Modeling permafrost impacts on vegetation and carbon dynamics in northern high latitudes

Yueyang Jiang, Purdue University

Abstract

In the northern high latitudes, vegetation distribution and carbon cycling have been continuously changed in the past and could change more rapidly as the climate warming. The purpose of my PhD dissertation is to quantify the uncertainty in modeling vegetatidynamics and to assess the effect of permafrost on vegetation dynamics and carbon cycling in the northern high latitudes under different levels of warming conditions. The uncertainty in current modeling of vegetation dynamics is considerably large. The first part of this study is to assess how high-latitude vegetation may respond under various climate scenarios during the 21st Century with a focus on analyzing model parameters induced uncertainty and how this uncertainty compares to the uncertainty induced by various climates. The analysis was based on a set of 10,000 Monte Carlo ensemble LPJ simulations for the 45°N polewards region from 1900 to 2100. LPJ-DGVM was run under contemporary and future climates from four Special Report Emission Scenarios (SRES), A1FI, A2, B1, and B2, based on the Hadley Centre General Circulation Model (GCM), and six climate scenarios, X901M, X902L, X903H, X904M, X905L, and X906H from the Integrated Global System Model (IGSM) at the Massachusetts Institute of Technology (MIT). In the current dynamic vegetation model, some parameters are more important than others in determining the vegetation distribution. Parameters that control plant carbon uptake and light-use efficiency have the predominant influence on the vegetation distribution of both woody and herbaceous plant functional types. The relative importance of different parameters varies temporally and spatially and is influenced by climate inputs. In addition to climate, these parameters play an important role in determining the vegetation distribution in the region. The parameter-based uncertainties contribute most to the total uncertainty. The current warming conditions lead to a complexity of vegetation responses in the region. Temperate trees will be more sensitive to climate variability, compared with boreal forest trees and C3 perennial grasses. This sensitivity would result in a unanimous northward greenness migration due to anomalous warming in the northern high latitudes. Temporally, boreal needle-leaved evergreen plants are projected to decline considerably, and a large portion of C3 perennial grass is projected to disappear by the end of the 21st century. In contrast, the area of temperate trees would increase, especially under the most extreme A1FI scenario. As the warming continues, the northward greenness expansion in the Arctic region could continue. Permafrost is a key component that largely affects the vegetation dynamics and carbon cycling, however it is slowly incorporated into current ecosystem modeling. The second part of this study applied a well-developed numerical algorithm to simulate the thawing and freezing processes at daily time steps across multiple sites that vary with vegetation cover, disturbance history, and climate. The model performance was evaluated by comparing modeled and measured soil temperatures at different depths for both boreal forest stands and tundra stands. We used the model to explore the influence of climate, fire disturbance, and topography (north- and south-facing slopes) on soil thermal dynamics. Modeled soil temperatures agree well with measured values for both boreal forest and tundra ecosystems at the site level. Combustion of organic soil horizons from wildfire alters the surface energy balance and increases the downward heat flux through the soil profile, resulting in the warming and thawing of near-surface permafrost. A projection for the 21 st century indicates that as the climate warms, the active layer thickness could possibly increase more than three meters in the boreal forest site and deeper than one meter in the tundra site, which are both rates faster than expectations of previous studies. We concluded that the presented soil thermal model is able to simulate the soil thermal dynamics in permafrost regions well and could be used as a tool to analyze the influence of climate change and wildfire disturbance on permafrost thawing. As climate warming continues, permafrost degradation could be exacerbated and consequently exert considerable effects on terrestrial ecosystem in the region. The third part of this dissertation incorporates the soil thermal model into a dynamic global vegetation model (LPJ-DGVM) to improve the simulations of soil thermal, vegetation and carbon dynamics. The coupled model was applied to assess the impact of permafrost on ecosystem and carbon dynamics in the region during the 21st century. We found that (i) the near-surface permafrost would degrade significantly with the southern boundary of permafrost moving northward in the northern north America and the western boundary moving eastward in the northern Eurasia, especially under extreme climate scenarios (e.g., A1FI); (ii) the incorporation of permafrost into LPJ affects the distribution of boreal forests; (iii) climate variability and elevated CO2 fertilization both exert significant effects on vegetation distribution and carbon cycling with and without permafrost, while the CO2 fertilization plays a larger effects than climate; (iv) the overall effect of the addition of permafrost into LPJ results in an increase in net ecosystem production (NEP); however the permafrost degradation would lead to a faster decreasing NEP by 3 - 6 Tg C yr-1 because of more rapid increase in heterotrophic respiration (3 - 13 Tg C yr-1) than net primary production (0 - 8 Tg C yr-1); (v) permafrost largely suppresses soil carbon loss by 56.5 - 90.8 Pg C but slightly change vegetation carbon gain by -2.2 - 2.7 Pg C by year 2100. As the climate warms, the northern high latitudes region is predicted to turn into a substantial carbon source. Coupling the soil thermal model into LPJ improves the simulation of the seasonality of carbon cycles and constrains the uncertainty in modeling future vegetation distribution and carbon dynamics in the region.

Degree

Ph.D.

Advisors

Zhuang, Purdue University.

Subject Area

Climate Change|Atmospheric sciences

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