Bruno Rapidel

Simulation of leaf transpiration and sap flow in virtual plants: model description and application to a coffee plantation in Costa Rica

Abstract Computer representations of plants (virtual plants) are used as the basis for a model simulating leaf transpiration and sap flow. The virtual plants provide a detailed description of plant geometry and topology and, once positioned in a scene, enable a highly realistic reconstruction of a portion of the canopy. Stomatal conductance as well as energy balance are simulated by the model at the level of individual leaves in order to calculate their transpiration. Leaf transpiration is then cumulated to get the sap flow throughout the plant. Owing to its structure, the model can take into account feedbacks such as the effect of the temperature of a leaf on its stomatal conductance, transpiration and water potential, and in return, the effect of water potential of a leaf on its stomatal conductance. The model has been validated on a coffee tree stand in Costa Rica. The geometry (i.e. 3D position, area and diameter of organs) of six adult coffee trees in a row was measured in the field in order to generate a computer scene. Stomatal conductance, sap flow, water potential and wood hydraulic conductivity were measured for model parameterisation and validation. Analysis of model outputs lead to a correction of leaf boundary layer thickness. After calibration, the model exhibited correct values of transpiration and water potential in different microclimatic conditions.

Experiment-based prototyping to design and assess cotton management systems in West Africa

Designing innovative combinations of techniques to improve the sustainability of cropping systems in poor countries is a major challenge. Here, we developed a prototyping methodology to design, assess and adapt a crop management system for a specific set of constraints. It was applied in Mali with the aim of designing innovative prototypes of cotton management systems to be further tested and adapted by farmers. The prototype aimed at shortening the cotton cycle to overlap the rainy season. The prototype is particularly suited for late sowing and for regions where rainfall is often insufficient. We propose a conceptual model that organizes technical interventions to shorten the cotton cycle. We developed a set of indicators to evaluate the relevance of each modified technical intervention, by comparison with current farmer practices. We evaluated the overall performances of the prototype by taking into account economic, environmental and social factors. The prototype was tested and adjusted in six trials between 2002 and 2004. Our results show that the cotton growth cycle was reduced by 15 days on average, mainly through the shortening of the flowering period. The combination of much higher stand density than currently applied in the region and application of the growth regulator mepiquat chloride produced a much higher number of bolls per hectare of +69%. The prototype produced higher seed-cotton yields, of +44% on average, and much higher gross margin than the standard cotton management system. The method was therefore successful in designing a new cotton management system that helps farmers to adapt to diverse cropping conditions.

Vegetation as a driver of temporal variations in slope stability: The impact of hydrological processes

Although vegetation is increasingly used to mitigate landslide risks, how vegetation affects the temporal variability of slope stability is poorly understood, especially in earthquake-prone regions. We combined 3-year long soil moisture monitoring, measurements of soil physical properties and plant functional traits, and numerical modeling to compare slope stability under paired land uses with and without trees in tropical, sub-tropical, and temperate landslide- and earthquake-prone regions. Trees improved stability for 5-12 months per year from drawdown of soil moisture and resulted in less interannual variability in the duration of high-stability periods compared to slopes without trees. Our meta-analysis of published data also showed that slopes with woody vegetation were more stable and less sensitive to climate and soil factors than slopes with herbaceous vegetation. However, estimates of earthquake magnitude necessary to destabilize slopes at our sites suggest that large additional stabilization from trees is necessary for meaningful protection against external triggers.

Farmer perception and utilization of leaf functional traits in managing agroecosystems

Using knowledge of leaf functional traits, such as those forming the leaf economics spectrum (LES), to understand plant responses to environmental change is well-established and now being more widely applied to agroecosystems. Yet, little is known about how farm managers invoke leaf functional traits to inform management decisions. The objectives of this research were to (1) evaluate whether farmers use knowledge of intraspecific trait variation (ITV) in LES traits (or trait proxies) of target crops as response indicators of management conditions; (2) determine whether LES trait values are ranked consistently among multiple farmers along a “Farmer Leaf Economics Spectrum” (FES); (3) evaluate how a FES corresponds to the LES; and (4) identify the farmer and farm attributes that best predict the agreement between the FES and the LES. We collaborated with coffee (Coffea arabica) farmers in the Turrialba Valley, Costa Rica. We used a visual elicitation tool of fresh leaves along an intraspecific spectrum of leaf size, leaf thickness and leaf colour (as a proxy for leaf nutrients); respondents were asked to rank leaves in response to shade and nutrient scenarios as well as yield potential. On-farm biophysical data, management practices and socio-economic attributes were also collected. The majority of farmers demonstrated a developed system of utilizing coffee leaf and whole-plant ITV as indicators of management practices. Farmers managing smaller farms tended to more commonly acknowledge ITV in LES chemical–morphological traits, as compared to those managing large farms. The agreement between a respondent-identified ranking of leaf thickness ITV as a function of light environment and an empirically defined thickness-to-light ranking was partially explained by farmers’ physical engagement with plants. Synthesis and applications. In scientific literature, analyses of crop intraspecific trait variation have provided important insights into the mechanistic bases of multiple key agroecological processes. We demonstrate that farmers use crop leaf trait variation as an indicator to both evaluate management prescriptions and to initiate management actions including shade-tree species selection and abundance, crop- and shade-tree pruning regimes and fertilization treatments. These findings signify that functional traits represent a key nexus between scientific and local knowledge.

Simulation of Ecophysiological Processes on 3D Virtual Stands with the ARCHIMED Simulation Platform

Most classic ecophysiological models rely on crude representations of canopies as stacks of vegetation layers. Therefore, their use in complex canopies implies complicated adaptations as well as simplifying assumptions that are difficult to validate. Alternatively, the ARCHIMED simulation platform uses 3D virtual stands as a support for numerical simulations of biophysical processes such as leaf irradiation, transpiration and temperature and ultimately carbon assimilation. By doing so, detailed information can be integrated from the individual leaf scale up to the individual plant scale, even within complex stands such as agroforestry systems. Simple numerical methods are used for solving multiple feedbacks between light, energy, water and CO2 transfers at leaf, plant and plot scales. Numerical calculations applied at different scales allow simple implementation of complex models involving intricate processes.