United States Global Change Research Program

Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas, and microorganisms play a crucial role in its metabolism. While soil microbial roles in N 2 O cycling are well studied, there is a major knowledge gap regarding the distribution and diversity of these microbes within tree ecosystems. In this study, we aimed to comprehensively assess the nitrogen (N) cycling gene abundance and the diversity of N 2 O-reducing microorganisms in shoots (leaves and terminal branches) and wood cores of four tree categories — European beech ( Fagus sylvatica ), European hornbeam ( Carpinus betulus ), birch ( Betula pendula and Betula pubescens ) and Norway spruce ( Picea abies ) across long transect. We assessed N 2 O exchange through shoot incubation experiments and measured internal N 2 O concentrations in stem wood. Inorganic N compounds were studied as indicators of microbial transformation, and a targeted metagenomic approach was used to analyze the relative abundance of N-cycling genes and nosZ clade I and II diversity. Our study revealed that hornbeam shoots showed potential N₂O emissions, while beech shoots indicated N₂O consumption in the incubation study. Birch had the highest internal stem wood N₂O concentration, and beech the lowest when compared to the ambient concentration. Metagenomic analysis confirmed the presence of key nitrification and denitrification genes in both tissues, with nosZ genes abundant in spruce shoots, birch wood cores, and beech wood cores—clade I dominating over clade II and Rhizobiales prevalent within clade I. These findings provide new insights into tree microbiome and its contribution to N 2 O exchange in tree-associated environments.

Abstract. Limiting global warming to well below 2 ∘C by the end of the century is an ambitious target that requires immediate and unprecedented emission reductions. In the absence of sufficient near-term mitigation, this target will only be achieved by carbon dioxide removal (CDR) from the atmosphere later during this century, which would entail a period of temperature overshoot. Aside from the socio-economic feasibility of large-scale CDR, which remains unclear, the effects on biogeochemical cycles and climate are key to assessing CDR as a mitigation option. Changes in atmospheric CO2 concentration and climate alter the CO2 exchange between the atmosphere and the underlying carbon reservoirs of the land and the ocean. Here, we investigate carbon cycle feedbacks under idealized and more realistic overshoot scenarios in an ensemble of Earth system models. The responses of oceanic and terrestrial carbon stocks to changes in atmospheric CO2 concentration and changes in surface climate (the carbon–concentration feedback and the carbon–climate feedback, quantified by the feedback metrics β and γ, respectively) show a large hysteresis. This hysteresis leads to growing absolute values of β and γ during phases of negative emissions. We find that this growth over time occurs such that the spatial patterns of feedbacks do not change significantly for individual models. We confirm that the β and γ feedback metrics are a relatively robust tool to characterize inter-model differences in feedback strength since the relative feedback strength remains largely stable between phases of positive and negative emissions and between different simulations, although exceptions exist. When the emissions become negative, we find that the model uncertainty (model disagreement) in β and γ increases more strongly than expected from the assumption that the uncertainties would accumulate linearly with time. This indicates that the model response to a change from increasing to decreasing forcing introduces an additional layer of uncertainty, at least in idealized simulations with a strong signal. We also briefly discuss the existing alternative definition of feedback metrics based on instantaneous carbon fluxes instead of carbon stocks and provide recommendations for the way forward and future model intercomparison projects.

Abstract. The taiga ecosystem in northeastern Siberia, a nitrogen-limited ecosystem on permafrost with a dry climate, changed during the extreme wet event in 2007. We investigated the normalized difference vegetation index (NDVI) as a satellite-derived proxy for needle production and compared it with ecosystem parameters such as soil moisture water equivalent (SWE), larch foliar C/N ratio, δ13C and δ15N, and ring width index (RWI) at the Spasskaya Pad Experimental Forest Station in Russia for the period from 1999 to 2019. Historical variations in NDVI showed a large difference between typical larch forest (unaffected) and the sites affected by the extreme wet event in 2007 because of high tree mortality at affected sites under extremely high SWE and waterlogging, resulting in a decrease in NDVI, although there was no difference in the NDVI between typical larch forest and affected sites before the wet event. Before 2007, the NDVI in a typical larch forest showed a positive correlation with SWE and a negative correlation with foliar C/N. These results indicate that not only the water availability (high SWE) in the previous summer and current June but also the soil N availability likely increased needle production. NDVI was also positively correlated with RWI, resulting from similar factors controlling them. However, after the wet event, NDVI was negatively correlated with SWE, while NDVI showed a negative correlation with foliar C/N. These results indicate that after the wet event, high soil moisture availability decreased needle production, which may have resulted from lower N availability. Foliar δ15N was positively correlated with NDVI before 2007, but foliar δ15N decreased after the wet event. This result suggests damage to roots and/or changes in soil N dynamics due to extremely high soil moisture. As a dry forest ecosystem, taiga in northeastern Siberia is affected not only by temperature-induced drought but also by high soil moisture (led by extreme wet events) and nitrogen dynamics.

Understanding the impact of reclamation measures on the quality of timber produced in post-mining areas is crucial for the proper establishment of future forest cultures on such sites. We studied European larch trees (Larix decidua Mill.) grown since 1981 on the external dump of a brown coal mine in Bełchatów (Central Poland). In particular, the effects of stand admixture and the intensity of mineral fertilization, applied in the initial phase of tree growth, on the increments and quality of larch wood were evaluated. Total tree height and stem diameter of 4-meter sections were measured. Qualitative timber classification took into account the share of wood defects, which excluded the timber from a higher quality-dimensional class. Larch trees grown in a monoculture had a better quality of wood compared to larch grown mixed with other species. Fertilization, applied in the initial afforestation period, had only a limited effect on the growth of larch trees. Curvature and knots determined timber quality most significantly. While knots had a decisive importance in larch growing in the monoculture stands, curvature determined wood quality in the admixed larch stands.

Abstract Impacts of climate change as well as adaptation and mitigation policies have been studied through their effects on individuals, societies, the economic sector, agriculture, and ecosystems. Since the environment is expected to change significantly as a consequence of global warming, the role of local communities in shaping a more resilient landscape is crucial. This paper focuses on regions that are highly susceptible to climate change impacts, with the aim to develop and test an interdisciplinary mixed-methods approach to the development of future landscape scenarios, combining: i) qualitative and quantitative methods; and ii) research-driven (exploratory) and participatory (normative) processes of scenario development. A research-driven business-as-usual scenario accounts for adopted and planned mandatory adaptation policies, while an optimistic scenario, developed through a combination of research-driven and participatory approaches, includes additional measures stemming from feasible local activities. While the differences between the impacts of both scenarios on the land use and landscape structure seem to be relatively subtle, the optimistic scenario driven by active stakeholders would in fact lead to a considerably more resilient landscape. The results show that the mixed-method approach is limited to some extent due to its labour/effort intensity, uncertainty in the projections of particular landscape element changes, and unpredictability in the durability of the participatory process and local activism. However, it encourages citizens in thinking about the future of the landscape threatened by the impacts of climate change, and increases their willingness to actively manage the landscape in a way leading to enhancing the resilience to climate change effects.

Electroantennography (EAG) is a simple, yet powerful technique that can be used to indicate that chemical compounds are detected by mosquito antennae. When coupled with gas chromatography (GC), it allows for the identification of detectable compounds within complex mixtures, including those in plant and animal headspaces that might influence mosquito behavior. EAGs and GC-EAGs reveal compounds that can be tested in behavioral assays with the goal of developing new repellents or attractants for mosquito control.

Wetlands in the United States are an important terrestrial carbon sink and are impacted by management decisions and land use change. This chapter presents general patterns of wetland carbon management and land use, and identifies the data needs and scientific investigations necessary to improve research in this important field. Background information in contemporary and historical land use change and land management decisions, such as changes to hydrology and wildfire, and effects on wetlands are discussed, along with the need to further investigate and create more precise maps of wetland location, type, and carbon stocks.