The aim of this thesis was to evaluate the survival, growth, and phytoremediation potential of Populus and Salix seedlings grown in polluted soils. More specifically, the following topics were studied: (1) the survival and growth of two European aspen clones and four hybrid aspen clones grown in control soil (pristine), old creosote soil polluted with hydrocarbons, and pristine soil spiked with fresh diesel oil at three different planting densities in a greenhouse over two growing seasons (Article I); (2) the survival, growth, and hydrocarbon removal of three European aspen clones and seven hybrid aspen clones grown in hydrocarbon-contaminated soil (including polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs)) under field conditions over 4 years (Article II); and (3) the growth and metal accumulation ability of Salix psammophila seedlings with bamboo biochar (BBC) amendment at ratios of 0–7% in soils heavily contaminated by Cd and Zn in a pot experiment over 180 days (Article III).
In study I (Article I), the survival rates of European aspen and hybrid aspen clone seedlings were 70–100% in control soil, 99% in the old creosote-contaminated soil, and 22–59% in the diesel-contaminated soil across all planting densities. The heights of aspen seedlings were 5–44% and 9–38% lower and the stem dry biomass was 9–93% and 34–63% lower in diesel-contaminated and creosote-contaminated soils, respectively, compared to the control. Low plant density increased survival rates and growth compared to higher density treatments. Of all the clones, hybrid aspen clones 14 and 291 and European aspen clone R3 showed reasonable survival and growth across all treatments. Soil treatment, planting density, and clone type significantly affected survival rate, height, and stem dry biomass (p < 0.05).
In study II (Article II), the highest survival rates in old creosote-contaminated soils were in clone 291 (72%) among hybrid aspen clones and clone R3 (70%) among European aspen clones. Hybrid aspen clones 14 and 34 had 16–211% greater heights than other hybrid aspen clones. The height of European aspen clone R3 was also 25‒35% greater than that of other European aspen clones. However, clone type did not significantly affect seedling survival or height (p > 0.05). Among hybrid aspen clones, clone 134 had the largest hydrocarbon removal at a depth of 5–10 cm and clone 191 at a depth of 10–50 cm. Clone 14 also showed potential for removing hydrocarbons at both soil depths. In European aspen clones, clone R2 had the highest hydrocarbon removal at both soil depths. However, all clones showed an ability to remove total PAHs and TPHs from the soil (but p < 0.05 only at a soil depth of 5–10 cm). The reduction in hydrocarbon levels in the soil was more prominent at a soil depth of 5–10 cm than at a depth of 10–50 cm. Based on studies I and II, European aspen and hybrid aspen clones can be considered candidates for the remediation of soils polluted with PAHs and TPHs.
In study III (Article III), BBC ratios of 1% and 5% resulted in only slight decreases in characteristics, especially height (0.6‒1.3%) but also total dry biomass (2‒10%), of S. psammophila seedlings compared to the control, whereas BBC 3% increased these measurements slightly (2% increase). BBC 7% reduced the height (16%) and total dry biomass (26%) of seedlings compared to the control. BBC amendment increased the accumulation of Cu, Cd, and Zn in different plant tissues, especially Cd and Zn accumulation (23‒30% and 13‒24%, respectively), in the BBC 3% treatment compared to the control. Based on these findings, S. psammophila with BBC amendment can be considered a candidate for phytoremediation. However, metal accumulation in the roots, stems, and leaves was not significantly affected by the BBC 1‒7% treatments (p > 0.05), except for Pb accumulation in the roots and Cu accumulation in the stem (p < 0.05).
Overall, hybrid aspen, European aspen, and S. psammophila seedlings showed reasonable survival and growth, photosynthetic activity, efficient hydrocarbon removal from soil and metal accumulation ability both under greenhouse conditions and in a field experiment. Therefore, these species could be used to depollute areas affected by a range of hydrocarbons or Cd and Zn. However, future research should be conducted in the field to verify the abilities of hybrid and European aspens and S. psammophila to remediate soil contaminated by hydrocarbons, Cd, or Zn, and such studies should also use different planting densities and soil amendments over longer periods.
To address the issue of climate change, the EU’s climate and energy framework has set targets to improve energy efficiency. Reducing greenhouse gas (GHG) emissions requires higher energy efficiency in the wood supply of forest industries. The aim of the study was to clarify the energy-efficiency baseline for wood-harvesting operations, define useful measures and follow up the total fuel consumption and resulting emissions.
The results indicated that wood-harvesting entrepreneurs have a positive attitude towards energy efficiency. The fuel consumption of wood-harvesting machines was the lowest for the final fellings, while in first thinnings, the consumption was highest per cubic metre harvested. The average cubic metre-based fuel consumption and GHG emissions in respect of wood harvesting were more than double in the first thinning compared to the final felling. Better allocation of harvesting machines could reduce fuel consumption and GHG emissions while improving work efficiency. Hour-based fuel consumption is most affected by machines’ engine power and wood-harvesting conditions of forest stands. Fuel consumption per cut cubic metre is affected by wood-harvesting conditions and machine units.
The calculated energy efficiency was highest in final fellings. A more significant factor than fuel consumption (input) is the amount of harvested wood (output) in the energy-efficiency equation. Energy efficiency can also be improved by operator education. Trucks which are used for harvesting-machine relocation have a significant impact on wood-harvesting operations' total fuel consumption and emissions. It is therefore essential to minimise the number of relocations and operational and resource planning should be developed. In the future, the examination of fuel consumption and GHG emissions should be extended to the entire wood-harvesting chain, including long-distance transportation and timber trade, and for example the effect of operator should be investigated in more detail.
Boreal forest soils are globally one of the most extensive carbon storages, whereas soil respiration (CO2 efflux) forms the largest carbon flux from the ecosystem to the atmosphere. Current changes in the world climate may have unpredictable effects on belowground carbon processes, and thereby, on the carbon balance of boreal forests.
To better understand the various processes in soil and to quantify the potential changes in the carbon cycle, forest-floor respiration (RFF) was partitioned into five different components, and tree-root respiration (RR) was estimated, using four different methods in a mature boreal Scots pine (Pinus sylvestris L.) stand in southern Finland. Non-structural carbohydrate (NSC) concentrations in tree roots were determined, and carbon allocation to belowground by trees was estimated with the whole-tree carbon model ‘CASSIA’. In addition, RR and heterotrophic soil respiration (RH) were separated using root exclusion in seven coniferous forests along a latitudinal gradient in Northern and Central Europe.
The RR comprised almost half of the RFF, the RH almost a third, and ground vegetation and respiration of mycorrhizal hyphae the remaining fifth in the boreal Scots pine stand. While the annual RR decreased throughout the first three study years, the RH increased when the mycorrhizal roots were excluded from the treatments. The RR and most of the NSC concentrations were higher in the warmer years and lower in the cooler, as estimated with most of the methods. Three methods resulted in rather similar RR estimations, while the RR estimated with root incubation was significantly lower. The RR was over 50% of the annual photosynthesis in the northernmost forest stand, whereas in the southernmost stand it was only up to 15%. Carbon allocation to the belowground, as modelled with CASSIA was a third of the annual photosynthesis on average and almost 5% for the symbiotic mycorrhizae.
The growing conditions in northern boreal forests have remained similar for millennia. However, amplified climate change may cause higher mean annual temperatures and precipitation sums, longer growing seasons, along with increased occurrence of extreme weather events (drought, heavy rain, or summertime frost) in the region. The relationship that forest vegetation has with soil nutrients and the exchange of carbon dioxide (CO2) between the forest and atmosphere may change. This dissertation focuses on quantifying the baseline status of northern boreal forests from these aspects, to be able to predict the upcoming changes more precisely. Soil total phosphorus (P) and nitrogen (N) contents were important factors in explaining the community composition of understory vegetation in the study site. The site was located in a region near a phosphate ore, where soil nutrient contents are highly variable. The number of herb, grass, and sedge species increased with N and P contents in the humus, especially with P. The increasing P content, on the other hand, positively correlated with downy birch (Betula pubescens Ehrh.), which was the dominant tree species of the research plot.
The understory vegetation had an important role in the CO2 exchange rates of a northern boreal Scots pine (Pinus sylvestris L.) forest site. The annual CO2 dynamics varied between the canopy and understory, so that when the canopy began photosynthesizing in the spring, the understory was still under snow cover. The cumulative temperature sum had a higher positive correlation with photosynthesis than the total ecosystem respiration (TER) rate of the pine site. Overall, the pine site was a weak carbon sink during the growing season, although it temporarily turned into a carbon source during a cold and rainy summer.
Extreme weather events, and their effects on the CO2 dynamics of forests, were studied on a Scots pine site and a Norway spruce (Picea abies (L.) Karst.) site. Both sites had experienced extreme summers during the studied times, but the CO2 flux rates in the Norway spruce site responded more clearly to them. The TER rates of the Norway spruce forest declined when it was warm and dry. This likely happened because of decreased decomposition of organic matter. The decline was, however, only temporary, and TER returned to normal when the temperature and precipitation returned to their average levels. Thus, the studied forest sites seemed to, so far, be rather resilient towards extreme weather events.
Several studies have found that N availability will increase because of warmer temperatures, which speeds up decomposition and nutrient mineralization. However, decomposition may potentially slow down in some spruce forests due to heat. Local variation may thus be high when it comes to the availability of nutrients or to the CO2 dynamics of forests. While modeling studies are important for predicting the responses of northern forests to climate change on the large scale, our research reminds that local-scale studies are also inevitable for gaining a more precise picture.
This dissertation examines how carbon storage in forests may be increased by changing forest management at the stand level. To extend the economics of forest carbon storage beyond single-species even-aged stands, this dissertation develops a bioeconomic model framework that incorporates the internal structure of the stand, and the optimal choice between continuous cover forestry and forestry based on clearcuts. The studies apply empirically estimated growth models for boreal conifer and broadleaf tree species.
The first article presents an analytically solvable economic model for timber production and carbon storage. Continuous-time optimal control theory is utilized to solve the thinning path and the potentially infinite rotation age: if no optimal finite rotation age exists, thinnings are performed indefinitely while maintaining continuous forest cover. The second article extends this model by applying a size-structured growth model for Norway spruce, a detailed description of revenues and costs, and several carbon pools. The timing and intensity of thinnings, the rotation age, and the management regime are optimized numerically. In the third article, the optimization approach of the second article is extended to mixed-species size-structured stands. Species mixtures include the commercially valuable Norway spruce and birch, and other broadleaves (e.g. Eurasian aspen and maple) that have no market value.
Optimal rotation age is shown to either increase or decrease with carbon price depending on interest rate and the speed of carbon release from harvested wood products. Given empirically realistic assumptions, carbon pricing increases the rotation period and eventually causes a regime shift from rotation management to continuous cover management. Hence, carbon pricing heightens the importance of determining the management regime through optimization.
Optimal thinnings are targeted to the largest size classes of each tree species. Carbon pricing postpones thinnings and increases the average size of harvested and standing trees. Without carbon pricing, commercially non-valuable other broadleaves are felled during each harvesting operation. When carbon storage is valued, some of the other broadleaves are retained standing until they are large, thus increasing tree species diversity and deadwood quantity.
The results suggest that moderate carbon price levels increase timber yields, especially of sawlog. Increasing carbon storage through changes in forest management is shown to be relatively inexpensive, and the marginal abatement cost is the lower, the higher the number of tree species in the stand.
Process-based soil carbon models can simulate small short-term changes in soil organic carbon (SOC) by reconstructing the response of soil CO2 and CH4 emissions to simultaneously changing environmental factors. However, the models still lack a unifying theory on the effects of soil temperature, moisture, and nutrient status on the boreal landscape. Thus, even a small systematic error in modelled instantaneous soil CO2 emissions and CH4 emissions may increase bias in the predicted long-term SOC stock.
We studied the environmental factors that control CO2 and CH4 emissions in Finland in sites along a continuum of ecosystems (forest-mire ecotone) with increasing moisture and SOC (I and II); soil CO2 emissions and SOC in four forest sites in Finland (III); and SOC sequestration at the national scale using 2020 forest sites from the Swedish national forest soil inventory (IV). The environmental controls of CO2 and CH4 emissions, and SOC were evaluated using non-linear regression and correlation analysis with empirical data and by soil C models (Yasso07, Q and CENTURY).
In the upland forest-mire ecotone, the instantaneous variation in soil CO2 emissions was mainly explained by soil temperature (rather than soil moisture), but the SOC stocks were correlated with long-term moisture. During extreme weather events, such as prolonged summer drought, soil CO2 emissions from the upland mineral soil sites and CH4 emissions from the mire sites were significantly reduced. The transition from upland forest to mire did not act as a hot spot for CO2 and CH4 emissions. The CO2 emissions were comparable between forest/mire types but the CH4 emissions changed from small sinks in forests to relatively large emissions in mires. However, the CH4 emissions in mires did not offset their CO2 sinks. In the Swedish data, upland forest SOC stocks clearly increased with higher moisture and nutrient status. The soil carbon models reconstructed SOC stocks well for mesotrophic soils but failed for soils of higher fertility and wetter soils with a peaty humus type. A comparison of measured and modelled SOC stocks and the seasonal CO2 emissions from the soil showed that the accuracy of the estimates varied greatly depending on the mathematical design of the model’s environmental modifiers of decomposition, and their calibration.
Inaccuracies in the modelling results indicated that soil moisture and nutrients are mathematically underrepresented (as drivers of long-term boreal forest soil C sequestration) in process-based models, resulting in a mismatch for both SOC stocks and seasonal CO2 emissions. Redesigning these controls in the models to more explicitly account for microbial and enzyme dynamics as catalysts of decomposition would improve the reliability of soil carbon models to predict the effects of climate change on soil C.
Climate change mitigation aims to reduce greenhouse gases in the atmosphere. Forest mitigates climate change by accumulating atmospheric carbon to biomass. This biomass can be used to various products which also act as a carbon sink. Carbon sequestration is the opposite of carbon emission, but not fully. Forest carbon storages are uncertain and temporal but the role of forests as temporary carbon storages still has value. However, climate policy must take this into account both in the implementation of policies and in the valuation of carbon sinks.
The thesis consists of four articles and a summary chapter. Articles represent different perspectives of the forest sector and the use of forests and wood products to mitigate climate change. They cover the use of forests from the growth of trees to the use of wood products.
In the first article we analyze with an age-class model how forest owners will change their forest management if there is a subsidy based on the forest carbon storage. The results show that enhancing investments for forest growth increases and that forest rotation will be longer. We also investigate how subsidies for silvicultural investment will affect carbon sequestration of the forest. The second article analyses wood consumption and HWP carbon stock in Finland until 2050. The main HWP carbon pool consists of products made of sawn wood. The HWP carbon pool in Finland seems to increase until 2050 even in the case of decreasing consumption of sawn wood. The third article deals with optimal forest management where the growth of the forest is described by a size-class model. The results show a feature on size-classified matrix models that significantly reduces the comparability of forest management results of these models. The optimal thinning intensity and rotation length of forest are highly dependent of the specification of the model. The fourth article analyzes the existing climate policy for forestry in the EU. Because the policy only applies to one period, we can use a simple two-period model to describe the impact of the policy. The results show that constraints on current climate policy design reduce the potential of using forests to mitigate climate change.
The framework in the summary of the articles complements the conclusions in the articles and builds a view towards a more comprehensive conclusion for governance of forest sector to mitigate climate change.
Human land use affects the climate through various channels. This thesis focuses on the optimal (i.e. welfare-maximizing) regulation of land use sector climate impacts using market-based instruments, such as taxes and subsidies. The thesis consists of four articles and a summary chapter. Each article focuses on a separate aspect of land use sector climate policy.
The first article outlines a comprehensive tax policy for jointly regulating carbon storage in biomass, soils and products. Considerations regarding soil carbon storage are emphasized.
The second article concerns the regulation of CO2 emissions from the energy use of logging residues. The harmfulness of these emissions is compared with that of fossil emissions. A way to harmonize the carbon taxation of the both energy sources is presented.
The third article regards the application of the additionality principle to forest carbon subsidies. In the stand-level context it appears that the additionality principle can be implemented without distorting the optimal rotation, by reclaiming subsidies for baseline carbon storage by a site productivity tax on forests. However, at the market-level such a tax distorts the optimal rotation and the optimal land allocation. These distortions can be avoided, if the excess subsidies are eliminated by general land taxation (which also targets other land use).
The fourth article presents a new concept: the Social Cost of Forcing (SCF), which is the social cost of the marginal unit of radiative forcing at a given moment. It is a fundamental price that can be used to value different forcing agents. Forcing agents’ prices that are based on the SCF are consistent with the Social Cost of Carbon, and can therefore be consistently applied in cost-benefit analysis or utilized to harmonize the regulation of non-CO2 forcing agents.
Together the four articles contribute to our understanding of land use sector climate policy design.
Human-induced disturbances may change vegetation and carbon (C) and nitrogen (N) processes in the forest floor and the soil beneath it. The aim of this dissertation was to study the effects of physical and chemical disturbance on boreal forest soil and vegetation. The specific aims were to evaluate the rate and direction of the forest ecosystem recovery from the disturbance and to assess how C and N processes are affected by different disturbances regimes. Two contrasting soil-affecting treatments – stump harvesting and sprinkling infiltration – were studied as case studies representing a disturbance. Sprinkling infiltration alters the chemical composition of forest soil, whereas stump harvesting results in changes especially in the physical structure of the forest soil. Furthermore, in contrast to stump harvesting where C and nutrients are removed from the soil with the removed biomass, sprinkling infiltration adds large quantities of C and nutrient-rich surface water into the forest soil. As stump harvesting and sprinkling infiltration are relatively newly introduced land use practices, very little is known of their long-term effects on boreal forest soil and vegetation.
The effects of stump harvesting on forest soil surface disturbance, C and N pools and mineralization rates, understory vegetation, seedling growth and coarse woody debris (CWD) were studied in Norway spruce (Picea abies (L.) Karst.) stands located in Central and Southern Finland. The results of this study indicate that stump harvesting causes soil mixing and relocation of organic matter in the soil profile, which in turn is reflected to the soil C and N dynamics as soil C and N pools tended to be lower following stump harvesting. Stump harvesting combined with site preparation tends to cause more extensive soil surface disturbance than site preparation alone, and the mixing effect of stump harvesting persists on soil surface after a decade since harvest. Furthermore, this study underlines that stumps, coarse roots and fine coarse roots represent a significant portion of the CWD, belowground biomass and nutrients in a forested stand, and thus their extraction results in substantial and direct removal of biomass, C and nutrients from the stand.
The effects of sprinkling infiltration on forest soil, tree growth and understory vegetation and their respective recovery were studied in an experimental stand that had been infiltrated with surface water in order to produce artificial groundwater. The study revealed that the previously observed changes soil chemistry had persisted in the experimental stand; soil pH and base cation concentration as well as the rate of net N mineralization were still significantly higher at the infiltrated plots after a 12–15-year recovery period. These results lead to the conclusion that sprinkling infiltration results in the long-term neutralization of the forest soil. In contrast to tree growth, theunderstory vegetation had not benefited from the added nutrients and organic matter, instead the large amounts of added water had created conditions unfavorable to certain plant species. In conclusion, sprinkling infiltration is an environment altering treatment which, based on the findings of this study, can have short-term effects on tree growth and long-term effects on soil processes and understory vegetation and ultimately, ecosystem recovery.
The results of this study demonstrate that disturbances affect the function and structure of forest soil and these changes can persist for at least a decade on the surface of the soil in the organic layer and deeper in the mineral soil. Furthermore, this dissertation highlights the need for long-term perspectives in ecosystem management and planning.
This dissertation aims to develop the economics of even-aged Scots pine (Pinus sylvestris L.) management. In our economic-ecological model, a detailed process-based forest growth model is connected to an economic description of stand management. The process-based growth model is able to describe forest growth in management regimes and climate conditions previously not experienced, because it applies causal relationships and feedbacks instead of statistical correlations. Optimization is carried out with an effective general pattern search algorithm. The optimized variables include rotation length, initial stand density, and the timing, type, intensity, and number of thinnings. Essential model details include the quality pricing of timber and detailed harvesting cost functions. Integration of carbon subsidy systems into the model enables the determination of the economically optimal carbon storage with various carbon price levels. Finally, the growth model is extended to include a direct link between climate change and tree growth, to optimize stand management in a changing climate.
The dissertation thesis is composed of a summary section and three articles, which produce a coherent and comprehensive picture on the optimal stand management of Scots pine in the relevant growth conditions of Fennoscandia. The results demonstrate the necessity to simultaneously optimize all stand management variables, and the advantages of having a detailed model. Optimal stand management is shown to be sensitive to growth conditions, interest rate, and management objective, along with the design of the carbon subsidy system and the subsidy level. The stand-level analysis is additionally extended to the national level, and adapting forest management was found to potentially be a cost-efficient method for carbon abatement in Finland. Furthermore, the optimal adaptation of stand management in a changing climate remarkably improves the economic surplus from forestry.
This study aims to quantify how the spatially varying vegetation modifies the carbon sink of a boreal bog. Photosynthesis, respiration, biomass composition, biomass production and net ecosystem exchange were studied on three levels: plant species, community and ecosystem.
There was a clear plant species turnover and a strong decrease in standing biomassfrom dry to wet plant communities. Biomass production was even along the water table gradient due to higher biomass turnover rate of wet habitat species than hummock species. Both respiration and gross photosynthesis were the highest in dry plant communities, but their symmetrical water table responses resulted in no differences in net ecosystem exchange among plant communities. However, this evenness did not hold in the absence of Sphagna; sparsely vegetated bare peat surfaces were mostly carbon sources. The small difference in water table between Sphagnum-covered hollows and bare peat surfaces suggests that even a small change in water table could induce shifts between them.
The observed spatially-even carbon sink contradicts earlier studies. However, the components behind that spatial evenness showed high variability and responded to environmental conditions as previously observed. The site-specific relative abundances of functionally varied species appeared to have a larger effect on the overall carbon sink than anticipated.
Different plant species and communities had the highest photosynthesis and carbon sink at distinct times of the growing season, decreasing the ecosystem-level seasonal variation. Over the three studied years, the roles of plant communities in the ecosystem-level carbon sink changed. This indicates that the presence of species with different seasonal growth patterns and responses to environmental conditions could increase ecosystem resiliency in changing conditions. To verify this, the responses of functionally different components to environment, either based on natural variation or experimentally defined, should be included in processmodels predicting the fate of bog carbon sink in changing climate.
The main aim of this study was to investigate the dynamics and biophysical controls of carbon, water and energy exchange over a semiarid shrub ecosystem in the Mu Us desert, northern China, using continuous eddy-covariance (EC) measurements. Specific objectives were as follows: (1) To examine intra-annual variations in net ecosystem CO2 exchange (NEE) and its biophysical controls (Paper I); (2) To quantify the diurnal and seasonal variations in surface energy-balance components, and to examine the partitioning of net radiation (Rn) among different energy components at diurnal and seasonal timescales (Paper II); and (3) To examine how ecosystem production and water use efficiency (WUE) vary inter-annually with contrasting precipitation (PPT) and soil moisture patterns (Paper III).
The results showed that, soil water content (i.e. at 30 cm depth, SWC_30), or water deficit, imposed a major control on the seasonal dynamics of carbon assimilation and energy partitioning. Water deficit (i.e. SWC_30 < 0.10 m3 m-3) was a major constraint over daytime NEE, and also interacted with other stresses, e.g. heat stress and photoinhibition (Paper I). Low soil moisture reduced the temperature sensitivity (Q10) of total ecosystem respiration (TER). Rain events triggered immediate pulses of carbon release from the ecosystem, followed by peaks of CO2 uptake 1–2 days later. Leaf area index (LAI) accounted for 45 and 65% of the seasonal variation in NEE and gross ecosystem production (GEP), respectively. On the other hand, sensible heat flux (H) exceeded latent heat flux (λE) during most time of the year (Paper II). The evaporative fraction (EF, i.e. λE/Rn), Priestley-Taylor coefficient (α), surface conductance (gs) and decoupling coefficient (Ω) all correlated positively with SWC_30 and LAI. The direct enhancement of λE by high vapor pressure deficit (VPD) was buffered by a concurrent suppression of gs, which controlled EF and α by mediating the effects of LAI, SWC_30 and VPD.
At the annual scale, net ecosystem production (NEP, here defined as −NEE) indicated a rapid shift from an annual sink of carbon in 2012 (NEP = 77 ± 10 g C m-2 yr-1) to a source of carbon in 2014 (NEP = -22 ± 5 g C m-2 yr-1), with the year 2013 being close to carbon neutral (NEP = -4 ± 10 g C m-2 yr-1) (Paper III). GEP, TER and evapotranspiration (ET) also declined over the three years. Suppressed annual carbon and water fluxes were observed in years with low spring soil moisture. GEP declined more than TER and ET, leading to reduced carbon sequestration and WUE (i.e. GEP/ET). Neither annual nor growing-season PPT amount could explain the year-to-year variation in carbon fluxes. ET was a better proxy for water available to ecosystem carbon exchange on an annual basis. Autumn soil moisture levels were carried over winter to spring, and affected the rates of leafout, plant growth and carbon uptake in the early- to mid-growing season.
Environmental factors have a dual effect on growth as they affect both the momentary growth rate and the rate of ontogenetic development. Photosynthesized carbon on the other hand is needed for growth, respiration and other purposes. According to opposite theories, growth rate is determined by: 1) the availability of carbon for growth (source limitation) or 2) limitation that environmental factors cause on tissue ability to grow (sink limitation). Understanding the responses of wood, needle and root growth to environmental and other factors allows us to predict changes in tree growth and carbon balance in changing climatic conditions.
The purpose of this study was to define the effects of temperature on Scots pine growth at different temporal scales and to estimate the relative importances of the source and sink effects on growth. For that, a dynamic growth model CASSIA (Carbon Allocation Sink Source InterAction) was constructed.
CASSIA was able to predict daily primary, secondary and needle growth rate variation with indirect and direct effects of temperature. In addition, warm previous late summer was observed to lead to enhanced length (in temperature accumulation units) of shoot growth period in the following year. Growth onset during spring was a continuous process determined by temperature accumulation instead of momentary temperatures.
Short-term growth variations in normal conditions were concluded to be sink limited because the within year growth was satisfyingly predicted with temperature and without direct effect of photosynthesis or stored carbon. On the other hand carbon source effect (GPP) was needed to produce the between year variation in growth.
The results suggest that growth is limited by a complex combination of sink and source effects. Furthermore, environmental factors affect growth at different time scales varying from instantaneous to delayed effects from previous year(s). More research is needed to determine carbon flows to different processes.
In this study, the effects of restoration of forestry-drained peatlands on the nutrient and organic carbon exports and methane dynamics of the restored sites are explored. The study consists of four sub-studies. Two of the sub-studies are concerned with the effects on water quality and export of elements of restoration and were conducted on a catchment scale. One of the studies was conducted in the laboratory, and assessed the release of elements from peat samples under anaerobic inundation simulating the effects of a rising water table after restoration or logging. The fourth study was again a field study, in which the differences in methane emissions between undrained, drained and restored spruce swamp forests were assessed. In all, 24 different pristine, drained and restored sites are featured in the study, one site being present in two of the sub-studies.
The results indicate potentially large effects of restoration especially on the nutrient rich spruce-dominated sites, which had the highest restoration-induced increases in organic carbon and nutrient exports in the catchment studies, and which also exhibited high methane emissions after restoration, higher than in the undrained or drained state. The results should prompt research into the techniques applied in restoration of such sites and into the processes which lie behind these large effects.
The aims of the study were to identify factors related to temporal and spatial variation in forest soil CO2 efflux(Fs), compare measurement chambers, and to test effects of a climate change experiment. The study was based on four-year measurements in upland Scots pine forests.
Momentary plot averages of Fs ranged from 0.04 to 1.12 gCO2m−2 h−1 and annual estimates for the forested area from1750 to 2050 gCO2 m−2. Soil temperature was a dominant predictor of the temporal variation in Fs (R2=76–82%). A temperature and degree days model predicted Fs of independent data within 15% on the average but underestimated it during the peak efflux period (July–August), possibly because of seasonal pattern in growth of roots and mycorrhiza. A comparison sub-study indicated that the reliability of the measurement chambers was not related to the principle i.e. non-steady-state through-flow, non-steady-state non-through-flow or steady-state through-flow.
Spatial variability of Fs within 400 m2 plots in four stands was large; coefficients of variation (CV) ranged from 0.10 to 0.80, with growing season averages of 0.22–0.36. A positive spatial autocorrelation was found at short distances (3–8 m). In data from several stands, thickness of the humus layer explained 28% of the variation in Fs, and with the distance to the closest trees it explained 40%. Fs also correlated with root mass of the humus layer. Between-plot differences in Fs were small.
In the climate change experiment, CO2 enrichment and air warming consistently, but not always significantly,increased Fs in whole-tree chambers. Their combined effect was additive, with no interaction; i.e. +23–37% (elevated CO2), +27–43% (elevated temperature), and +35–59% (combined treatment), depending on year. Air warming was a significant factor in the 4-year data according to ANOVA. Temperature sensitivity of Fs under the warming, however, decreased in the second year.
In this study, 1) a model to estimate soil carbon dioxide (CO2) balance for forestry-drained peatlands was tested on site and countrywide levels in Finland. 2) A dataset of annual soil–atmosphere fluxes of CO2, methane (CH4) and nitrous oxide (N2O) from 68 sites was collected, and models fitted for their upscaling to a countrywide level. 3) The current greenhouse gas impact of the 68 study sites, including soil CO2, CH4 and N2O balances and the CO2 sink function of tree biomass increment, was estimated.
The soil CO2 balance estimation, as the difference between litter input to soil and CO2 efflux from soil, was straightforward to apply, but considerable uncertainty was caused by the inadequate level of knowledge on belowground plant–soil carbon fluxes. Soil–atmosphere gas fluxes could be upscaled to a countrywide level utilizing readily available forest inventory results and weather statistics. Soils in nutrient-rich study sites were sources of greenhouse gases while those in nutrient-poor study sites were sinks, on average. The current greenhouse gas impact, when no forest fellings occurred, was nevertheless climate cooling for both the nutrient-rich and poor sites due to the considerable CO2 sink formed by increasing tree biomass.
The aim of this work was to analyse how the seasonal biomass growth and allocation in a boreal bioenergy crop (Phalaris arundinacea L., hereafter RCG) were affected by elevated temperature and CO2 under different levels of groundwater. For this purpose, plants in peat monoliths representing young and old cultivations were grown in auto-controlled environmental chambers over two growing seasons (April-September, 2009 and 2010) under elevated temperature (ambient + 3.5°C) and CO2 (700 μmol mol−1). Three levels of groundwater, ranging from high (0 cm below the soil surface), to normal (20 cm below the soil surface) and low (40 cm below the soil surface), were used.
Compared to growth under ambient conditions, elevated temperature (ET) enhanced leaf development and photosynthesis in the RCG plant. Consequently, ET enhanced biomass growth during early growing periods. It also reduced photosynthesis and caused earlier leaf senescence during later growing periods. ET therefore reduced total biomass growth across the entire growing season. Elevated CO2 (EC) significantly increased biomass growth throughout the growing period primarily because of increased leaf area and photosynthesis. Lower groundwater level (LW) decreased the growth of RCG, mainly because of lower leaf area and photosynthesis. Furthermore, LW accelerated the cessation of growth, thus making the growing season shorter compared with the effects of higher groundwater levels. The LW- induced reductions in biomass growth were exacerbated by ET and partially mitigated by EC. The combination of elevated temperature and CO2 (ETC) slightly increased plant growth. The age of cultivation did not affect the biomass growth among the three major organs (leaf, stem and root) and thus did not affect total biomass. Biomass allocation was clearly controlled by plant phenology.
Biomass growth was mainly allocated to leaves and stems in the early growing season, to stems in the middle of the growing season and to roots later in the growing season. Under EC, root growth contributed more to total biomass growth compared to the leaf and stem growth in biomass, regardless of groundwater levels. The opposite was observed under ET and ETC and well-watered conditions (which was opposite to what was observed under LW). Our results show that climatic treatments affected biomass growth and biomass allocation to each of the three plant organs, while the direction and extent of climate-related changes in biomass growth and allocation depended on the availability of groundwater. The influence of groundwater level appeared to be crucial for the carbon gain regarding the production of RCG biomass for energy purposes and the concurrent sequestration of carbon in soils under changing climates in the mire sites used to cultivate RCG.