Moss and Lichen within the Boreal Forest Ecosystem

摘要

Observations taken in the mid 1990’s during the Boreal Ecosystem-Atmosphere Study (BOREAS) have shown the importance of moss and lichen to the dynamics within the North American boreal forest ecosystem. This study directly led to an increased interest in modeling the effects of moss and lichen to understand their effects on the regional and global climates. In comparison with bare soil, the presence of moss or lichen covering much or the entire surface greatly reduces the evaporation and reduces the heat exchange between the atmosphere and the soil. This dryness leads to the development of a very deep boundary layer, much like that of a desert. Through this mechanism, the presence or absence of moss and lichen can have far reaching effects and may influence the global circulation patterns. As moss is quite sensitive to pollution and to increased heat, its presence in the boreal regions may change in time. The ability to model the thermal and hydrological effects of a moss or lichen layer within a climate model was limited in the past. One of the most versatile methods of treating the moss or lichen is to include it either within the “soil” or as a layer that grows atop the soil. Either way the moss and lichen layer must be assigned thermal and hydraulic properties or a method to treat heterogeneous soil interfaces must be used. Past treatments for heterogeneous interfaces have largely focused on soil matric potential or headbased solutions to Richards’ Equation. However, head-based equations are too inefficient numerical to be used within a climate model, even for homogeneous soils. Furthermore, most past models (head-based and water-content-based) have serious flaws. We developed a water-content based solution to Richards’ Equation that is efficient enough for climate modeling. It features the first physically correct treatment of the heterogeneous interface within a water-content based model. Furthermore, it introduces a marker for nonconvergence. This marker is used to create a physically based short-cut that allows the heterogeneous interface to be modeled efficiently under otherwise difficult conditions. Our model can be applied to any type of discrete soil heterogeneity. Our model has been incorporated into a version of the Community Land Model (CLM) and was used to model the North American boreal forest. We successfully reproduced the decreased evaporation and observed moist sub-surface soil conditions. Coupling the CLM to a Global Climate Model (GCM) will allow us to estimate the effects moss and lichen have on the global climate. We will discuss the key features of treatment of the heterogeneous interface and present results from our modeling in the BOREAS area