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Agroforestry is a land-use system where woody perennials are deliberately combined with agricultural crops and/or livestock on the same land-management units in some form of spatial arrangement or temporal sequence. Agroforestry has the potential to respond to multiple challenges related to soil carbon sequestration including soil fertility improvement, land restoration, food security and adaptation and resilience to climate change. In this chapter, we show how agroforestry systems (AFSs) address several of the above-mentioned challenges, and play a key role in boosting soil carbon sequestration (SCS) and improving soil functions. Other co-benefits are also considered, i.e., soil and ecosystem services, and the wellbeing of rural populations (increase in income, access to non-timber products, etc.), mainly in the less developed countries. Attention is also paid to the main barriers, which may lessen or halt SCS in AFSs. We make recommendations for implementing and improving AFSs to foster SCS and to meet the challenges faced by human societies in the 21st century.

In recent years, plantations of fast-growing tree species have emerged as a possible way to meet the increasing demand for biomass for renewable energy in Europe. Agroforestry plantations including fast-growing tree species could be an attractive option because they reduce land competition for biomass and food production while providing forest benefits. Today, the species interactions that determine whether a given mixture will be more productive than the corresponding monocultures are still poorly understood. Our objective was to assess the performance of fast-growing poplar and alder trees in an experimental plantation set up in 2014 including different treatments, i.e. poplar and alder (1) in monocultures, (2) in association together (forest mixture) or (3) in association with herbaceous species (agroforestry) either N2-fixing species (succession alfalfa and clover) or graminoids (succession of wheat, triticale and temporary grassland) respectively. During eight growing seasons, tree growth was monitored monthly, while leaf (eco)physiological traits (e.g. photosynthetic capacities) were investigated since 2017.During the early stages, negative interspecific competition between crops and poplars predominated and required management actions. A facilitation process in the poplar agroforestry treatment, due to a significant enrichment in soil nitrogen by the N2-fixing crops, led to a higher stem height in agroforestry poplars than in the forest mixture and the monoculture at the end of the 6th growing season*. For alders, tree growth did not appear to be affected by either an association with graminoids or with poplar. No overall significant treatment effect was observed on leaf traits but intra-annual dynamics revealed different tree responses to specific seasonal conditions, with for instance agroforestry poplars maintaining higher growth rates and photosynthetic performances during the 2018 summer drought compared to poplars in forest treatments. This could highlight a better acclimation capacity to potential constraining conditions for poplars in agroforestry stands.

As a renewable energy resource, woody biomass has a role to play in reducing worldwide fossil fuel consumption. Species in the genera Salix and Populus are widely used for this purpose through short rotation tree plantations under temperate latitudes. In warmer climates, common short rotation species include genera such as eucalyptus or black locust (Robinia) although this latter is also cultivated in temperate climates. The objective of our study was to compare the potential of poplar (Populus deltoides × P. nigra, clone 'Dorskamp') and black locust (Robinia pseudoacacia, provenances 'Nagybudmerii' and 'Nyirseg') for biomass production in the Northwest of France. We estimated biomass production and efficiency of water use of poplar compared to those of the two provenances of black locust, (1) at two planting densities (short rotation forestry: SRF, 1428 trees per ha vs. short rotation coppice: SRC, 7272 trees per ha), and (2) 2 and 4 years after planting. Only few effects of the initial planting density on plant growth and physiology were visible two years after planting. Stem height and circumference became higher in SRF than in SRC for both species after four years. Yield at plantation level of the first SRC rotation was almost twice as high for black locust than for poplar, while both species exhibited similar stem height or stem increase rate likely due to the development of more sylleptic branches in black locust. At the end of the second rotation, plantation yield was significantly higher for poplar due to black locust mortality. The combination of higher leaf N and greater WUE probably enhanced black locust performance under the dry conditions of our study site in 2010. During the fourth year (2012), the gap between the two species in terms of Δ 13 C, used as a surrogate of water-use efficiency, was less marked than during the first years due to the fact that poplar Δ 13 C decreased (increase in WUE), while black locust Δ 13 C slightly increased (decrease in WUE). To conclude, black locust appears as an interesting option for biomass production at sites prone to water shortage. However, at high planting density, poplar exhibited a lower mortality after harvest leading to an improved biomass production at the end of the second SRC rotation compared to black locust. The choice of species/cultivar must be adapted to the production system (SRC vs. SRF) and to the specific site conditions.

Le rôle des Salicacées et autres arbres à croissance rapide dans la reprise économique, l'approvisionnement durable en bois et l'atténuation des changements climatiques

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Because of the current scarcity of fossil fuels, woody biomass appears to be an environmentally satisfactory alternative. Under temperate latitudes, in addition to what the forest can provide, intensive cropping systems dedicated or semi-dedicated to the production of biomass for energy are often short-rotation poplar plantations. However, these systems deplete soils in nutrients because of frequent exports of young biomass. This risk can be limited (1) by the use of resource-efficient genotypes or (2) by the association of poplar with nitrogen-fixing species. Concerning the first point, the large diversity demonstrated in terms of use efficiency of water and nitrogen in poplar, the plasticity of these traits in response to the environment (pedoclimate and plantation management), as well as their lack of antagonistic relationship with productivity, portends the possibility of selecting genotypes combining the desired properties. About Poplar association with an atmospheric nitrogen fixator such as black locust, fixation of substantial amounts of nitrogen was observed in the mixture. Nevertheless, the expected facilitation effect was offset by increased interspecific competition in the mixture as compared to monocultures of both species, leading to reduced productivity of the mixture. In the future, the study of carbon allocation changes resulting from poplar association with other nitrogen-fixing species, woody (alder) or herbaceous (alfalfa, clover), will be initiated. The aim will be to describe the effects of species interactions, in the forest and agroforestry mixtures, on carbon and nitrogen allocations, and the growth and productivity of the plantation.; À l’heure de la raréfaction des ressources énergétiques fossiles, la biomasse ligneuse représente une alternative écologiquement satisfaisante. En milieu tempéré, en complément de ce que fournit la forêt, les systèmes de culture intensifs dédiés ou semi-dédiés à la production de biomasse-énergie sont souvent des peupleraies à courte rotation. Néanmoins, ces systèmes appauvrissent les sols en raison des exportations fréquentes de bois jeune. Ce risque peut être limité (1) par l’utilisation de génotypes utilisant efficacement les ressources ou (2) par l’association du peuplier avec une espèce fixatrice d’azote. Concernant le premier point, la très grande diversité mise en évidence en termes d’efficiences d’utilisation de l’eau et de l’azote chez le peuplier, la plasticité de ces caractères en réponse à l’environnement (pédoclimat et gestion), ainsi que leur absence de relation antagoniste avec la productivité, laisse augurer la possibilité de sélectionner des génotypes combinant les propriétés voulues. Concernant l’association du peuplier avec un fixateur d’azote atmosphérique tel que le robinier, la fixation de quantités d’azote conséquentes a été observée dans le mélange. Néanmoins, l’effet de facilitation attendu était contrebalancé par la compétition interspécifique accrue en mélange par rapport aux monocultures des deux espèces, entrainant une productivité réduite du mélange. À l’avenir, l’étude des changements d’allocation du carbone résultant de l’association du peuplier avec d’autres espèces fixatrices d’azote, ligneuses (aulne) ou herbacées (luzerne, trèfle), va être entreprise. L’objectif sera de décrire les effets des interactions entre espèces de mélanges forestier et agroforestier sur l’allocation du carbone et de l’azote et sur la croissance et la productivité de la plantation.

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N° CABI:20163000802; National audience; For three species (poplar, black locust and willow) cultivated as short or very short rotation coppices (SRC/ VSRC), the project relied on monitoring growth and efficiency with which trees use water and nitrogen in a network of four experimental sites, located in contrasting stations. The relevance of DNA methylation levels as an early marker of the level of productivity was also evaluated. For short-rotation plantations (SRP), the project was based on the collection of growth and biomass data in experimental networks, to build compartmented biomass yield tables (trunk, bark, branches and leaves) for forest species for which the literature is scarce. Significant differences appear, both in SRC and VSRC, between the three species, and between intraspecific genotypes for biomass production, its phenology, architecture, leaf structure, and resource use efficiency. The pedoclimatic conditions and the planting density modulated the complex relationships between these traits. Measurement of apex or leaf DNA methylation rate may be a good predictor for the growth potential in poplar. Mean annual biomass production ranging from 7 to 13 dry tons/ha can be expected at age 20 years with fast-growing conifers grown in SRP on site types other than those used for SRC and VSRC. Moreover, very significant genetic gains on biomass production in SRP are also offered by selection of efficient genotypes.; Dans le cadre d’un traitement en taillis à courte ou très courte rotation (TCR et TTCR), le projet SYLVABIOM s’est appuyé sur le suivi de la croissance et de l’efficacité avec laquelle les arbres de trois espèces (Peuplier, Robinier et Saule) utilisent l’eau et l’azote dans un réseau de quatre sites ateliers, situés dans des stations contrastées. La pertinence du taux de méthylation de l’ADN en tant que marqueur précoce du niveau de productivité a également été évaluée. Dans le cadre d’un traitement en futaie à courte révolution (FCR), le projet a reposé sur la collecte de données de croissance et de biomasse dans les réseaux expérimentaux, pour construire des tarifs compartimentés (bois de tronc, écorce, branches et feuilles) d’essences forestières à croissance rapide peu ou pas documentées. Des différences significatives ont été mises en évidence, tant en TCR qu’en TTCR, entre les trois espèces, et entre génotypes au sein d’une espèce, pour la production en biomasse, ses déterminants phénologiques, foliaires et architecturaux et l’efficience d’utilisation des ressources. Les relations complexes entre ces caractères sont modulées en fonction des conditions pédoclimatiques et de la densité de plantation. La mesure du taux de méthylation de l’ADN d’apex ou de feuilles pourrait constituer un bon prédicteur du potentiel de croissance chez le Peuplier. Des productions moyennes annuelles de biomasse comprises entre 7 et 13 tonnes de matière sèche par hectare et par an peuvent être espérées autour de 20 ans avec des conifères à croissance rapide cultivés en FCR sur d’autres gammes de stations que les espèces cultivées en TCR ou TTCR. Des gains génétiques très importants sur la production de biomasse sont par ailleurs offerts par sélection de génotypes performants

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