<p> Forest ecosystems play an important role in the global carbon (C) cycle by exchanging large amounts of carbon dioxide (CO₂) with the atmosphere. Their potential to act as significant sink for atmospheric CO₂ has been recognized and is relevant to current efforts in reducing atmospheric CO₂ concentrations. Besides the most important greenhouse gas CO₂, forests also emit and consume methane (CH₄) and nitrous oxide (N₂O) as the two other important atmospheric greenhouse gases (GHGs). To date, few attempts have been made to quantify the net effect of forest GHG exchange on the global warming potential. Furthermore, a better understanding of successional and environmental effects on forest processes is required to improve large scale estimates of forest C and GHG exchange. </p> <p> This thesis examines C dynamics and the exchange of the three major greenhouse gases (CO₂, CH₄, and N₂O) in an age-sequence (7-, 20-, 35-, and 70-years-old as of 2009) of afforested pine forests, in southern Ontario, Canada. The impacts of environmental controls on these GHG exchanges were also evaluated. Forest C exchange was determined for 2003 to 2008 using the eddy-covariance (EC) technique and inventory-based biometric measurements. Soil CH₄ and N₂O measurements were conducted from 2006 to 2007 using the static closed-chamber method. In addition, concentrations and fluxes of dissolved organic carbon (DOC) throughout the vertical profile in forest canopy and soil were determined from 2004 to 2005 using throughfall buckets and lysimeters. </p> <p> During periods without climatic constraints, monthly gross ecosystem productivity (GEP) and ecosystem respiration (RE) corrected for differences in site index increased with stand age, whereas monthly net ecosystem productivity (NEP) peaked at the 35-year-old site. In contrast, during constrained periods (e.g. seasonal drought events), monthly GEP and NEP at the 20-year-old site were higher compared to the 35-year-old site because trees may have benefited from sustained availability of soil water in deeper layers. This study further demonstrates that differences in site quality may affect the interpretation of age-related C flux dynamics in chronosequence and synthesis studies (Chapter 2). </p> <p> The temperature-RE relationship was an important control on daily NEP anomalies under optimum growing conditions, whereas constrains on GEP primarily determined NEP during environmentally constrained periods. Furthermore, effects from single environmental variable constrains on NEP anomalies were enhanced as well as outbalanced under multiple environmental variable constrains. The results further indicate that future changes in temperature and precipitation patterns towards drier and warmer conditions as well as greater cloud cover may result in reduced C sequestration potentials in these temperate pine forests (Chapter 3). </p> <p> Early summer drought and heat events in 2005 caused NEP to decrease by approximately 100 g C m⁻² y⁻¹ at each site compared to the other years. This decrease was primarily driven by a decrease in photosynthesis, while the effect of these events on ecosystem respiration was small. Overall, for the years 2003-2007, annual NEP was 219, 155, 36, 148, and 120 g C m⁻² y⁻¹ at the 68-year-old site, 666, 318, 346, 511 and 366 g C m⁻² y⁻¹ at the 33-year-old site, 768, 885, 684, 708 and 826 g C m⁻² y⁻¹ at the 18-year-old site, and-18, 145, 125, 34 and 164 g C m⁻² y⁻¹ at the 5-year-old seedling site, respectively (negative numbers indicating net C source (Chapter 4). </p> <p> Four-year mean values of biometric NEP_(B) and EC-based NEP_(EC) were similar at the 7-year-old seedling (77 and 66 g C m⁻² y⁻¹) and the 70-year-old mature site (135 and 124 g C m⁻² y⁻¹), but differed considerably at the 20-year-old (439 and 736 g C m⁻² y⁻¹) and the 35-year-old sites (170 and 392 g C m⁻² y⁻¹). Integrating NEP across the age-sequence resulted in a total net C sequestration of 137 and 229 t C ha⁻¹ over the initial 70 years as estimated by the biometric and EC method, respectively. The total ecosystem C pool at the 70-year-old site suggested an accumulation of 160 t C ha⁻¹. These three estimates resulted in a mean C sequestration of 175 ± 48 t C ha⁻¹ (Chapter 5). </p> <p> For both CH₄ and N₂O, we observed uptake and emission ranging from -160 to 245 μg CH₄ m⁻² hour⁻¹ and -52 to 21 μg N₂O m⁻² hour⁻¹, respectively (negative values indicate net uptake). Mean N₂O fluxes from mid-April to mid-December across the 7-, 20-, 35-, 70-years old stands were -3.7, 1.5, -2.2, and-7.6 μg N₂O m⁻² hour⁻¹, without age-related pattern, whereas the uptake rates of CH₄ increased with stand age from 6.4 to -7.9, -10.8, and-23.3 μg CH₄ m⁻² hour⁻¹, respectively. For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated forest floor exchanges of CH₄ and N₂O was on average <4% (Chapter 6). </p> <p> DOC concentration in forest floor leachates was positively correlated to stand age, aboveground biomass and forest floor carbon pools. From the period of Mid-April to December, DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon were in the range of ~1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m⁻², respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge decreased with increasing stand age from ~7 to 4, 3, and 2 g DOC m⁻² (Chapter 7). </p> <p> This thesis improved our understanding of C and GHG exchange dynamics and their environmental, physical, and physiological controls in forest ecosystems. This study will also contribute to efforts being made to better predict future forest C and GHG dynamics and their feedbacks on climate under changing environmental conditions. <p> / Thesis / Doctor of Philosophy (PhD)

Understanding how habitat amount and configuration affect species richness, occurrence or abundance has been one of the major foci of research in ecology and biogeography, given its central importance for conservation planning and landscape management. We conducted bird point counts within clearcuts and mature pine stands of different sizes and configuration in working pine forest in north-central Mississippi to determine species associations. Early-successional and mature pine focal species showed varying response to the proportion and proximity of vegetation conditions in the landscape. While elements of configuration exhibited a greater influence on predicted avian abundance in this landscape, meaning many species require a mosaic of habitat conditions that come from both early-successional and mature vegetation types. Efforts to combine management of timber and conservation of songbirds must consider both species’ habitat requirements and the distribution of these requirements in the landscape.

Darbo tikslas – ištirti dotidėjomicetų (Dothideomycetes) ir sordarijomicetų (Sordariomycetes) įvairovę bei ekologines ypatybes aliuviniuose juodalksnynuose ir kormoranų pažeistame pušyne. Darbe pirmą kartą Lietuvoje detaliai ištirta aliuviniuose juodalksnynuose ir kormoranų pažeistame pušyne dotidėjomicetų ir sordarijomicetų rūšių įvairovė ir paplitimas. Sudarytas šiuose miškuose aptiktų grybų rūšių sąvadas. Išaiškintos 72 naujos Lietuvai tirtų aukšliagrybūnų rūšys ir pateikti originalūs jų morfologijos aprašymai. Pirmą kartą Lietuvoje įvertinta aplinkos veiksnių įtaka sumedėjusių bei žolinių augalų dotidėjomicetų ir sordarijomicetų įvairovei. Darbo rezultatai pagilino žinias apie šių grybų paplitimo dėsningumus Europoje saugomuose aliuviniuose miškuose, kas gali būti panaudota šių buveinių biologinės įvairovės išsaugojimui. Kormoranų pažeistame pušyne nustatyti tirtų grybų rūšinės sudėties ir paplitimo ypatumai leidžia įvertinti šių paukščių sukeliamos hipertrofikacijos poveikį pušyno mikobiotai. / The aim of the study was to investigate the diversity and ecological properties of Dothideomycetes and Sordariomycetes in the alluvial black alder forests and the pine forest affected by cormorants. For the first time in Lithuania, detailed investigations were carried out on the diversity and distribution of Dothideomycetes and Sordariomycetes in the above-mentioned forests. The checklist of the study fungi was complied. 72 Dothideomycetes and Sordariomycetes species new to Lithuania were identified and original descriptions of their morphology were provided. The influence of abiotic factors on the diversity of woody and herbaceous plants fungi was evaluated for the first time in Lithuania. The results of the study enhanced the knowledge about the distribution of Dothideomycetes and Sordariomycetes in the alluvial forests protected in Europe, which may be used to protect the biologic diversity of these habitats. The properties of the diversity and distribution of studied fungi in the pine forest affected by cormorants allow assessing the impact of the hypertrophication, caused by these birds on the pine forest mycobiota.

The aim of the study was to investigate the diversity and ecological properties of Dothideomycetes and Sordariomycetes in the alluvial black alder forests and the pine forest affected by cormorants. For the first time in Lithuania, detailed investigations were carried out on the diversity and distribution of Dothideomycetes and Sordariomycetes in the above-mentioned forests. The checklist of the study fungi was complied. 72 Dothideomycetes and Sordariomycetes species new to Lithuania were identified and original descriptions of their morphology were provided. The influence of abiotic factors on the diversity of woody and herbaceous plants fungi was evaluated for the first time in Lithuania. The results of the study enhanced the knowledge about the distribution of Dothideomycetes and Sordariomycetes in the alluvial forests protected in Europe, which may be used to protect the biologic diversity of these habitats. The properties of the diversity and distribution of studied fungi in the pine forest affected by cormorants allow assessing the impact of the hypertrophication, caused by these birds on the pine forest mycobiota. / Darbo tikslas – ištirti dotidėjomicetų (Dothideomycetes) ir sordarijomicetų (Sordariomycetes) įvairovę bei ekologines ypatybes aliuviniuose juodalksnynuose ir kormoranų pažeistame pušyne. Darbe pirmą kartą Lietuvoje detaliai ištirta aliuviniuose juodalksnynuose ir kormoranų pažeistame pušyne dotidėjomicetų ir sordarijomicetų rūšių įvairovė ir paplitimas. Sudarytas šiuose miškuose aptiktų grybų rūšių sąvadas. Išaiškintos 72 naujos Lietuvai tirtų aukšliagrybūnų rūšys ir pateikti originalūs jų morfologijos aprašymai. Pirmą kartą Lietuvoje įvertinta aplinkos veiksnių įtaka sumedėjusių bei žolinių augalų dotidėjomicetų ir sordarijomicetų įvairovei. Darbo rezultatai pagilino žinias apie šių grybų paplitimo dėsningumus Europoje saugomuose aliuviniuose miškuose, kas gali būti panaudota šių buveinių biologinės įvairovės išsaugojimui. Kormoranų pažeistame pušyne nustatyti tirtų grybų rūšinės sudėties ir paplitimo ypatumai leidžia įvertinti šių paukščių sukeliamos hipertrofikacijos poveikį pušyno mikobiotai.

Dry, boreal, forest ecosystems have been shown to alternate due to reindeer presence by changing the field layer. This affects the temperature of the soil, changes the microclimate and the nutrients of soil and field layer. By removal of biomass and trampling stress the ground lichen mats reduce in cover and thickness. In this thesis two dry, boreal pine forests in Northern Finland were studied, to see how the nutrient stocks in the field layer are impacted by reindeer presence. A comparison of the same site, with &gt;20 years in between was studied to see how year-round grazing affect the carbon storage in the field layer compared to winter grazing.    I found that reindeer grazing reduces the vegetation carbon storage, however, the impact on the vegetation nitrogen storage differs with site. Furthermore, the lichens and shrubs had lower C:N ratios because of increased nitrogen content in the plant tissue and unaffected carbon content. These results show that reindeer actively changes the vegetation quantity and quality which will have implications for the soil carbon storage.

The productivity of agricultural and forest ecosystems in regions at higher latitudes is to a large extent governed by low temperature and moisture conditions. Environmental conditions are acting both above- and below-ground and regulating carbon fluxes and evapotranspiration. However, the understanding of various feedbacks between vegetation and environmental conditions is still unclear. In this thesis, two studies were conducted to understand the physical and biological processes. In the first study, the aim was to simulate soil temperature and moisture dynamics in the bare soil with seasonal frost conditions in China. In the second study, the aims were to model seasonal courses of carbon and evapotranspiration and to examine the responses of photosynthesis, transpiration and respiration on environmental conditions in a boreal Scots pine ecosystem in Finland. In both studies the CoupModel was applied to simulate the dynamic responses of the systems. Both sites represented investigations from which a high number of measurements were available. To understand to what extent the data could be used to increase the understanding of the systems, the Generalized Likelihood Uncertainty Estimation (GLUE) was applied. The GLUE method was useful to reduce basic uncertainties with respect to parameter ranges, model structures and measurements. The strong interactions between soil temperature and moisture processes have indicated by a few behavioral models obtained when constrained by combined temperature and moisture criteria. Model performance on sensible and latent heat fluxes and net ecosystem exchange (NEE) also indicated the coupled processes within the system. Seasonal and diurnal courses were reproduced successfully with reduced parameter ranges. However, uncertainties on what is the most general regulation for transpiration and NEE are still unclear and need further systematic investigations. / QC 20101206

In Valencia region (SE Spain), many post-fire communities are dominated by non-resprouting (seeder) species, because of the long history of land exploitation and subsequent abandonment during the last half of 20th century. These communities accumulate fine dry biomass and, therefore, can burn again easily. In fact, Mediterranean forests are suffering from an increase in wildfire frequency since the early 1970s. Wildfires shape the composition and functioning of Mediterranean ecosystems, but we do not know how these ecosystems respond to both the higher fire recurrence and shorter recovery times expected for future climatic scenarios. In this sense, Aleppo pine forest (Pinus halepensis) is one of the most fire affected vegetation of this type in the Mediterranean Basin and to know how it respond to fire is fundamental to design management plans. After fire, regeneration of this forest can be highly variable, and it can go from extremely dense tree stands (overstocked pine) to treeless shrublands dominated by seeder species. All these regenerated stands are fire prone with limited ability to deliver multiple ecosystem services. Although several management techniques are applied to redirect these post-fire ecosystems towards less vulnerable and more functional communities, we do not know yet which amongst them could serve to foster more diverse and multifunctional landscapes. Therefore, the general objective of this thesis is to investigate the functioning of these Mediterranean ecosystems as consequence of shifts in fire regime and forest management application, using different techniques, in different post-fire regenerated ecosystems (overstocked pine forests and dense shrublands). To do so, we calculate, within Mediterranean Pinus halepensis forests affected by wildfires, the supply of multiple ecosystem services (biodiversity conservation, carbon sequestration, disturbance regulation, food production, supporting services, and multifunctionality), through up to 25 aboveground and belowground attributes. Our main findings are (1) High fire recurrence and time since last fire interacted to determine ecosystem services but did not affect their synergies and trade-offs between them. Their combined effects reduced carbon sequestration and multifunctionality. Disturbance regulation diminished drastically with the first fire, with no effect of further fires. However, their effects dampened, and even became positive, for biodiversity conservation and food production services if provided enough time to recover. (2) Thinning in overstocked pine stands enhances ecosystem attributes associated with biodiversity conservation without compromising the provision of carbon sequestration. After 10 years, two levels of thinning, (600 and 1200 trees·ha-1), similarly affected ecosystem attributes, which suggest that 1200 trees·ha-1 suffice to enhance individual ecosystem attributes. (3) Clearing within dense shrubland dominated by seeder species enhances ecosystem attributes associated with biodiversity conservation without compromising the capacity of ecosystem to sequester carbon. (4) Plantation of resprouting species combined with thinning and clearing, in overstocked pine forests and dense shrublands respectively, can enhance the provision of ecosystem services of disturbance regulation, food production and ecosystem multifunctionality. (5) Prescribed burning reduces the amount of dead fuel, increases biodiversity conservation, and improves food production. However, these effects become negative, in addition to the decline in disturbance regulation and multifunctionality, if prescribed burning is applied frequently. (6) Combining different management activities can enhance the supply of multiple ecosystem services simultaneously by reducing the trade-offs in between them and therefore, establish multifunctional Mediterranean landscapes.