There is an important new paper that further examines the role of vegetation processes within the climate system. The study documents a complex interaction between local vegetation and larger scale atmospheric circulations.
The paper is
Isabella M. Angelini, Michael Garstang, Robert Davis, Bruce Hayden, David R. Fitzjarrald, David R. Legates, Steven Greco, Stephen Macko, and Vickie Connors, 2010: On the Coupling Between Vegetation and the Atmosphere. Theor. Appl. Climatol. DOI 10.1007/s00704-010-0377-5 (in press)
The abstract reads
“Recent studies suggest that vegetation can drive large-scale atmospheric circulations and substantially influence the hydrologic cycle. We present observational evidence to quantify the extent of coupling between vegetation and the overlying atmosphere. Within the context of vegetation-atmospheric interactions, we reanalyze existing climatological data from springtime leaf emergence, emissivity, dew point temperatures, and historical records of precipitation and forest coverage. We construct new rainfall transects based on a robust global climatology. Using isotopic analysis of precipitation, we find that rain in Amazonia comes primarily from large-scale weather systems coupling interior regions to the ocean and is not directly driven by local evaporation. We find that changes in vegetative cover and state influence the temperature and moisture content of the surface and atmospheric boundary layer, but are not reflected in observable precipitation changes. This analysis reaffirms the view that changes in precipitation over continental reaches are a product of complex processes only partly influenced but not controlled by local water sources or vegetation.”
The conclusion reads
“Vegetation-atmosphere interactions involve complex processes on multiple time and space scales. Attempts to synthesize these interactions within the framework of a model have been severely limited by the inability to adequately represent nonlinear interactions on scales not easily included in the model framework.
Our results suggest that the rain-producing processes in the atmosphere ranging from the cloud elements through the ensemble of storm clouds within the larger-scale parent system have profound effects upon how the total rain-producing entity is coupled to the surface and its constituents. While there is an essential coupling of the surface to the atmosphere in the hydrologic cycle, there is no simple relationship between the supply of water from the surface to the atmosphere and the return of that water to the surface in the form of rain.
We show that major changes in the amount of surface vegetation as well as changes from passive to active production of water by plants are not dramatically reflected by changes in precipitation but are more subtly evident in changes in water content and temperature of the air. While air temperature and water vapor content are related, the relationship of air temperature to plants may involve more complex radiative and stability considerations of the air column than simple feedbacks between water phases and temperature might imply. Careful analysis of rainfall distributions over vegetated and non-vegetated surfaces extending over large distances inland from the ocean show no evidence of the gradients in rainfall suggested by M&G nor do they support, from an observational point of view, the existence of any vegetation-induced driving mechanism. Instead, analysis of stable isotopes of oxygen and hydrogen collected in a manner compatible with the production of the observed rain show a strong dependence upon water drawn from the ocean. These results suggest that any numerical simulation of the pathways connecting the source of the water required to produce the observed rainfall must not only include careful consideration of the moist air thermodynamics and the physiological processes governing transpiration but must be equally aware of the dynamic forces driving the three-dimensional circulation fields of the atmosphere on a diverse range of time and space scales. “