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Background Deciduous forests in eastern North America experienced a widespread and intense spongy moth (Lymantria dispar) infestation in 2021. This study quantified the impact of this spongy moth infestation on carbon (C) cycle in forests across the Great Lakes region in Canada, utilizing high-resolution (10 × 10 m2) Sentinel-2 satellite remote sensing images and eddy covariance (EC) flux data. Study results showed a significant reduction in leaf area index (LAI) and gross primary productivity (GPP) values in deciduous and mixed forests in the region in 2021. Results Remote sensing derived, growing season mean LAI values of deciduous (mixed) forests were 3.66 (3.18), 2.74 (2.64), and 3.53 (2.94) m2 m−2 in 2020, 2021 and 2022, respectively, indicating about 24 (14)% reduction in LAI, as compared to pre- and post-infestation years. Similarly, growing season GPP values in deciduous (mixed) forests were 1338 (1208), 868 (932), and 1367 (1175) g C m−2, respectively in 2020, 2021 and 2022, showing about 35 (22)% reduction in GPP in 2021 as compared to pre- and post-infestation years. This infestation induced reduction in GPP of deciduous and mixed forests, when upscaled to whole study area (178,000 km2), resulted in 21.1 (21.4) Mt of C loss as compared to 2020 (2022), respectively. It shows the large scale of C losses caused by this infestation in Canadian Great Lakes region. Conclusions The methods developed in this study offer valuable tools to assess and quantify natural disturbance impacts on the regional C balance of forest ecosystems by integrating field observations, high-resolution remote sensing data and models. Study results will also help in developing sustainable forest management practices to achieve net-zero C emission goals through nature-based climate change solutions. 

ABSTRACT Evapotranspiration (ET) from temperate forests plays a significant role in the regional and global water cycles. However, extreme weather events such as heat and drought are affecting the water use and water use efficiency (WUE) of these forests. Climate change impacts may be more severe in plantation forests where the age of the forest plays a significant role, causing differences in their responses to environmental stresses. This study presents 14 years (2008–2021) of water flux data measured using the eddy covariance technique in an age sequence (83, 48 and 20 years as of 2021) of eastern white pine (Pinus strobusL.) forests in the Great Lakes region in southern Ontario, Canada. The mean annual ET was 465 ± 41, 466 ± 32 and 403 ± 21 mm year⁻¹in the 83‐, 48‐ and 20‐year‐old stands, respectively, with the highest annual water flux observed in the 83‐year‐ old stand, which was similar to that of the 48‐year‐old stand. Mean annual gross ecosystem productivity (GEP) was 1585 ± 100, 1660 ± 115 and 1634 ± 331 g C m⁻² year⁻¹in the 83‐, 48‐ and 20‐year‐old stands, respectively, while mean annual WUE was 3.4 ± 0.4, 3.6 ± 0.4 and 4.0 ± 0.8 g C kg H₂O year⁻¹in the respective stands. Lower ET and relatively higher GEP resulted in the highest WUE in the youngest stand, even though the highest GEP was observed in the middle‐aged stand. Air temperature (Tair) was the dominant control on ET, GEP and WUE in all three different‐aged stands, while drought, characterised as the relative extractable water (REW) in the soil, had a significant impact on ET in the late summer. The results of this study further showed that forest age significantly influenced how forests responded to drought stresses. The younger stand was more efficient in carbon sequestration and water use despite exhibiting greater sensitivity to water stress and higher drought coupling. The long‐term eddy covariance measurements analysed in this study have helped to enhance our understanding of water exchange processes in the temperate conifer forest ecosystems in Eastern North America. Specifically, this work contributes to a better understanding of how different‐aged forests respond to extreme weather events, aiding in the development of new strategies for managing water resources and ensuring water security in the region under a changing climate. 

Abstract Forests significantly influence regional and global water cycles through transpiration, which is affected by meteorological variables, soil water availability, and stand and site characteristics. Variable retention harvesting (VRH) is a forest management practice in which varying densities of trees, such as 55% and 33%, are retained after thinning or harvesting. These trees can be grouped together or evenly distributed. VRH aims to enhance forest growth, improve biodiversity, preserve ecosystem functions, and generate economic revenue from harvested timber. Application of VRH treatment in forest ecosystems can potentially impact the response of forest transpiration to environmental controls. This study analyzed the impacts of four different VRH treatments on sap flow velocity (SV) in an 83‐year‐old red pine (Pinus resinosa Ait.) plantation forest in the Great Lakes region in Canada. These VRH treatments included 55% aggregated (55A), 55% dispersed (55D), 33% aggregated (33A), and 33% dispersed (33D) basal area retention, and an unharvested control (CN) plot, 1 ha each. Analysis of counterclockwise hysteresis loops between SV and meteorological variables showed larger hysteresis areas between SV and photosynthetically active radiation (PAR) than vapor pressure deficit (VPD) and air temperature (T_air), particularly in clear sky and warm temperatures in the summer. It demonstrated that PAR was the primary control on SV across VRH treatments, followed by VPD andT_air. Larger hysteresis loop areas and higher SV values were observed in the CN and 55D treatments, with lower values found in the 55A, 33D, and 33A plots. This suggests that maintaining dispersed retention of 55% basal area (55D) is the optimal forest management practice that can be utilized to enhance transpiration and forest growth. These findings will assist forest managers and other stakeholders to adopt sustainable forest management practices, thereby enhancing forest growth, water use efficiency, and resilience to climate change. Additionally, these practices will contribute to nature‐based climate solutions. 

Temperate deciduous forests are an important contributor to the global carbon (C) sink. However, changes in environmental conditions and natural disturbances such as insect infestations can impact carbon sequestration capabilities of these forests. While, insect infestations are expected to increase in warmer future climates, there is a lack of knowledge on the quantitative impact of these natural disturbances on the carbon balance of temperate deciduous forests. In 2021, a record-breaking defoliation, caused by the spongy moth (Lymantria dispar dispar (LDD), formerly knows as the gypsy moth) occurred in eastern North America. In this study, we assess the impact of this spongy moth defoliation on carbon uptake in a mature oak-dominated temperate forest in the Great Lakes region in Canada, using eddy covariance flux data from 2012 to 2022. Study results showed that the forest was a large C sink with mean annual net ecosystem productivity (NEP) of 207 ± 77 g C m–2 yr−1 from 2012 to 2022, excluding 2021, which experienced the infestation. Over this period mean annual gross ecosystem productivity (GEP) was 1,398 ± 137 g C m–2 yr−1, while ecosystem respiration (RE) was 1,209 ± 139 g C m–2 yr−1. However, in 2021 due to defoliation in the early growing season, annual GEP of the forest declined to 959 g C m–2 yr−1, while annual RE increased to 1,345 g C m–2 yr−1 causing the forest to become a large source of C with annual NEP of -351 g C m–2 yr−1. The forest showed a rapid recovery from this major disturbance event, with annual GEP, RE, and NEP values of 1,671, 1,287, and 298 g C m–2 yr−1, respectively in 2022 indicating that the forest was once again a large C sink. This study demonstrates that major transient natural disturbances under changing climate can have a significant impact on forest C dynamics. The extent to which North American temperate forests will remain a major C sink will depend on the severity and intensity of these disturbance events and the rate of recovery of forests following disturbances. 

Climate and hydrologic change across the Great Lakes region and other transboundary watersScott Steinschneider, M. Altaf Arain, Paulin Coulibaly, Andrew Gronewold, and Gail Krantzberg, explore climate and hydrologic change across the Great Lakes region in North America and other transboundary waters. Hydroclimate extremes are transforming water landscapes in transboundary regions. These systems are particularly susceptible to hydroclimatic variability due to shared governance structures, interconnected ecosystems, and a wide range of water users. The Great Lakes basin – one of the world’s largest freshwater systems, shared by Canada, the United States, and numerous Indigenous sovereign nations – exemplifies how shifting hydroclimatic conditions are challenging conventional approaches to water management across borders. In this region, the impacts of these changes are evident in increased flooding, shoreline erosion, economic disruption, ecosystem stress, and rising uncertainty surrounding water availability and quality. 

ABSTRACT Climate change and extreme weather events affect hydrology and water resources in catchments worldwide. This study analysed Blue Water (BW) and Green Water (GW) scarcity in the McKenzie Creek watershed in Ontario, Canada, and explored how changes in temperature and precipitation may impact water scarcity dynamics. The McKenzie Creek is the main water source for agricultural activities for the Six Nations of the Grand River reserve (the largest Indigenous community in Canada) and other non‐Indigenous communities in the watershed. Data from the water use surveys and streamflow simulations performed using the Coupled Groundwater and Surface‐Water Flow Model (GSFLOW) under the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathways (RCP) scenarios 4.5 and 8.5, representing moderate and high greenhouse gas emissions and climate warming, respectively, were used to calculateBWandGWscarcity. Study results showed thatBWscarcity may increase to ‘moderate’ levels if water users extract the maximum permitted water withdrawal allocation. This level of scarcity has the potential to cause ecological degradation and water quality issues in the watershed.GWscarcity will steadily increase throughout the 21st century due to climate warming with the western portion of the McKenzie Creek watershed projected to experience slightly higher levels ofGWscarcity. This may cause users to withdraw more water resources, thereby decreasingBWavailable for downstream communities, including the Six Nations of the Grand River. This study provides water resource managers and regional planners with important information about potential challenges facing the watershed due to increased water use and changing climate conditions. 

Abstract We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (Start_GPPand End_GPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO₂to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity ∼3 weeks before end of snowmelt, while DBF forests achieved that capacity ∼4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d⁻¹) than ENF (1.10% d⁻¹), and their active season length (End_GPP–Start_GPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in Start_GPP, End_GPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier Start_GPP(1.3–2.5 days °C⁻¹) or later End_GPP(1.5–1.8 days °C⁻¹, depending on forest type and month). Finally, we tested whether existing phenological models could predict Start_GPPand End_GPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for Start_GPP, while a chilling‐degree‐day model was best for End_GPP. The root mean square errors (RMSE) between predicted and observed Start_GPPand End_GPPwere 11.7 and 11.3 days, respectively. For DBF forests, temperature‐ and daylength‐based models yielded the best results (RMSE 6.3 and 10.5 days). 

Environmental observation networks, such as AmeriFlux, are foundational for monitoring ecosystem response to climate change, management practices, and natural disturbances; however, their effectiveness depends on their representativeness for the regions or continents. We proposed an empirical, time series approach to quantify the similarity of ecosystem fluxes across AmeriFlux sites. We extracted the diel and seasonal characteristics (i.e., amplitudes, phases) from carbon dioxide, water vapor, energy, and momentum fluxes, which reflect the effects of climate, plant phenology, and ecophysiology on the observations, and explored the potential aggregations of AmeriFlux sites through hierarchical clustering. While net radiation and temperature showed latitudinal clustering as expected, flux variables revealed a more uneven clustering with many small (number of sites < 5), unique groups and a few large (> 100) to intermediate (15–70) groups, highlighting the significant ecological regulations of ecosystem fluxes. Many identified unique groups were from under-sampled ecoregions and biome types of the International Geosphere-Biosphere Programme (IGBP), with distinct flux dynamics compared to the rest of the network. At the finer spatial scale, local topography, disturbance, management, edaphic, and hydrological regimes further enlarge the difference in flux dynamics within the groups. Nonetheless, our clustering approach is a data-driven method to interpret the AmeriFlux network, informing future cross-site syntheses, upscaling, and model-data benchmarking research. Finally, we highlighted the unique and underrepresented sites in the AmeriFlux network, which were found mainly in Hawaii and Latin America, mountains, and at under- sampled IGBP types (e.g., urban, open water), motivating the incorporation of new/unregistered sites from these groups.