Vom 11. bis zum 13. September 2023 fand die Forstwissenschaftliche Tagung (FowiTa) 2023 in Dresden statt. Die FowiTa findet regelmäßig alle zwei Jahre rotierend an einer der forstlichen Universitäten Deutschlands (Göttingen, Tharandt, Freiburg, München) statt. Sie bietet den im Bereich der Forstwissenschaften Tätigen die Möglichkeit, aktuelle Erkenntnisse aus ihrer Forschung einem breiten forstwissenschaftlichen Publikum vorzustellen. Neben in parallel stattfindenden Sessions organisierten Vorträgen waren Poster-Ausstellungen inkl. Vorstellung der einzelnen Poster und Vorträge von renommierten Keynote-Speakern Teil der FowiTa 2023. Weiterhin bekamen Nachwuchswissenschaftler aus dem Bereich der Forstwissenschaften die Möglichkeit, im Rahmen eigener Sessions ihre Beiträge zur Forschung zu präsentieren. Im vorliegenden Tagungsband sind die Abstracts der gehaltenen Vorträge sowie die ausgestellten Poster aufgeführt.:Programm 10 11. September 2023 14 BEG | Begrüßung 15 K-01 | Keynote - Das Selbst und die Anderen - Waldpolitik zwischen Identitäten und Institutionen (Prof. Dr. Daniela Kleinschmit) 19 K-02 | Keynote - Forestry's Eduring Debt to German Science (Prof. Dr. William Hyde) 21 V-01 | Waldschutz I: Borkenkäfer 23 V-02 | Waldwirtschaft und Klimawandel I: Umwelt, Stress, Mortalität 30 V-03 | Ökologie, Biodiversität, Naturschutz I 39 V-04 | Forstgenetik, Forstpflanzenzüchtung I 46 V-05 | Forstbotanik I 53 V-06 | Waldbau I 61 V-07 | Waldschutz II: Borkenkäfer 69 V-08 | Waldwirtschaft und Klimawandel II: Waldanpassung, Baumartenwahl 77 V-09 | Ökologie, Biodiversität, Naturschutz II 87 V-10 | Forstgenetik, Forstpflanzenzüchtung II 96 V-11 | Eschen-Session 104 V-12 | Waldbau II 116 P 1-3 | Poster Session 126 P-1 | Management mit sozialwissenschaftlichem Schwerpunkt & Sozialwissenschaften 127 P-2 | Naturwissenschaften mit Schwerpunkt Waldschutz 142 P-3 | Naturwissenschaften 1 159 12. September 2023 180 V-13 | Waldschutz III: Krankheiten 181 V-14 | Waldwirtschaft im Klimawandel III: Forest Restoration 187 V-15 | Fernerkundung I 192 V-16 | Forsttechnik, Verfahrenstechnik I 196 V-17 | Forstrecht 201 V-18 | Forstpolitik, Umweltkommunikation I 205 V-19 | Waldschutz IV: Grundsätzliches und Säugetiere 210 V-20 | Waldwirtschaft im Klimawandel IV: Klimaschutz, Kohlenstoffsenke, Wasserhaushalt 215 V-21 | Fernerkundung II 227 V-22 | Forsttechnik, Verfahrenstechnik II 236 V-23 | Holzkunde und Verwendung 245 V-24 | Forstpolitik, Umweltkommunikation II 255 K-03 | Keynote - Über den Wald als chemisches Element (Prof. Dr. Peter Annighöfer) 262 K-04 | Keynote - Verschwommene Grenzen: Zur Handhabung und Wirkung wissenschaftlicher Aussagen in öffentlichen Auseinandersetzungen (Prof. Dr. Senja Post) 264 PV | Preisverleihung Deutscher Forstwissenschaftspreis 266 13. September 2023 267 P 4-6 | Poster Session 268 P-4 | Management mit naturwissenschaftlichem Schwerpunkt 269 P-5 | Naturwissenschaften mit Schwerpunkt Waldbau 287 P-6 | Naturwissenschaften 2 304 V-25 | Waldschutz V: Sturm und Waldbrände 328 V-26 | Bodenkunde I 334 V-27 | Fernerkundung III 341 V-28 | Forsttechnik, Verfahrenstechnik III 351 V-29 | Forstökonomie I: Ökonomik des Kleinprivatwaldes 361 V-30 | Bioökonomie 373 YS-01 | Young Speakers 381 YS-02 | Young Speakers 383 YS-03 | Young Speakers 385 YS-04 | Young Speakers 387 YS-05 | Young Speakers 389 YS-06 | Young Speakers 391 V-31 | Waldschutz VI 392 V-32 | Bodenkunde II 398 V-33 | Fernerkundung IV 404 V-34 | Forsttechnik, Verfahrenstechnik IV 411 V-35 | Forstökonomie II: Landnutzung und Forsteinrichtung 416 V-36 | Wissenschaftskommunikation I 423 V-37 | Agroforstsysteme 429 V-39 | Waldwachstum 435 V-40 | Bioklimatologie und Meteorologie 444 V-41 | Forstökonomie III: Klimawandel und Waldschäden 451 V-42 | Wissenschaftskommunikation II 459 V-43 | Ökologie, Biodiversität, Naturschutz III 466 Register 468 Moderatorenindex 469 Autorenindex 470 Schlagwortindex 479

Sustainable water resource management requires understanding how hydrologic processes are impacted by environmental management and land-use decisions across multiple spatial and temporal scales. A key concept in this respect is hydrologic ecosystem services (HES), which are the water related ‘goods’ produced by the environment which are valuable to humans. This dissertation assesses a range of topics concerning HES in the Vouga basin (north-central Portugal), and their connection with land-cover and land-use practices. Specifically, the relationship between changes in forest and agricultural land-cover and management practices, and associated changes in HES were examined using a range of statistical and modeling approaches. To quantify the effects of different agricultural scenarios on both HES and potential stakeholders, the ‘Soil and Water Assessment Tool’ (SWAT) was utilized, in conjunction with economic assessment methods. The first research section (Section 6) of the dissertation assesses the trends in streamflow quantity and yield in the Águeda watershed (a sub-basin of the Vouga) over a 75-yr period which coincided with large-scale afforestation of Pinus pinaster and (later) Eucalyptus globulus. Counter to the findings from meta-analysis studies of the effect of forest change on water availability, this study did not detect statistically significant trends in streamflow. By contrast, these findings support the view that there are prerequisite climatic, pedological, and eco-physiological watershed conditions that are necessary to observe hydrologic impacts at the watershed scale (which are not present in the Águeda watershed). By contrast, the significant changes which were detected are related to baseflow, which correspond with different periods of afforestation, and may be attributable to the promotion of soil water repellency under the mature pine and eucalypt stands. In the second research section (Section 7), an assessment is carried out on the hydrologic and nitrate dynamics at the whole basin scale, using the SWAT model. This assessment indicated that there is a high degree of variability in nitrate export from the different parts of the basin, with the highest rates coming from the lower (agriculturally dominated portion) of the basin. The main flow pathways for nitrate export were found to be leaching from agricultural land-cover types, which consistently had the highest export for all land-use and pathways. These findings indicate that the water bodies at the highest risk of nitrate pollution in the Vouga basin are the groundwater aquifers. The final research section (Section 8) utilizes the SWAT model to examine how reduced rates of fertilizer inputs would affect nitrate leaching, crop yields, and agricultural profitability in the lower Vouga basin. This research found that reduced rates of fertilization would reduce the amount of leached nitrate substantially, but that this would also lead to a large decrease in crop yield and profitability. A large difference in the inefficiency (i.e. crop production vs. nitrate export) between different HRUs was found, which could provide a focus for potential management action. This research strongly indicates that such actions may be needed to reduce the negative impacts of this pollution on the value of the groundwater aquifers, and to avoid associated costs which are otherwise passed on to local water users (e.g. through higher water treatment costs). The overall findings of the dissertation highlight the importance of the upper (forested) basin as a drinking water supply area, given the prevalence of nitrate pollution in the lower basin. However, the historic afforestation in the Vouga basin has resulted in a reduction in baseflow, which is negative from a drinking water supply perspective. Therefore, while the forested uplands are beneficial from water quality standpoint (compared to intensive agriculture), they also have altered flow patterns in a manner which will reduce available supply. The findings from the upper basin contrast sharply with the lower basin, where there are potentially large negative HES impacts due to current agricultural practices. These practices will primarily impact groundwater aquifers, and therefore the water quality within the lower basin receive little benefit from the relatively high-quality water from the upper basin. This highlights the importance of considering the interconnectivity of HES across spatial scales, which will depend on the specific site characteristics of the river basin.

The marine environment provides essential ecosystem services that are critical to the functioning of the earth’s life support system and the maintenance of human well-being. Marine Protected Areas (MPAs) are recognised as being the mechanism though which marine natural capital may be conserved. This thesis focuses on the value associated with marine conservation in a case study area, Lyme Bay, England where a ‘closed area’ was created in 2008. A review of literature spanning 20 years shows that despite sound ecological knowledge of a marine area, the reliance on traditional neo-classical economic valuations for marine spatial planning can obscure other issues pertinent to the ecosystem approach. A further valuation of the marine leisure and recreation industry shows that the industry is of economic significance and that the MPA enables the protection of the most valuable sites but has limited benefits for protecting the full resource base. In terms of ecological value, a ‘service orientated framework’ was developed to enable decision makers to understand the links between benthic species, ecological function and indirect ecosystem services. Results spatially identify which ecosystem services occur and demonstrate the value of the MPA in ensuring delivery of these ecosystem services. In relation to the social value of the MPA the research reveals that support for the MPA is strong amongst the majority of stakeholder groups. Values are expressed as the economic, environmental and social benefits of the MPA. However, there have been clear social costs of the MPA policy and these have been borne by mobile and static gear fishermen and charter boat operators. Each valuation methodology can inform decision making. Though, if ecosystem service valuation is to become a deliberative tool for marine conservation and planning, then there is a need for a larger societal discussion on what activities and trade-offs society considers acceptable.

Die Doktorarbeit untersucht institutionelle Aspekte von staatlichen Payments for Ecosystem Services (PES, positive Anreizinstrumente i.F.v. Zahlungen für Ökosystemleistungen) in der EU und den USA. Die Untersuchung ist zweiteilig: Zunächst werden Design und Performance von bestehenden agrarumweltpolitischen Anreizinstrumenten analysiert, welche vermehrt als PES verstanden werden. Dann wird die Relevanz des Ecosystem Services (ES) Konzeptes für die Ausgestaltung der bestehenden Instrumente identifiziert. Die generellen Forschungsfragen werden hinsichtlich institutionellem Design, institutioneller Performance und institutionellem Interplay konkretisiert und im Rahmen von fünf individuellen Veröffentlichungen beantwortet. Hinsichtlich der Anreizinstrumente werden Regeln für eine effektive Gestaltung hervorgehoben. Ferner wird herausgearbeitet, wie Anreizinstrumente mit bestehenden institutionellen Strukturen interagieren. Hinsichtlich der Politikrelevanz des ES Konzeptes wird dargelegt, dass es bisher nicht in die Umweltgesetzgebung integriert wurde, eine Integration aber fortschreitet. Der größte zukünftige Einfluss des Konzeptes wird für die Klima- und Agrarpolitik vorausgesagt, insbesondere für bestehende Zahlungsinstrumente. Generell wird aufgezeigt, dass Zielgerichtetheit und Integration von staatlichen PES wichtig sind. Für einen funktionierenden Instrumentenmix von PES und Regularien muss die jeweilige Eigentumsrechtsituation transparent gemacht werden und verschiedene Akteure müssen auf einer gemeinsamen Basis zusammenarbeiten. Demgemäß wird diskutiert, welches Potential das ES Konzept dafür bietet, die Kommunikation zwischen den Akteuren zu verbessern und neue Impulse für eine Kooperation zu geben. Schließlich wird argumentiert, dass eine systematische ES Untersuchung und Quantifizierung bessere Möglichkeiten für Zieldefinition und Monitoring bieten könnte, der Nutzen einer ökonomische Bewertung von ES aber immer sehr sorgfältig geprüft werden sollte. / The doctoral thesis identifies and analyses institutional aspects of governmental Payments for Ecosystem Services (PES) in the EU and US. The analysis is twofold: First, the design and performance of existing governmental agri-environmental payment schemes that are increasingly labelled as PES have been investigated. Second, the influence of the ecosystem services (ES) concept on such payments has been identified. The general research objectives are structured in terms of institutional design, performance, and interplay. Five individual research papers shed light on the raised issues. Regarding payment schemes, findings indicate certain design rule sets that can be crucial for environmentally effective governmental payments. Furthermore, scheme interactions with other institutional arrangements have been shown. In terms of ES concept driven policies, the identified design principles have hardly been included but their integration is proceeding. The greatest future ES impact on policy design is predicted regarding climate and agricultural policies, especially on existing payment schemes. In summary, effective targeting and integration of governmental PES have been outlined as important. To create and achieve a sound mix of PES with regulations and policies, the property rights situation, the reference point for application of the ‘provider-gets’ and ‘beneficiary-pays’ principles, and any deviations therefrom should be made transparent. Furthermore, different actors must collaborate on basis of common denominators. The potential of ES to enhance communication among actors and provide new impulses for cross-sectoral and cross-level cooperation in existing governmental payments schemes implementation has been discussed. Moreover, it has been argued that regarding governmental payments, systematic ES definition and quantification may offer the opportunity to enhance targeting, and economic valuation and monetarization of ES should be very carefully considered.

Zahlungen für Ökosystemdienstleistungen (Payments for EcosystemServices, PES) sind in den letzten Jahren zum Aushängeschild von Umweltorganisation geworden. Der Gedanke, die Bereitstellung von Ökosystemdienstleistungen durch PES abzusichern, ist in praktischen Diskursen von Vermittlern zu finden, die an potentiell Zahlende gerichtet sind. Praktikern ist bisher jedoch schwer gefallen, zu zeigen, dass PES tatsächlich zu den vorgesehenen Zielen führen können. Forscher haben darauf hingewiesen, dass zahlreiche PES-Schemata, insbesondere diejenigen mit Bezug auf Wasser, auf unsicheren Annahmen beruhen und außerdem gewichtige Kausalzusammenhänge zwischen Eingriffen in die Landnutzung und Ökosystemdienstleistungen vermissen lassen. Diese Unsicherheit in PES-Schemata geht nicht nur aus praktischen Schwierigkeiten hervor, sondern aus der Komplexität von Mensch-Umwelt-Systemen (human-environment systems) und aus der Begrenztheit des Wissens über diese Systeme. Forscher sind zwar in derLage, diese wesentlichen Herausforderungen zu beschreiben und zu diskutieren. In der Fachliteratur mangelt es jedoch an empirischen Studien,die die zusätzliche Wirksamkeitvon PES-Schemata untersuchen, d.h. ob diese Schemata zusätzliche Wirkungen zeigen, die anderen Faktoren nicht zurechenbar sind, bzw. Studien, die die Bedeutung von Nachweisen für ihre Wirksamkeit für die Interessengruppen (stakeholders) untersuchen. Die Dissertation trägt dazu bei, diese empirische Lücke zu schließen: Dazu untersucht sie vier wasserbezogene Zahlungsschemata, hier auch Zahlungen für Wassereinzugsgebietsleistungen genannt, in Kolumbien. Sie vergleicht die vier Fälle hinsichtlich der Bestrebungen, durch Beobachtung (monitoring) und Evaluation Nachweise für die Wirksamkeit zu erbringen, sowie hinsichtlich der damit verbundenen Herausforderungen. Eines der Kapitel enthält auch drei Fallstudien aus Brasilien, die als Vergleich zu den Fällen aus Kolumbien und der Darstellung von Unterschieden und Gemeinsamkeiten dienen. / Payments for Ecosystem Services (PES) have become the flagship of conservation organizations in recent years. The idea of securing ecosystem service (ES) provision through PES has been present in practical discourses of intermediaries directed at potential payers. However, demonstrating that PES can actually achieve the intended goals has been difficult for practitioners. Researchers have pointed out that many PES schemes, particularly water-related ones, are based on unreliable assumptions and lack strong causal links between land use interventions and ecosystem services. This uncertainty in PES schemes arises not only from practical difficulties, but from the complexity of human-environment systems (HES), and the limits of knowledge about them. Researchers have been able to describe and discuss these major challenges. However, the literature is still poor on empirical studies exploring the additionality of PES schemes, that is, if those schemes produce additional effects not attributable to other factors, as well as studies exploring the importance of impact evidence for stakeholders involved. This dissertation contributes to filling this empirical gap by exploring four water-related payments schemes (here also called payments for watershed services, PWS) in Colombia, comparing the cases in terms of their efforts to produce impact evidence through monitoring and evaluation, and their associated challenges. Three cases from Brazil are also included in one of the chapters and compared with the Colombian cases by illustrating differences and similarities.

This study aimed to investigate the spatial extent of Land-Use Land-Cover (LULC) change and the implications for ecosystem services in order to develop a sustainable land-use management framework for traditional authorities in South Africa. Effectively, this study undertook an insightful examination of the impacts that arise from policy decisions and practices, which unfortunately were found to be ineffective. The methodologies and approaches used in this study included both quantitative and qualitative techniques. The critical quantitative method employed in this research was the use of survey questionnaires to collect primary data. Qualitative approaches, such as one-on-one and key informant interviews, were used to triangulate the findings. Remote sensing and geographic information system (GIS) methods were used to investigate changes in LULC from 1990 to 2018 through the use of data obtained from the South African National Land-Cover project. Stochastic models were used to predict future LULC changes from 2018 to 2050. The Co$ting Nature Policy Support System was used to identify and undertake economic valuation of services provided by ecosystems. Statistical analysis using the Statistical Package for the Social Sciences was used to identify correlations and the reliability of the data, while graphs and tables were generated to identify patterns and lessons from the research. Between 1990 and 2018, significant changes in land cover were noticed for thickets and dense bush, woodlands, waterbodies, subsistence agriculture, and built-up areas. Woodlands changed by over 1 000 hectares (ha) per year, while thickets decreased by over 900 ha per year. Drivers of these changes include deforestation, among others. Future predictions for LULC revealed that between 2018 and 2050, almost 500 ha of woodlands would be lost to built-up areas. The aggregate value of the services flowing from ecosystems was found to be R9 509 044 608.00. A significant issue was that 90% of the traditional leaders interviewed could not positively respond to whether they knew the extent of the land they presided over, which raised questions regarding the effectiveness of their management systems. Recommendations were made in this study to address the limitations identified in the land-use management practices by adapting elements of the main theoretical frameworks, namely the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services framework; the Drivers, Pressures, States, Impacts and Responses framework; the sustainability theory; and the hierarchy of plans, into a new framework designed specifically for traditional leaders, titled the “Traditional leaders land-use decision support framework”. / Environmental Sciences / D. Phil. (Environmental Management)

Humans make decisions within ecosystems to enhance their well-being, but choices can lead to unintended consequences. The ecosystem services (ES) approach supports decision-making that considers all environmental goods and services. Many challenges remain in the implementation of the ES approach like how specific ES vary through space and time. We address this research problem using the Tualatin and Yamhill river basins in northwestern Oregon as a study area. Freshwater ES are quantified and mapped with the spatially-explicit ES modeling tool, Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST). In chapter II, we develop a simple urban land cover change modeling approach with selected stakeholder input. The products of this analysis are used in part to answer the question of how the freshwater ES of water yield, nutrient retention, and sediment retention will change in the future, and how their distribution potentially will change? In chapter III, these ES are modeled in InVEST using the land cover scenarios and three downscaled global climate models. The base period is 1981 to 2010 and the future period is 2036 to 2065. The models are calibrated to empirical estimates, and display different sensitivities to inputs. Water yield increases with higher rainfall but decreases with the highest temperature scenario. Nutrient export and retention estimates are positively correlated. In the Tualatin basin, more urban lands generally lead to increases in nutrient exports and retention. The effect is reversed in the Yamhill basin from much larger agricultural exports. Sediment exports and retention increase with higher winter rainfall but are negatively spatially correlated due to topographic effects. Simulation of a landscape scale installation of riparian buffers leads to decreases in exports and increases in retention. The distribution of the provision of freshwater ES remains unchanged throughout the scenarios. With few parameters in each InVEST model, all display a high degree of sensitivity. Parameterization is subject to high uncertainty even with calibrated values. We discuss the assumptions and limitations of InVEST's freshwater models. The spatially explicit nature of InVEST is its main advantage. This work coupled with other analyses in the study area can facilitate the identification of tradeoffs amongst ES leading to better ecosystem management.

<p>The purpose of this thesis is to analyse interactions between freshwater flows, terrestrial ecosystems and human well-being. Freshwater management and policy has mainly focused on the liquid water part (surface and ground water run off) of the hydrological cycle including aquatic ecosystems. Although of great significance, this thesis shows that such a focus will not be sufficient for coping with freshwater related social-ecological vulnerability. The thesis illustrates that the terrestrial component of the hydrological cycle, reflected in vapour flows (or evapotranspiration), serves multiple functions in the human life-support system. A broader understanding of the interactions between terrestrial systems and freshwater flows is particularly important in light of present widespread land cover change in terrestrial ecosystems. </p><p>The water vapour flows from continental ecosystems were quantified at a global scale in Paper I of the thesis. It was estimated that in order to sustain the majority of global terrestrial ecosystem services on which humanity depends, an annual water vapour flow of 63 000 km3/yr is needed, including 6800 km3/yr for crop production. In comparison, the annual human withdrawal of liquid water amounts to roughly 4000 km3/yr. A potential conflict between freshwater for future food production and for terrestrial ecosystem services was identified. </p><p>Human redistribution of water vapour flows as a consequence of long-term land cover change was addressed at both continental (Australia) (Paper II) and global scales (Paper III). It was estimated that the annual vapour flow had decreased by 10% in Australia during the last 200 years. This is due to a decrease in woody vegetation for agricultural production. The reduction in vapour flows has caused severe problems with salinity of soils and rivers. The human-induced alteration of vapour flows was estimated at more than 15 times the volume of human-induced change in liquid water (Paper II). </p>

Public water utilities are tasked with providing high quality, inexpensive water often sourced from watersheds representing a diverse mix of public and private land ownership. There is increasing recognition amongst water resource managers of the role that private landowners play in determining downstream water quality, but bringing together landowners with a wide variety of land management objectives under the umbrella of watershed stewardship has proven difficult. Recently, a large number of "Payment for Watershed Services" programs have aimed to engage private landowners in watershed stewardship initiatives by offering financial incentives for adopting watershed best management practices. However, a growing field of research suggests that financial incentives alone may be of limited utility to encourage widespread and long-standing behavior change, and instead understanding landowner attitudes and non-financial barriers to stewardship program enrollment has become a focus of research. This research examines a population of rural landowners representing a diversity of agricultural, forestry, recreational, and investment objectives in the Clackamas River watershed, Oregon. I designed and distributed a mail and web-based survey instrument intended to measure land uses and land ownership objectives, attitudes towards watershed stewardship programs, barriers to enrollment in stewardship programs, and preferred incentives and goals that would promote enrollment. I received 281 valid responses for a response rate of 29%. I conducted two primary analyses: one focused on relating attitudes and barriers to intent to enroll in a watershed stewardship program, and one focused on identifying how diverse landowners differ according to factors influencing enrollment in stewardship programs. I found that landowners did not report financial considerations to be a primary barrier to enrollment and expressed low interest in receiving financial incentives. Instead, landowners reported that primary barriers related to lack of trust, ecological understanding, and concerns that stewardship program enrollment would be incompatible with their land management objectives. I do not discount the potential utility of financial incentives under certain circumstances, but emphasize the importance of addressing these other considerations before incentives can make a meaningful impact. I compared how barriers to enrollment were perceived by landowners with different land management objectives relating to production, investment, and conservation. I found that landowner attitudes were differentiated from one another primarily by their use of land for production purposes; however, I found a large amount of diversity between producers and non-producers in the degree to which they considered investment and conservation objectives in their land management, and these two variables added further explanatory power to understanding fine-scale differences in how landowner typologies relate to conservation programs.

Les rivières reçoivent de l'azote de leurs bassins versants et elles constituent les derniers sites de transformations des nutriments avant leur livraison aux zones côtières. Les transformations de l’azote inorganique dissous en azote gazeux sont très variables et peuvent avoir un impact à la fois sur l’eutrophisation des côtes et les émissions de gaz à effet de serre à l’échelle globale. Avec l’augmentation de la charge en azote d’origine anthropique vers les écosystèmes aquatiques, les modèles d’émissions de gaz à effet de serre prédisent une augmentation des émissions d’oxyde nitreux (N2O) dans les rivières. Les mesures directes de N2O dans le Lac Saint-Pierre (LSP), un élargissement du Fleuve Saint-Laurent (SLR) indiquent que bien qu’étant une source nette de N2O vers l'atmosphère, les flux de N2O dans LSP sont faibles comparés à ceux des autres grandes rivières et fleuves du monde. Les émissions varient saisonnièrement et inter-annuellement à cause des changements hydrologiques. Les ratios d’émissions N2O: N2 sont également influencés par l’hydrologie et de faibles ratios sont observés dans des conditions de débit d'eau plus élevée et de charge en N élevé. Dans une analyse effectuée sur plusieurs grandes rivières, la charge hydraulique des systèmes semble moduler la relation entre les flux de N2O annuels et les concentrations de nitrate dans les rivières. Dans SLR, des tapis de cyanobactéries colonisant les zones à faible concentration de nitrate sont une source nette d’azote grâce à leur capacité de fixer l’azote atmosphérique (N2). Étant donné que la fixation a lieu pendant le jour alors que les concentrations d'oxygène dans la colonne d'eau sont sursaturées, nous supposons que la fixation de l’azote est effectuée dans des micro-zones d’anoxie et/ou possiblement par des diazotrophes hétérotrophes. La fixation de N dans les tapis explique le remplacement de près de 33 % de la perte de N par dénitrification dans tout l'écosystème au cours de la période d'étude. Dans la portion du fleuve Hudson soumis à la marée, la dénitrification et la production de N2 est très variable selon le type de végétation. La dénitrification est associée à la dynamique en oxygène dissous particulière à chaque espèce durant la marée descendante. La production de N2 est extrêmement élevée dans les zones occupées par les plantes envahissantes à feuilles flottantes (Trapa natans) mais elle est négligeable dans la végétation indigène submergée. Une estimation de la production de N2 dans les lits de Trapa durant l’été, suggère que ces lits représentent une zone très active d’élimination de l’azote. En effet, les grands lits de Trapa ne représentent que 2,7% de la superficie totale de la portion de fleuve étudiée, mais ils éliminent entre 70 et 100% de l'azote total retenu dans cette section pendant les mois d'été et contribuent à près de 25% de l’élimination annuelle d’azote. / Rivers receive nitrogen (N) from their watershed and are the final sites of nutrient processing before delivery to coastal waters. Transformations of dissolved inorganic N (DIN) to gaseous N are highly variable and can impact both coastal eutrophication and greenhouse gas emissions. With anthropogenic N loading to aquatic ecosystems on the rise, nitrous oxide (N2O) emission from rivers should increase. Direct measurements of N2O from lake St. Pierre (LSP), an enlargement of the St. Lawrence River (SLR) indicate that although LSP is a net atmospheric source of N2O to the atmosphere fluxes are low compared to others rivers. Emissions are seasonally and inter-annually highly variable due to changes in hydrological conditions. N2O: N2 is also influenced by hydrology and lower ratios are observed in conditions of higher water discharge and elevated N charge into the ecosystem. In a cross system analysis, hydraulic load mitigates the relation between annual N2O flux and nitrate concentrations in rivers. In SLR, cyanobacterial mats colonizing low nitrate areas are a net source of N with high negative di-nitrogen (N2) fluxes. Given that fixation occurred during daylight and that oxygen concentrations in the water column were supersaturated, we hypothesize that N2 fixation is performed by the dominant cyanobacteria in anoxic micro-zone of the mat and/ or possibly by heterotrophic diazotrophs. Our estimates indicate that N fixation in the mats account for the replacement of up to 33% of the N loss via denitrification in the entire ecosystem during the study period. In the tidal Hudson River N2 production is highly variable between vegetated shallows and was associated with species-driven differences in dissolved oxygen (DO) dynamics during the ebb tide. N2 production was extremely high in invasive floating-leaved plants (Trapa natans) but was insignificant in submersed native vegetation. An estimate of summertime N2 production in Trapa beds suggests that these beds are a major seasonal hotspot for N removal. Large Trapa beds represent only 2.7% of the total area of the tidal Hudson but they remove between 70 and 100% of the total N retained in this section of the river during summer months and contribute to as much as 25% of the annual N removal.