## 4 Discussion
### Conversion to Forest  
When comparing L2F to I2F, a primary reason that carbon storage and productivity increase more for the same percentage change in L2F is due to the fertilization of lawns. Since the lawn cohort is fertilized, the new cohorts created in those areas continue to receive nitrogen, allowing them to grow better than I2F. In an additional model run to examine this effect, deciduous trees grew better with nitrogen fertilization and reached a higher mature biomass. While coniferous trees are also not nitrogen-saturated, the fertilization effect was not as impactful in these trees and will require further examination. In a real-world scenario, the soils in formerly impervious areas have fewer nutrients than the soils in greenspaces [39]. In order to maximize carbon storage and productivity in formerly impervious areas, fertilization would be required. For both cases in which new trees are grown, the future period of 80 years is not enough to reach maximum vegetation carbon. To enter the old-growth stage, forests often need to exceed the 100-year stand age [40]. Based on the definition of old-growth forest, the newly created forest from both lawns and impervious areas will need more time to reach full maturity. However, existing trees in Lehigh Valley are not old-growth, so the biomass of the trees at initialization may be larger than reality, which would further enhance the loss of biomass due to deforestation. The relation between vegetation and soil carbon is sometimes counterintuitive. In the L2F case, there is increasing vegetation carbon but decreasing soil carbon. While the percentage differences are great for the reduced soil carbon, the absolute differences are actually less for the changes in soil carbon due to the larger absolute value of vegetation carbon. However, turflawn is calibrated to a much larger value of soil organic carbon (16,260 gCm\({}^{-2}\) from [PERSON] [41], 1995 vs. 8290 gCm\({}^{-2}\) for temperate forests from [PERSON] et al. [42]). Therefore, when lawn is converted to forest, the heterotrophic respiration rates are much higher for the new forest cohort, leading to a reduction in soil carbon in spite of the increase in forest litter. On the other hand, when forest is converted to impervious surface, heterotrophic respiration rates are set to 0, so there is no loss of any existing soil below the surface and the amount of soil carbon is larger than a run without the conversion. Studies have shown that urban soils, especially in temperate and colder climates, accumulate 3-5 times more carbon than natural soils [43]. The conversion of impervious surface to forest would release carbon due to years of sealed and cultural layers, which is not considered in this study. Altering the amount of area covered by lawns or impervious surfaces can cause profound changes to carbon and water dynamics. Overall biomass, productivity, and carbon storage do not show a strong increase when converting impervious surfaces to forests but show a strong decrease when converting the forests to impervious surfaces (Figure 2, Table 4b,d). The experiments in which forests are being converted to impervious surfaces show a larger impact than when impervious surfaces are converted to forests. However, it is important to note that more total area is changed when forests are converted due to Lehigh Valley having more forested area than impervious area. The newly created forests are also less productive due to their young stand age, while the conversion of forest to impervious produces an immediate result. Similarly, converting forest to lawn has a larger effect than converting lawn to forest. Due to the treatment of lawn being irrigated in the model, the experiment with a higher proportion of lawn has higher soil moisture, evapotranspiration, and runoff than the newly created forests. Through the experiments, the conversion of lawns and impervious areas to forests has only a small and often insignificant increase in carbon stocks and fluxes within Lehigh Valley.