thesis
The treatment of vegetation in land surface models: implications for predictions of land-atmosphere exchange
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Abstract
Plant processes affect fluxes of energy, moisture and CO2 between the land and the atmosphere.
Land surface models need to correctly represent the vegetation functioning and
its response to environmental conditions. Due to anthropogenic carbon emissions rising,
and global warming, plant processes are being affected and in turn modulate the terrestrial
carbon sink. However, models still disagree on the response of plants to changing
conditions. This work analyses how vegetation is treated in two land surface models: the
Joint UK Land Environment Simulator (JULES) and Carbon Hydrology Tiled ECMWF
Scheme for Surface Exchanges over Land (CTESSEL). The aim is to analyse how environmental
variables control the vegetation processes at daily and seasonal timescales at
present day climate and the changes that arise in a scenario of double atmospheric CO2
and higher temperature. The analyses are carried out at the leaf level and at the canopy
level. To investigate the responses at the leaf level, the photosynthesis scheme used in each
model was extracted, thereby providing a submodel that can be run in stand alone mode.
The photosynthesis submodel provides a means to analyse the leaf level response of each
photosynthesis model to environment variables as well as the internal model parameters
that characterise each plant type. In JULES the environmental controls on photosynthesis
are explicitly introduced by three limiting regimes: light, rubisco (carbon) or export
limiting regime. In CTESSEL the carbon and light limitations are implicitly represented
but there is no export limitation. Due to the lack of export limiting regime, CTESSEL
presents higher sensitivity to CO2 concentration resulting in a stronger CO2 fertilization
effect. The carbon and energy fluxes produced by the full land surface models were tested
and compared at 10 European FLUXNET sites. The main differences between modellled
carbon fluxes were found to be the treatment of soil moisture stress and the lack of export
limiting regime in CTESSEL. The optimum temperature for photosynthesis in models is
the result of model parameters’ dependence on temperature and the combination of limiting
regimes. The optimum temperature for photosynthesis was found to be a determining
element in the strength and sign of the vegetation modelled feedback to climate change