Climate change impacts: In the perspective of soil organic carbon, above-ground biomass and vegetation shifts (A case study from Nepal)

Abstract

Climate change impact has been examined with regard to different aspects of forest ecosystems such as soil organic carbon (SOC), above-ground tree biomass (AGTB) and vegetation shift (broadleaved and coniferous forests). The SOC and AGTB were assessed in terms of their change in amount with future climate change whereas vegetation shift was assessed in terms of shifting altitudinal ranges, natural habitat area and species in future climate change. Nepal was selected as a test region to study the impact of climate change on these aspects due to the large differences in altitude and temperature in the country and also it is one of the most vulnerable countries to climate change. The objectives of this study were, i.e. 1) Are climatic variables important for the estimation of SOC and AGTB? 2) Does future climate change contribute to an increase in SOC and AGTB stock in the forest ecosystem? 3) Does climate change affect the vegetation shift? Multiple linear regression, Random forest and Maximum Entropy (MaxEnt) models were applied to investigate the objectives. The availability of forest resource assessment (2010-2014) data of Nepal, accompanied by topographic and bioclimatic variables, provided the opportunity to study the impacts of climate change on forest ecosystems at a national scale.

Climatic variables (temperature and precipitation) show a strong relation with SOC and AGTB. The climatic variables do not only explain SOC and AGTB at present, but they can also predict SOC and AGTB under future climate change scenarios. This study found a higher amount of SOC existed at higher altitudes compared to lower altitudes and the rate of accumulation of SOC increasing at a higher rate with the increase in altitude. The lower latitude has a higher temperature and vice versa. In the projected climate change scenario, i.e. CMIP6, SSP2 4.5 for 2040-2060, the amount of SOC was found to decrease by 3.85% in general with the increasing temperature and precipitation against near current period (1970-2000). In Contrast, the study indicated a positive relation between climatic variables (temperature and precipitation) and AGTB that found the amount of AGTB to increase by 2.96% in general in the same projected climate change scenario against same near current period. Moreover, vegetation shifts from one forest to another are likely to occur over a longer period, influenced by climatic variables. This study found the vegetation shift in terms of areas, i.e. coniferous to broad-leaved forests (1579 km2) and its reverse (232 km2), in terms of altitudinal shift, i.e. 77m higher for broad-leaved forests and 54m lower for coniferous forests, and in terms of species, i.e. broad-leaved forests to coniferous forests and its reverse in the future climate change scenario.

The result confirms that a higher amount of SOC stock exists in the forests at a higher altitude compared to the forests at lower altitude. However, the amount of SOC is likely to decrease while the amount of AGTB is likely to increase in the future climate change scenarios. Moreover, the result shows that vegetation shift from coniferous to broad-leaved forest is more dominant than the broad-leaved to coniferous forests and the area of broad-leaved forest will likely to expand while the area of coniferous forest is likely to shrink in the future climate change scenario.

Therefore, this study highlights the need to retain SOC amount thus reducing carbon emission from the soil. It also highlights the significance of, particularly, high altitude forest in sequestrating atmospheric carbon in the future climate change scenario. Moreover, the study highlights that the expansion of broad-leaved forests due to vegetation shift may benefit in terms of species diversity, SOC amount and forest resilience and also may affect coniferous forest- dependent people and enterprise due to lower supply of the forest products. Thus, the study suggests to adopt sustainable management of high altitude forests to increase mitigation potential of the forests (increase carbon sequestration and reduce carbon emission) and also suggests to assess adaptation measures for vulnerable communities due to climate change.