## 3 Results
### Future Simulations
The left-hand column in Figure 8 shows the seasonal cycle of the NPP for the present day compared with the the future and no adaptation of roots. For the deep-rooting species, \(CN_{low}Root_{60}\), there is some increase in NPP in the future but the plants remain nitrogen-limited. However, for the shallow-rooting species, \(CN_{high}Root_{25}\), there is only a very small increase in NPP in the future although the plants also remain nitrogen-limited. As might be expected with warmer temperatures, the future projections of NPP and potential NPP show that there is a longer growing season. This is particularly visible at the start of the growing season. The right-hand column shows the NPP in the future for plants with the same characteristics but deeper roots. These are compared with the \(CN_{low}Root_{60}\) and \(CN_{high}Root_{25}\) present-day simulations. These vegetation types with deeper roots show a marked increase in productivity in the future when the roots can access the mineral nitrogen deeper in the soil, and they are no longer nitrogen-limited. Despite the large amount of extra inorganic nitrogen available in these \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multicolumn{2}{c}{**Present Total Inorganic N**} & \multicolumn{2}{c}{**Future Total Inorganic N**} \\  & **Std Roots** & **Deeper Roots** & **Std Roots** & **Deeper Roots** \\ \hline \(CN_{high}Root_{25}\) & 0.2 & 5.1 & \(-\)0.01 & 2.0 \\ \(CN_{low}Root_{60}\) & 1.8 & 34.4 & 1.6 & 29.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Change in plant-available inorganic nitrogen (g[N] m\({}^{-2}\)year\({}^{-1}\)) derived using Equation (4) from monthly JULES output, taken as 10-year annual averages for present (2000-2010) and future (2090–2100). In all cases, the maximum summer thaw depth is defined using the future simulations (i.e., >1.2 m). The left two columns show the change in available inorganic N using the present-day nitrogen cycle; so, they do not include any simulated change in total inorganic nitrogen as a result of a changing climate. The right two columns use the total inorganic nitrogen simulated for the future time period. simulations, \(CN_{low}Root_{60}\) is still nitrogen-limited in the future. This is likely because the additional nitrogen becomes available near the end of the growing season (Table 4 and Figure 7). The increasing temperature, increasing CO\({}_{2}\) concentration, increasing plant-available nitrogen leads to an increase in productivity and therefore in total vegetation carbon. Figure 9 shows the increase in aboveground biomass is \(\sim\)22% from present-day to the end of the century for both species. If the vegetation is allowed to adapt in the future and increase its root depth, the increase in aboveground biomass from present is 30% for the low C:N species with the deep roots and 38% for the high C:N species with the shallow roots. Therefore, it is more beneficial for \(CN_{high}Root_{25}\) to increase its roots and create more plant-available inorganic nitrogen, even though it is less nitrogen-limited in the present day than \(CN_{low}Root_{60}\) (Table 3). The increase in aboveground biomass will lead to more litter fall and replenish the nutrients at the soil surface. In each of the simulations discussed in this section, there is only one species present. However, if the species co-existed, the nutrients released at the thaw front may well become available to the shallow-rooting species via the increase in litter fall and the deep-rooting species act as a nutrient pump. Figure 8: Potential NPP (NPP\({}_{pet}\)) and nitrogen-limited NPP for the period 2000–2010 (present) and the period 2090–2100 (future under RCP8.5 scenario) displayed as a 10-year monthly average. The left-hand column shows the future projections of NPP for the original vegetation types and the right-hand column shows that for a proposed future vegetation type, where the root depths are able to increase because the permafrost has thawed.