## 1 Introduction  
From 2011 to 2020, the global surface temperature increased by 1.09 \({}^{\circ}\)C compared to the period from 1850 to 1900 [1]. This warming rate was about 50% faster in the United States and two to three times faster across the Eurasian continent, while the Arctic and Antarctic Peninsula experienced warming rates three to four times greater [2]. Additionally, significant changes in the hydrological cycle, such as precipitation and evapotranspiration, were observed in high-latitude regions [3]. Climate change has led to widespread alterations and successions in biological communities, with alpine forest-tundra ecotones shifting to higher elevations, deciduous and boreal forests moving toward polar regions, an increase in woody vegetation in subarctic tundra, and a significant reduction in global grassland areas. In temperate and polar zones, approximately half of treelines are shifting toward the poles or higher altitudes, and alpine meadows are decreasing [4]. Climate warming has advanced the spring phenological phases of species in temperate regions and has also enhanced rice and corn yields in the Kosy River Basin [5]. These changes in vegetation composition and productivity in high-latitude areas are further impacting the global carbon cycle [6, 7]. Arctic ecosystems have long been exposed to low temperatures and are highly vulnerable to climate change [8, 9]. According to the definition of the Arctic Monitoring and Assessment Programme (AMAP) [10], the Arctic land area accounts for 5.37% of the world's land area, and the vegetative area in the Arctic is 71.02% [11]. Vegetation changes in this region play an important role in regulating the global carbon balance because the vast areas in this region belong to permafrost zones, where the soil organic carbon content is high [12; 13; 14]. The changes in vegetation dynamics and species compositions in the Arctic have attracted many concerns. However, there are still great uncertainties in the net ecosystem carbon exchanges in this region due to the lack of long-term observational data [15; 16]. Vegetation is an important component of the global carbon cycle. Carbon assimilation by vegetation plays a key role in determining the roles of the ecosystem as a carbon source or sink [17]. Vegetation type shifts, particularly shrub expansion across the Arctic tundra, have been widely regarded as a result of rapid climate warming [18; 19]. The changes in vegetation can also potentially alter ecosystem carbon balances by affecting interactions among the soil-plant-atmosphere [20]. Presently, process models have been widely used to study vegetation growth and the carbon cycle in terrestrial ecosystems [20; 21]. For example, based on in situ observation data, ecological process-based models can assess how the carbon balance will be impacted by climate change in Arctic tundra ecosystems [22]. Among these models, the LPJ-GUESS (Lund-Potsdam-Jena general ecosystem simulator) is a coupled biogeochemical model that has a wide range of applications in vegetation productivity estimation and vegetation carbon cycle under the current and future climate scenarios [21; 23; 24; 25; 26; 27]. The purpose of this study is twofold. First, to explore the trends in vegetation GPP and carbon budget changes in the tundra and boreal forest ecosystems of Alaska from 1992 to 2014. Second, to analyze whether future reliance on vegetation to absorb more carbon is feasible by simulating the trends in vegetation GPP and carbon budget changes for the two aforementioned ecosystems under three future climate scenarios. To address these scientific inquiries, this study rigorously validates the LPJ-GUESS model using field monitoring data spanning 4 to 5 years to ensure its accuracy in simulating carbon cycling during historical periods. Subsequently, detailed simulations of vegetation carbon budgets from 1992 to 2014 are conducted, extending further to predictive analyses under future climatic conditions.