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The increase of tree canopy cover due to woody plant encroachment and tree plantations modifies both carbon and water dynamics. The tradeoffs between ecosystem net primary productivity (NPP) and water use with increasing tree cover in different climate conditions, particularly under future climate scenarios, are not well understood. Within the climate transition zone of the southern Great Plains, USA, we used the Soil and Water Assessment Tool+ (SWAT+) to investigate the combined impacts of increasing tree cover and climate change on carbon and water dynamics in three watersheds representing semiarid, subhumid, and humid climates. Model simulations incorporated two land use modifications (Baseline: existing tree cover; Forest +: increasing evergreen tree cover), in conjunction with two climate change projections (the RCP45 and the RCP85), spanning two time periods (historic: 1991-2020; future: 2070-2099). With climate change, the subhumid and humid watersheds exhibited a greater increase in evapotranspiration (ET) and a corresponding reduction in runoff compared to the semi-arid watershed, while the semi-arid and subhumid watersheds encountered pronounced losses in water availability for streams (>200 mm/year) due to increasing tree cover and climate change. With every 1 % increase in tree cover, both NPP and water use efficiency were projected to increase in all three watersheds under both climate change scenarios, with the subhumid watershed demonstrating the largest increases (>0.16 Mg/ha/year and 170 %, respectively). Increasing tree cover within grasslands, either through woody plant expansion or afforestation, boosts ecosystem NPP, particularly in subhumid regions. Nevertheless, this comes with a notable decrease in water resources, a concern made worse by future climate change. While afforestation offers the potential for greater NPP, it also brings heightened water scarcity concerns, highlighting the importance of tailoring carbon sequestration strategies within specific regions to mitigate unintended repercussions on water availability. 

Eastern redcedar (Juniperus virginiana, redcedar) is a major woody species encroaching upon the native grasslands and forests of the southern Great Plains (SGP), representing a significant threat to regional ecosystem services. Future climate change is anticipated to influence redcedar habitat suitability, changing the probability of further encroachment and reshaping its spatial distribution. In this study, we trained seven Species Distribution Models (SDMs) with redcedar records from the USDA Forest Inventory Analysis database and used the ensemble of these SDMs to simulate redcedar distribution probability under current and future climate conditions in Kansas, Oklahoma, and Texas. Results reveal a distinct east-to-west gradient of decreasing distribution probability in the study domain, primarily driven by climate aridity. Throughout the 21st century, the optimal range of aridity for redcedar habitat is projected to shift eastwards by 0.7◦ (≈ 58 km) under the RCP45 climate scenario and 1.3◦ (≈ 108 km) under the RCP85. Accordingly, the suitable habitat will shift eastward by 0.6◦ (≈ 49 km) in the RCP45 and by 1.2◦ (≈ 103 km) in the RCP85. The proportion of unsuitable habitat will increase from 40.2 % of the study domain during 2000 – 2019 to 48 % in the RCP45 and 54.2 % in the RCP85 during 2080 – 2099. Additionally, highly suitable land areas will decrease from 10.4 % of the study domain during 2000 – 2019 to 1.3 % in the RCP45 and 0 % in the RCP85 by the end of this century. This study suggests a low likelihood of further redcedar encroachment in the west of the SGP states under future climates, while anticipating continued expansion in the east, gradually replacing the existing oak forests and rangelands. The findings provide valuable insights for prioritizing WPE management resources and contribute to our understanding of future changes in the SGP vegetation composition and their impacts on ecosystem dynamics. 

The Arkansas River and its tributaries provide critical water resources for agricultural irrigation, hydropower generation, and public water supply in the Arkansas River Basin (ARB). However, climate change and other environmental factors have imposed significant impacts on regional hydrological processes, resulting in widespread ecological and economic consequences. In this study, we projected future river flow patterns in the 21st century across the entire ARB under two climate and socio-economic change scenarios (i.e., SSP2-RCP45 and SSP5-RCP85) using the process-based Dynamic Land Ecosystem Model (DLEM). We designed “baseline simulations” (all driving factors were kept constant at the level circa 2000) and “environmental change simulations” (at least one driving factor changed over time during 2001–2099) to simulate the inter-annual variations of river flow and quantify the contributions of four driving factors (i.e., climate change, CO2 concentration, atmospheric nitrogen deposition, and land use change). Results showed that the Arkansas River flow in 2080–2099 would decrease by 12.1% in the SSP2-RCP45 and 27.9% in the SSP5-RCP85 compared to that during 2000–2019. River flow decline would occur from the beginning to the middle of this century in the SSP2-RCP45 and happen throughout the entire century in the SSP5-RCP85. All major rivers in the ARB would experience river flow decline with the largest percentage reduction in the western and southwestern ARB. Warming and drying climates would account for 77%–95% of the reduction. The rising CO2 concentration would exacerbate the decline through increasing foliage area and ecosystem evapotranspiration. This study provides insight into the spatial patterns of future changes in water availability in the ARB and the underlying mechanisms controlling these changes. This information is critical for designing watershed-specific management strategies to maintain regional water resource sustainability and mitigate the adverse impacts of climate changes on water availability.