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Dissertation
Mechanistic modelling of cropland and grassland ecosystems: focus on the water cycle and on cattle grazing
Authors: --- --- --- ---
Year: 2019 Publisher: Liège Université de Liège (ULiège)

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Abstract

Climate change stirs up more and more citizens’ interest and concern, and the role of greenhouse gases (GHG)
in climate change has now been widely discussed. The agricultural sector is pointed as one of the main causes of
climate change, mostly for its emissions of biogenic GHG. However, the impact of ecosystems environment and management
practices on these emissions is yet not fully understood, and the response of agroecosystems to a changing
environment is still questioned. This context highlights the necessity of studying and understanding ecosystem
dynamics in order to design climate change mitigation strategies. To do so, mechanistic models reproducing the
carbon, nitrogen and water cycles of ecosystems can be developed.
This thesis aims at modifying the ASPECTS model, developed by Rasse et al. (2001) to simulate the evolution
of forest stands, into a cropland and grassland model. The main dissimilarities between forests and croplands
or grasslands were identified, and the required modifications were implemented in a new version of ASPECTS,
called the Terrestrial Agroecosystems Dynamics Analysis (TADA) model. This new model was then calibrated
against data acquired in two cropland and grassland sites, both equipped with eddy covariance (EC) systems and
meteorological stations.
In this work, the attention is paid to the water cycle. Soil evaporation and canopy transpiration were calibrated
against evapotranspiration fluxes (ET) measurements, and the dynamics of water infiltration and percolation within
the soil profile was compared to measures of soil water content (SWC). Soil evaporation was calibrated during bare
soil conditions and resulted in a calibration RMSE of 1.37 and 2.57 mm day-1 and a validation RMSE of 1.82 and
2.75 mm day-1 for, respectively, the cropland and the grassland sites. These results could not be transferred to soils
covered with vegetation, making plant transpiration impossible to calibrate. Canopy aerodynamic resistance was
identified as a possible cause of this problem and a new methodology is proposed to calibrate these two processes
with a wide and diverse dataset in terms of environmental conditions.
In addition to this calibration, the new grazing module was tested by comparing measured and modelled grass
height. The discrepancies are mainly due to the partitioning of assimilated carbon between the shoot and root
compartments and to uncertainties in the estimation of cattle intake capacity. Paths of improvement are provided
for a future calibration of this grazing module, considering both available data and potentially measurable variables.


Book
Afforestation and Reforestation: Drivers, Dynamics, and Impacts
Authors: --- --- --- ---
ISBN: 3039214489 3039214470 Year: 2019 Publisher: MDPI - Multidisciplinary Digital Publishing Institute

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Abstract

Afforestation/reforestation (or forestation) has been implemented worldwide as an effective measure towards sustainable ecosystem services and addresses global environmental problems such as climate change. The conversion of grasslands, croplands, shrublands, or bare lands to forests can dramatically alter forest water, energy, and carbon cycles and, thus, ecosystem services (e.g., carbon sequestration, soil erosion control, and water quality improvement). Large-scale afforestation/reforestation is typically driven by policies and, in turn, can also have substantial socioeconomic impacts. To enable success, forestation endeavors require novel approaches that involve a series of complex processes and interdisciplinary sciences. For example, exotic or fast-growing tree species are often used to improve soil conditions of degraded lands or maximize productivity, and it often takes a long time to understand and quantify the consequences of such practices at watershed or regional scales. Maintaining the sustainability of man-made forests is becoming increasingly challenging under a changing environment and disturbance regime changes such as wildland fires, urbanization, drought, air pollution, climate change, and socioeconomic change. Therefore, this Special Issue focuses on case studies of the drivers, dynamics, and impacts of afforestation/reforestation at regional, national, or global scales. These new studies provide an update on the scientific advances related to forestation. This information is urgently needed by land managers and policy makers to better manage forest resources in today’s rapidly changing environments.

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