which severely affected the performance of the predictions. For example, the commonly used topographic wetness index (TWI) performed best on a resolution of 16 m, while TWI calculated on DEM resolutions higher than 4 m gave inaccurate results. In contrast, depth to water (DTW) and elevation above stream (EAS) were more stable and performed best on 1–2 m DEM resolution. None of the terrain indices performed best on the highest DEM resolution of 0.5 m. In addition, this study highlights the challenges caused by heterogeneous soil types within the study area and shows the need of local knowledge when interpreting the modelled results. The results from this study clearly demonstrate that when using terrain indices to represent soil moisture conditions, modelled results need to be validated, as selecting an unsuitable DEM resolution or user-defined threshold can give ambiguous and even incorrect results

Soil moisture controls the partitioning of carbon stocks across a managed boreal forest landscape

Boreal forests sequester and store vast carbon (C) pools that may be subject to significant feedback effects induced by climatic warming. The boreal landscape consists of a mosaic of forests and peatlands with wide variation in total C stocks, making it important to understand the factors controlling C pool sizes in different ecosystems. We therefore quantified the total C stocks in the organic layer, mineral soil, and tree biomass in 430 plots across a 68 km2 boreal catchment. The organic layer held the largest C pool, accounting for 39% of the total C storage; tree and mineral C pools accounted for 38% and 23%, respectively. The size of the soil C pool was positively related to modelled soil moisture conditions, especially in the organic soil layer (R2 = 0.50). Conversely, the tree C pool exhibited a unimodal relationship: storage was highest under intermediate wetness conditions. The magnitude and variation in the total soil C stocks observed in this work were comparable to those found at the national level in Sweden, suggesting that C accumulation in boreal landscapes is more sensitive to local variation resulting primarily from differences in soil moisture conditions than to regional differences in climate, nitrogen deposition, and parent material

Use of multiple LIDAR-derived digital terrain indices and machine learning for high-resolution national-scale soil moisture mapping of the Swedish forest landscape

Spatially extensive high-resolution soil moisture mapping is valuable in practical forestry and land management, but challenging. Here we present a novel technique involving use of LIDAR-derived terrain indices and machine learning (ML) algorithms capable of accurately modeling soil moisture at 2 m spatial resolution across the entire Swedish forest landscape. We used field data from about 20,000 sites across Sweden to train and evaluate multiple ML models. The predictor features (variables) included a suite of terrain indices generated from a national LIDAR digital elevation model and ancillary environmental features, including surficial geology, climate and land use, enabling adjustment of soil moisture class maps to regional or local conditions. Extreme gradient boosting (XGBoost) provided better performance for a 2-class model, manifested by Cohen's Kappa and Matthews Correlation Coefficient (MCC) values of 0.69 and 0.68, respectively, than the other tested ML methods: Artificial Neural Network, Random Forest, Support Vector Machine, and Naive Bayes classification. The depth to water index, topographic wetness index, and `wetland' categorization derived from Swedish property maps were the most important predictors for all models. The presented technique enabled generation of a 3-class model with Cohen's Kappa and MCC values of 0.58. In addition to the classified moisture maps, we investigated the technique's potential for producing continuous soil moisture maps. We argue that the probability of a pixel being classified as wet from a 2-class model can be used as a 0-100% index (dry to wet) of soil moisture, and the resulting maps could provide more valuable information for practical forest management than classified maps

Changes in winter length and spring flood in a boreal forest in northern Sweden

Jordens klimat förändras, atmosfären och haven blir allt varmare. Dessa förändringar påverkar vattenbalansen. Kandidatarbetes syfte har varit att undersöka huruvida det skett förändringar i vinterns längd och basflöde samt vårflodens tidpunkt, högsta flöde och volym i Krycklans försökspark under perioden 1981-2015. Daglig vattenflödesdata från Krycklans Site 7 laddades ned från Krycklans öppna databas. Snödata användes för att kunna definiera vinterns början. För att upptäcka eventuella trender i datat utfördes en regressionsanalys och ett Mann-Kendall test. Resultatet från regressionsanalysen och Mann-Kendall testet visade på en signifikant trend mot kortare vintrar, lägre högsta flöde och att vårfloden startar tidigare. Ingen av analyserna visade på någon signifikant förändring i vårflodens volym, vinterns basflöde och tidpunkt för högsta flöde. Vinterns basflöde visade inte på någon förändring vilket skulle kunna bero på definitionen av vinter som användes. Studiens definition av vinter var tiden mellan snön har lagt sig till vårflodens början. Vinterbasflödet var det dagliga flödets medelvärde under vintern. Genom att studera tiden på vintern då flödet är som lägst skulle eventuellt en förändring av vinterbasflödet upptäckas. Den relativt korta period mellan 1981-2015 som studerats här kanske inte är tillräcklig för att upptäcka förändringar i ett Svenskt klimat på lång sikt.The Earth’s climate is changing, the atmosphere and oceans are getting warmer. These changes affect the water balance. The aim of this thesis have been to investigate whether there has been any changes in winter length and winter base flow as well as in spring flood timing, maximum flow and volume in the The Krycklan catchment study area during the period of 1981-2015. Daily water flow data from Site 7 in Krycklan was downloaded from Krycklans open database. Snow data was downloaded to help define the start of winter. A regression analysis and a Mann- Kendall test was performed to reveal any trends in investigated parameters. The results from the regression analysis and Mann-Kendall test showed a significant change towards shorter winters, lower maximum flows and earlier spring flood onset. There was no significant change in spring flood volume, winter base flow or timing of maximum flow in any of the statistical analysis. The winter base flow did not show any trend which might be due to the definition of winter that was used. In this study winter is defined as the time between snow arrival and spring flood onset. The winter base flow was defined as the average daily flow during the winter. A trend might been detected by only studying the time during winter where the base flow is at its lowest. The relatively short period 1981-2015 that was used in this study might not be a long enough time scale to show long term changes in a Swedish climate

What happens to trees and soils during five decades of experimental nitrogen loading?

High deposition of nitrogen was postulated to drive losses of NO3 - and nutrient base cations, causing soil acidification, nutrient deficiencies reducing tree growth and ultimately tree mortality. We tested these predictions in a uniquely long-term study involving three NH4NO3 addition treatments (N1-N3) in a boreal Pinus sylvestris forest. The lowest level (N1), 30 kg N ha− 1 yr− 1 was applied during 50 years. Twice this rate (N2) was added 38 years, followed by 12 years of recovery, while thrice this rate (N3) was added 20 years followed by 30 years of recovery. We compared tree growth, changes in foliar and soil chemistry among treatments including control plots without N additions. As predicted, the N treatments lowered soil pH and reduced soil base saturation by around 50 %. They also lowered foliar levels of Ca, Mg, K, P and B initially, but after 50 years only Ca and Mg remained lower than in the control. Lack of B motivated a single addition of 2.5 kg ha− 1 after ten years of N treatment. Tree stem growth became and then remained higher in N1 than in the other treatments through the 50 years of treatments. In N2 and N3, foliar δ15N increased during the N-loading phase, but declined during the recovery phase, indicating a return of ectomycorrhizal fungi and their role in tightening the N cycle in N-limited forests. In the terminated, initially highest N treatments, N2 and N3, the trees even show signs of returning to Nlimitation. In these treatments, the soil base saturation remains lower, while the pH was only lower at 0–10 depth in the mineral soil, but not in the 10–20 cm depth horizon or in the superficial organic mor-layer. Accurately documenting the effect of N additions on forest growth required a long-term approach, where reasonable rates of application could be compared with extreme rates. Such long-term experiments are necessary to support forest management in achieving goals for developing future forests as they shift in response to major, global-scale changes

Tree growth potential and its relationship with soil moisture conditions across a heterogeneous boreal forest landscape

Forest growth varies across landscapes due to the intricate relationships between various environmental drivers and forest management. In this study, we analysed the variation of tree growth potential across a landscape scale and its relation to soil moisture. We hypothesised that soil moisture conditions drive landscape-level variation in site quality and that intermediate soil moisture conditions demonstrate the highest potential forest production. We used an age-independent difference model to estimate site quality in terms of maximum achievable tree height by measuring the relative change in Lorey's mean height for a five year period across 337 plots within a 68 km2 boreal landscape. We achieved wall-to-wall estimates of site quality by extrapolating the modelled relationship using repeated airborne laser scanning data collected in connection to the field surveys. We found a clear decrease in site quality under the highest soil moisture conditions. However, intermediate soil moisture conditions did not demonstrate clear site quality differences; this is most likely a result of the nature of the modelled soil moisture conditions and limitations connected to the site quality estimation. There was considerable unexplained variation in the modelled site quality both on the plot and landscape levels. We successfully demonstrated that there is a significant relationship between soil moisture conditions and site quality despite limitations associated with a short study period in a low productive region and the precision of airborne laser scanning measurements of mean height

Ultrasonic Pretreatment of Corn Slurry in Batch and Continuous Systems

The effects of ultrasonication of corn slurry, on particle size distribution and enzymatic hydrolysis was studied for the dry-grind mill ethanol industry. Two independent ultrasonic experiments were conducted at a frequency of 20 kHz; in batch and continuous systems. The ground corn slurry (33% m/v) was pumped at flow rates 10-28 L/min in continuous flow experiments, and sonicated at constant amplitude (20µmpeak-to-peak(p-p)). Ultrasonic batch experiments were conducted at varying amplitudes of 192-320µmp-p. After ultrasonication, StargenTM001 enzyme was added to the samples and a short 3h hydrolysis followed. The treated samples were found to yield 2-3 times more reducing sugar compared to the control (untreated) samples. In terms of energy density, the batch ultrasonic system was found to deliver 25-times more energy than the continuous flow systems. Although the experiments conducted in continuous system released less reducing sugar than the batch system, the continuous system was more energy efficient. The particle size of the sonicated corn slurry (both batch and continuous) was reduced relative to the controls (without treatment). The reduction of particle size was directly proportional to the energy input during sonication. The study suggests that both batch and continuous flow ultrasonic systems enhances enzymatic hydrolysis yield, reduces particle size of corn slurry and could be a potential effective pretreatment for corn slurry