Modelling carbon responses of tundra ecosystems to historical and projected climate: a comparison of a plot‐ and a global‐scale ecosystem model to identify process‐based uncertainties

Abstract

Summary We are developing a process‐based modelling approach to investigate how carbon (C) storage of tundra across the entire Arctic will respond to projected climate change. To implement the approach, the processes that are least understood, and thus have the most uncertainty, need to be identified and studied. In this paper, we identified a key uncertainty by comparing the responses of C storage in tussock tundra at one site between the simulations of two models – one a global‐scale ecosystem model (Terrestrial Ecosystem Model, TEM) and one a plot‐scale ecosystem model (General Ecosystem Model, GEM). The simulations spanned the historical period (1921–94) and the projected period (1995–2100). In the historical period, the model simulations of net primary production (NPP) differed in their sensitivity to variability in climate. However, the long‐term changes in C storage were similar in both simulations, because the dynamics of heterotrophic respiration (RH) were similar in both models. In contrast, the responses of C storage in the two model simulations diverged during the projected period. In the GEM simulation for this period, increases in RH tracked increases in NPP, whereas in the TEM simulation increases in RH lagged increases in NPP. We were able to make the long‐term C dynamics of the two simulations agree by parameterizing TEM to the fast soil C pools of GEM. We concluded that the differences between the long‐term C dynamics of the two simulations lay in modelling the role of the recalcitrant soil C. These differences, which reflect an incomplete understanding of soil processes, lead to quite different projections of the response of pan‐Arctic C storage to global change. For example, the reference parameterization of TEM resulted in an estimate of cumulative C storage of 2032 g C m−2 for moist tundra north of 50°N, which was substantially higher than the 463 g C m−2 estimated for a parameterization of fast soil C dynamics. This uncertainty in the depiction of the role of recalcitrant soil C in long‐term ecosystem C dynamics resulted from our incomplete understanding of controls over C and N transformations in Arctic soils. Mechanistic studies of these issues are needed to improve our ability to model the response of Arctic ecosystems to global change.

Sensitivity of an ecosystem model to hydrology and temperature

“…2c). This is in agreement with other modelling studies that show a tight coupling of soil respiration to soil moisture (McGuire et al 2000b;Clein et al 2000).…”

“…2001). These factors include soil temperature (Lloyd and Taylor 1994;Rustad et al 2001), soil moisture (McGuire et al 2000b;Clein et al 2000), changes in NPP (net primary production), i.e. substrate availability (Raich and Potter 1995), population dynamics of plants (Raich and Schlesinger 1992) and vegetation substrate quality (Raich and Schlesinger 1992).…”

Evapotranspiration in Northern Agro-Ecosystems: Numerical Simulation and Experimental Comparison

“…All these changes will potentially alter the exchange of surface energy, water, and carbon cycles in high-latitude ecosystems [11] and consequently, the response at regional level to the atmosphere system. Soil moisture plays a critical role in the surface energy balance and water cycle in these regions [1,12,13]. It is widely recognized that the soil moisture confined in a thin layer underneath the land surface influences the partitioning of the surface energy fluxes simultaneously modifying surface thermal conductance and rates of evaporation [14].…”

Using Mechanistic Models to Scale Ecological Processes across Space and Time

“…The estimated spatial pattern of production using the GEM correlates well with satellite-based production estimates for years between 1992 and 1998 (r 2 = 0.88 to 0.94). The results of this spatialtemporal projection have been compared to results from the terrestrial ecosystem model (Clein et al 2000) to help understand changes in ecosystem C budgets throughout the Arctic.…”

“…To incorporate this feedback, Le Dizes and colleagues (2003) incorporated the ACM into the general ecosystem model (GEM). The GEM has been used in the past to analyze interactions between the C and N cycles in experimental plots of tussock tundra at the Arctic LTER site (McKane et al 1997) and to project responses of tussock tundra to changes in climate over the next century (Clein et al 2000).…”