## 4 Discussion
If the response of the shallow rooting species in JULES is extrapolated to the entire permafrost region, there is an extra 1.2-1.9 Gt C of carbon in the aboveground vegetation. Our results demonstrate an increase in future projections of carbon storage through enhanced biomass when including root adaptation and growth in models. This process has not previously been accounted for in Earth System Models. Therefore, it needs to be quantified for a range of Arctic vegetation types and its impact on climate-carbon feedbacks quantified in future Earth System models. We assume that the nitrogen cycling represented within JULES has a realistic response to a changing climate. However, it was developed specifically to capture the large-scale interaction of carbon and nitrogen in a simple way [53] and therefore has limitations. In particular, it assumes fixed plant stoichiometry. Recent analyses of field observations have observed changes in leaf nitrogen content in response to increased nitrogen availability [76]. The inclusion of flexible stoichiometry into JULES has the potential to alter the carbon and nitrogen cycling and the impact of climate change on the biogeochemical feedbacks in the future. Competition between species was not included in the simulations in this paper. Here, only Arctic C\({}_{3}\) PFT was allowed to grow. However, there is a high dominance of _Sphagnum fuscum_ moss throughout the site [40]. This is not accounted for in JULES and will impact both the surface water and energy balance and the biogeochemical cycling, including the soil net mineralisation. By only considering one PFT in this way, we are not considering the complete response of Arctic vegetation to future climate change such as plant species change. For example, although shrub expansion is often considered in tundra ecosystems, abrupt permafrost thaw can result in the formations of thaw ponds which can lead to vegetation shifts towards gramioid-dominated wetland [69]. Furthermore, shrub growth can be limited by wet soil conditions and nutrient-limitation whilst deep-rooting gramioid species may have the advantage of accessing newly available nutrients such as inorganic nitrogen under future permafrost thaw [69]. There are also uncertainties surrounding the interactions between Arctic vegetation response to increased nitrogen availability and impacts on nutrient cycling. As an example, as vegetation such as shrubs increase in height and cover as a result of warming, canopy overtopping might reduce the capacity to respond to warming induced changes [22]. However, such increases can also increase litter input to soils and enhance nitrogen mineralisation rates [33, 77, 78]. Therefore, any projections of vegetation responses in arctic ecosystems contain uncertainties surrounding overall response until these interactions are better understood and represented in models. Similarly, although an increase of root abundance of Arctic species at the thaw front is expected under future warming [43, 44], there is relatively little known regarding root distribution with depth. In our simulations, we assumed a linearly decreasing distribution as a function of depth; however, alternatives are suggested and may differ for different species or ecosystems [41]. Although peat processes have recently been added to JULES [62], they are not yet included in this model configuration, and will impact the simulation of the soil carbon and nitrogen distribution and hence the available inorganic nitrogen. Ultimately JULES is a model and we have tested a single process within that model: response to increased soil nitrogen-availability, both to test whether the representation of this process is realistic and to illustrate its potential role under future climate change.