7.2.6 Linking Biophysical to Biogeochemical and Ecohydrological Components
Soil moisture and surface temperatures work together in response to precipitation and radiative inputs. Vegetation influences these terms through its controls on energy and water fluxes, and through these fluxes, precipitation. It also affects the radiative heating. Clouds and precipitation are affected through modifications of the temperature and water vapour content of near-surface air. How the feedbacks of land to the atmosphere work remains difficult to quantify from either observations or modelling (as addressed in Sections 7.2.3.2 and 7.2.5.1). Radiation feedbacks depend on vegetation or cloud cover that has changed because of changing surface temperatures or moisture conditions. How such conditions may promote or discourage the growth of vegetation is established by various ecological studies. The question of how vegetation will change its distribution at large scales and the consequent changes in absorbed radiation is quantified through remote sensing studies. At desert margins, radiation and precipitation feedbacks may act jointly with vegetation. Radiation feedbacks connected to vegetation may be most pronounced at the margins between boreal forests and tundra and involve changes in the timing of snowmelt. How energy is transferred from the vegetation to underlying snow surfaces is understood in general terms but remains problematic in modelling and process details. Dynamic vegetation models (see Section 7.2.4.5) synthesize current understanding.
Changing soil temperatures and snow cover affect soil microbiota and their processing of soil organic matter. How are nutrient supplies modified by these surface changes or delivery from the atmosphere? In particular, the treatment of carbon fluxes (addressed in more detail in Section 7.3) may require comparable or more detail in the treatment of N cycling (as attempted by S. Wang, et al., 2002; Dickinson et al., 2003). The challenge is to establish better process understanding at local scales and appropriately incorporate this understanding into global models. The Coupled Carbon-Cycle Climate Model Intercomparison Project (C4MIP) simulations described in Section 7.3.5 are a first such effort.
Biomass burning is a major mechanism for changing vegetation cover and generation of atmospheric aerosols and is directly coupled to the land climate variables of moisture and near-surface winds, as addressed for the tropics by Hoffman et al. (2002). The aerosol plume produced by biomass burning at the end of the dry season contains black carbon that absorbs radiation. The combination of a cooler surface due to lack of solar radiation and a warmer boundary layer due to absorption of solar radiation increases the thermal stability and reduces cloud formation, and thus can reduce rainfall. Freitas et al. (2005) indicate the possibility of rainfall decrease in the Plata Basin as a response to the radiative effect of the aerosol load transported from biomass burning in the Cerrado and Amazon regions. Aerosols and clouds reduce the availability of visible light needed by plants for photosynthesis. However, leaves in full sun may be light saturated, that is, they do not develop sufficient enzymes to utilise that level of light. Leaves that are shaded, however, are generally light limited. They are only illuminated by diffuse light scattered by overlying leaves or by atmospheric constituents. Thus, an increase in diffuse light at the expense of direct light may promote leaf carbon assimilation and transpiration (Roderick et al., 2001; Cohan et al., 2002; Gu et al., 2002, 2003). Yamasoe et al. (2006) report the first observational tower evidence for this effect in the tropics. Diffuse radiation resulting from the Mt. Pinatubo eruption may have created an enhanced terrestrial carbon sink (Roderick et al., 2001; Gu et al., 2003). Angert et al. (2004) provide an analysis that rejects this hypothesis relative to other possible mechanisms.