Spatial organization of individuals and ecosystems services in tropical agroecosystems : COS 18-9

Background/Question/Methods Agroecology involves the optimization of ecological processes in agroecosystems. It has been identified as a sustainable alternative to the negative environmental impact of modern agriculture. A challenge in agricultural research is to design innovative "agroecological" systems: allowing to maintain an acceptable level of productivity; but also fostering on other ecosystem services. We worked on ecological concepts influencing the dynamics and ecological performances in ecosystems, and studied its application on agroecosystems. The spatial organization of individuals is fundamental in ecological theories. It is an important structural characteristics that influence ecosystem functioning and productivity. In agroecosystems, the spatial organization of individuals may influence key aspects influencing ecosystem services sought-after in sustainable agriculture. Our aim was to analyze the spatial organization of plant individuals in complex agroecosystems; and to highlight the links between spatial organization of plant individuals and selected ecosystems services: provisioning services (crop productivity), biodiversity conservation (trees species richness) and regulating services (pest and disease regulation). We used the Ripley function to analyze the spatial organization of shade and cacao trees in cacao agroforests in Costa Rica. We also assessed the species richness of shade trees; and cacao productivity and damages by Frosty Pod Rot, an important disease in Costa Rica. Results/Conclusions Three types of stands were identified: the first characterised by significant clustering of shade trees. The second type was characterised by random spatial organisation of shade trees. The third types showed a trend towards regular organisation. The clustered structure of shade trees appears to be a trade-off between biodiversity and productivity. Even if the damaged production (estimated by the number of damaged pods in the plots) was significantly higher in the clustered type, the potential (total number of pods) also tended to be higher, leading to an healthy (number of healthy pods) productions equivalent to the regular and the random spatial types. However, the clustered type had the highest shade tree species richness. The clustered type were located in remote places, closer to natural forest systems, and are managed for years by farmers who are very respectful of nature, and do not wish to disturb the natural process of tree regeneration. Manipulating spatial structure in complex agroecosystems appears as a lever for the ecological intensification of these agroecosystems. Indeed, the clustered spatial structure appears to favour a synergy between biodiversity conservation (tree species richness), and provisioning services (cacao production), taking into account a regulation services (pest and disease regulation). (Résumé d'auteur

Construction de modèles de structure spatiale permettant de simuler des peuplements virtuels réalistes : application aux peuplements mélanges chêne sessile - pin sylvestre de la région centre

La gestion forestière nécessite de pouvoir prédire l'évolution des différentes parcelles, en particulier en terme de production et de qualité du bois. La recherche forestière, et plus particulièrement la dendrométrie fournissent ainsi aux gestionnaires des outils de prédiction adaptés, à l'exemple des modèles de croissance. Les modèles "peuplement", bien adaptés aux contraintes de la gestion forestière, ne permettent pas toujours de simuler des peuplements hétérogènes (mélangés, irréguliers en âge). Pour tenir compte de la forte variabilité des arbres dans les peuplements hétérogènes, les dendrométriciens ont cherché à modéliser l'évolution de chaque individu arbre en fonction de ses caractéristiques propres et de son environnement local. Ces Modèles Individu-Centré (MIC) semblent plus pertinents pour simuler des peuplements complexes. Cependant, ces MIC prenant en compte l'environnement local des arbres nécessitent un important jeu de données en entrée. En effet, les MIC ont besoin pour lancer une simulation de connaître avec précision l'état initial du peuplement, à l'échelle de chaque arbre. Il faut dès lors fournir au simulateur un état initial avec les caractéristiques de chacun des arbres, et leur position dans le peuplement dont on veut simuler l'évolution. Ces données sont rares, coûteuses à obtenir, et l'acquisition de telles données ne peut en pratique être envisagée que pour quelques placettes expérimentales. Dans ce contexte, il est possible d'utiliser un peuplement virtuel, proche du peuplement réel dont on veut simuler l'évolution, comme état initial pour un MIC. Il apparaît alors important que le peuplement virtuel simulé soit réaliste, c'est-à-dire qu'il reproduise les caractéristiques du peuplement réel qu'il représente, particulièrement ses caractéristiques spatiales. L'objectif de cette thèse est de construire des modèles de structure spatiale, permettant de simuler des peuplements virtuels réalistes. Ce travail est appliqué aux peuplements mélangés chêne sessile - pin sylvestre de la région Centre. Pour atteindre cet objectif, nous avons dans un premier temps construit une typologie de peuplements mélangés chêne - pin. Cette typologie est basée sur la structure spatiale des arbres dans ces peuplements mélangés. Elle permet de synthétiser la grande diversité de structure spatiale de ces peuplements au sein des dispositifs étudiés. Nous avons ainsi mis en évidence 5 types de structure spatiale pour les arbres de la canopée et 3 types pour les arbres du sous-étage. Nous avons dans un deuxième temps construit des modèles de structure permettant de reproduire les structures spatiales identifiées. Pour cela, nous avons couplé notre typologie à des outils statistiques de simulation de peuplements virtuels, les processus ponctuels. Nous avons construit des modèles de structure pour les types spatiaux identifiés dans la canopée et dans le sous-étage. Notre travail basé la structure spatiale fournit une description précise des peuplements mélangés étudiés et nous permet ainsi de développer une approche réaliste de modélisation. La génération de peuplements virtuels réalistes est d'une grande importance pour l'utilisation de MIC, et améliorerait la qualité des résultats de simulations à l'échelle de l'arbre. (Résumé d'auteur

Spatial structure of forest trees in tropical agroforests

In the context of the ongoing destruction of tropical rain forests, the potential value of tropical agroforests as a model of agricultural sustainability has increased. Tropical agroforests consist in complex associations of trees and crops. However few studies dealt with spatial structure of forest trees in tropical agroforest systems and their similarity to spatial organisation of trees in forest ecosystems. Our aim is to analyse the spatial structure of forest trees in tropical agroforests. We used a classical method of spatial statistics: Ripley's K-function. We linked the different spatial organisation of trees with diversity of associated plants (ecological performance) and to pest and diseases pressure (agronomical performance). This paper focuses on 36 plots in a tropical cocoa based agroforest in the region of Talamanca (Costa Rica). Forest trees were not significantly present in 7 plots; randomly distributed in 15 plots; regularly distributed in 8 plots; clustered in 6 plots. The clustered structure of forest trees was correlated with a higher diversity in associated plants in the studied stand, and with a the highest pest and diseases pressure. We discuss advantage and drawback to introduce structural characteristics of forest in cropping systems. (Résumé d'auteur

Simulating realistic spatial structure for forest stands: a mimetic point process

The recent development of individual based and spatially explicit models raises the question of how to simulate relevant initial states for these models, and more particularly how to simulate realistic spatial structures. This is especially true for forest stand modelling. In this paper, we first review briefly the main simple steps in the simulation of forest stands with realistic spatial structure : (i) spatial structure analysis and ecological interpretation, (ii) expert simulation through classical point processes and ad hoc values of the parameters, and (iii) precise fitting of a spatial structure model using L(r) and L12(r) functions as criterion. We illustrate the advantages and limits of these steps on a real forest stand in mixed Oak - Scots pine forest near Orléans (France). We then propose a specific method - a mimetic point process - in order to simulate spatial structures similar to real patterns. This point process is based on a generalised Gibbs process, whose global cost function directly depends on the difference between the real measured values of L(r) and L12(r) functions, and the corresponding values computed on the simulated pattern at each iteration of the algorithm. We applied this method on a mixed Oak - Scots pine forest stand, and thus obtained an improved simulated pattern, that can be used as initial state for individual based forest growth models. We finally discuss the limits of this method, and its possible use to facilitate the valorisation of individual based models in the community of forest management. (Résumé d'auteur

Modelling the spatial structure of complex stands by point processes : [Abstract]

There is a growing interest for complex - uneven-aged and /or mixed- stands. The dynamics of these stands are more difficult to understand than those of pure and even-aged stands. Thus, new research questions have arisen in terms of stand description, stand dynamics and modelling. In terms of growth modelling, the use of individual based models seems more appropriate, because of the individual variability within complex stands. However, it is not easy to use these models, especially because they require knowing the localisation of each tree in order to run simulations. In this context, Spatial Structure analysis can be used to have a better knowledge of these stands. Indeed, spatial structure, resulting from past biological processes, defines the variety of local neighbourhoods of each tree, which influences future processes. Consequently, spatial structure analysis could be used to characterize these stands; moreover a good characterisation allows realistic modelling approaches. Our aim is to model the spatial structure of complex stands, in order to be able to simulate realistic virtual complex stands. Our first step is to characterize precisely complex real stands. We applied an analysis of spatial structure to 25 plots, set up in irregular mixed stands of Oak and Scots pine, of the Orleans forest (France). We used the classical Ripley function L(r) (for univariate and bivariate patterns). We used the spatial results to build a typology of these stands. We identified five main spatial types. In a second step, we used appropriate point processes to reconstruct the identified spatial types. We compare the simulated spatial structure to the real one, to investigate how realistic is our simulated stands. We illustrate this step for 2 identified types. Those simulated stands could be used as initial states for individual based models, when simulating the growth and the dynamic of mixed stands. (Résumé d'auteur)

Structures spatiales et régulation des bioagresseurs des agroforêts à cacaoyers au Cameroun et au Costa Rica

Dans ce travail, nous montrons que la structure spatiale des individus (arbres et cacaoyers) au sein d'agroforêts à cacaoyers influence l'intensité d'attaque de bioagresseurs des cacaoyers. À l'échelle de la parcelle, des organisations spatiales régulières ou aléatoires d'arbres forestiers réduisent les intensités d'attaques de la moniliose au Costa Rica et des mirides au Cameroun. À l'échelle de l'individu, le nombre de voisins cacaoyers (à 3,7 m) et fruitiers (à 4,3 m) influence négativement l'intensité d'attaque de la moniliose au Costa Rica. Nos résultats révèlent l'importance de la structure spatiale dans la description des agroforêts tropicaux et la compréhension des mécanismes influençant la régulation agroécologique des bioagresseurs au sein de ces systèmes complexes. La gestion de la structure spatiale des individus dans les systèmes agroforestiers pourrait ainsi constituer un levier intéressant pour le contrôle agroécologique des bioagresseurs

First typology of cacao agroforests in the Colombian Amazon, based on composition, structure and light availability

The cultivation of cocoa in Colombia are of key social importance. Indeed cacao plays a prime role in post conflict resolution as it is the legal crop to replace illicit crops. In the current context of the need of combating climate change, cacao agroforests are also expected to be a sustainable practice, promoting forest-friendly land use. In that context, it is necessary to describe accurately these systems, and especially accounting for their potential in terms of biodiversity conservation. Aim In this work, we present a first a typology of cacao agroforest systems in Colombian Amazonia, systems that had yet to be described in the literature. This typology is based on tree species richness, canopy structure and light availability. Material and Methods We worked in 50 agroforest plots of 2000m² each, in the Bajo Caguán area of the department of Caquetá, in the Colombian Amazonia. In each plot, we measured variables of composition (diversity of plants associated with cacao trees) and variables of vertical and spatial structure (height layers, Diameter at Breast Height, basal area, shape and area of the crown, (x, y) positions of each individual plant in each plot. We also measured variables of radiation transmitted to cacao trees in the understorey: above the cacao canopy layer, we took hemispherical photographs and measured the intensity of Photosynthetically Active Radiation (PAR, in umol m-2 s-1) using an AcuPAR LP-80 sensor. We included variables related to light availability to evaluate the amount of transmitted radiation to the cacao trees in each type, and its suitability for cacao ecophysiological development. We also use variables of spatial organization to model the distribution of light in each plot, using two models: Shademotion 4.0 to calculate the fraction of the average of shade hours and shade area in each agroforest plot and the Spatially individual-based Explicit Forest-Simulator to calculate the degree of canopy openness in each agroforest plot. We there used a cluster analysis to build a typology of cacao agroforest, based on 28 variables characterized in each plot, and related to diversity, composition, spatial structure and light availability for the cacao trees. Major results and Conclusions We identified 4 types of cacao agroforests. The typology was based on their differences in tree species diversity and the impact of canopy spatial structure on light availability for the cacao trees in the understorey. We also found 127 tree species in the dataset, with 3 out of the 4 types identified displayed an erosion of tree species diversity. This reduction in shade tree species may be linked to the desire to reduce shade, but we also found that all the types described were compatible with good ecophysiological development of the cacao trees. One challenging prospect will be to monitor and encourage the conservation of tree species diversity in cacao agroforest systems during the development of these cropping systems