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The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 3. Adaptation, resilience, conservation of resources and prevention of risk

Sub-chapter 3.3.2. Taming the Black Truffle (Tuber melanosporum)

Safeguarding Mediterranean food and ecological webs

Yildiz Aumeeruddy-Thomas, Elisa Taschen et Franck Richard

Texte intégral

Introduction

1Mediterranean landscapes are social-ecological systems (SES) shaped by interactions among highly diversified biotas at different ecological scales, coupled with human activities since the Neolithic (Grove and Rackham 2003). They host a large diversity of agrosylvopastoral systems that mimic dryland savannahs, and combinations of interconnected and complementary terraces, pastures and forests.

2Mediterranean SES provide food security through simultaneously producing fodder, pasture lands and related animal products, cereals and pulses, and honey as well as a large set of wild edible herbaceous species, saprophytic mushrooms, animal and tree-related products including acorns and nuts, leaves, bark, wood and a large diversity of associated ectomycorrhizal mushrooms such as chanterelles (Cantharellus spp), boletes (e.g. porcini Boletus edulis) (Zambonelli & Bonito 2012) and truffles (Tuber spp).

3Among the most esteemed food products in the Mediterranean region are two truffle species, the Périgord Black Truffle (Tuber melanosporum) and the Italian white truffle (Tuber magnatum). The Black Truffle has now been exported though cultivation outside the Mediterranean realm to Australia and South America. Desert truffles (Eremiomyces spp., Terfezia spp. and Tirmania spp.) are also prized in the Middle East and North Africa (Hall et al. 2007).

Black Truffle: an iconic product reflecting Mediterranean food webs

4The Périgord Black Truffle is generally portrayed as a delicacy of European gastronomy rather than a product linked to food security. Yet, the history of Black Truffle and its dependence on typical vegetation mosaics dominated by young oaks (early stages of forested garrigues) are emblematic of the slow and long transformations of Mediterranean landscapes by human societies. In most places, garrigues are the result of ancient use of fire in a form of swidden-fallow agriculture which has been largely abandoned, and long-term pastoralism that indirectly favored the production of the fruit bodies of Tuber melanosporum.

5Since the end of the 19th century, the decline of oak-based industries (e.g. charcoal, tannin) and of extensive pastoralism has led to strong landscape shifts, and to a general decline in the area covered by garrigues. This land abandonment induced a dramatic decrease in truffle production (Hall et al. 2007) and the disruption of coupled ecological webs associated with agro-sylvo-pastoral activities and food webs.

6The fall of truffle production in the Mediterranean region during the early 20th century has led to intensive efforts by truffle growers, social reorganizations and scientific research to recover truffle production since the 1950s, with results that show patterns of social-ecological resilience (Aumeeruddy-Thomas et al. 2012). These patterns offer insights into how coupled human food and ecological webs may withstand changes.

7During the last decade, the development of molecular biology tools propelled an unprecedented production of studies dealing with truffles, and allowed researchers to use modern tools to address ancient questions on the biology and the ecology of the Black Truffle (Le Tacon et al. 2014). On the practical front, the development of technical packages to inoculate host trees (establishing ectomycorrhizal relationships on seedlings in greenhouses) since the 70s enabled the settlement of large areas of planted orchards (Murat et al. 2015) following a classical agricultural paradigm that does not mimic the garrigue ecosystem. In parallel, truffle growers have been trying different techniques based on the memory of past garrigue management, as well as innovations through constant experiments (Aumeeruddy-Thomas et al. 2012).

The life cycle and ecology of the Black Truffle

8The Black Truffle belongs to the Pezizomycetes (Ascomycota). Truffle mycelia develop in soil and colonize tree roots to form ectomycorrhizas with their hosts (oaks, hazel, linden, etc). These plant-fungus chimeric structures are the location of mutualistic interactions: the plant host gains better access to water and soil nutrients captured by soil mycelia, and the fungal symbiont gains carbon produced by the host through photosynthesis. The presence of the truffle mycelium below ground is often reflected above ground by the presence of a brûlé extending from the trunk to the limit of the host canopy, and where the development of herbs and shrubs is strongly affected. The mechanisms underlying the formation of brûlé are still unclear and may include phytotoxic effect of volatile compounds synthetized by truffle (Streiblová et al. 2012).

9The fruitbody (hypogeous and tuberculate ascocarp) of the Tuber species, the so-called truffle, is the product of the sexual reproduction of two parents differing by a mating type gene (Rubini et al. 2011b). Millions of meiotic spores are produced in truffles, where one parent – the maternal one – forms the flesh of the fruitbody and feeds it through neighboring mycorrhizas, and the other parent – the paternal one – is only present in spores, and has never been observed as mycelium and/or ectomycorrhiza near the fruitbody (Rubini et al. 2011a). At the population level, the spatial aggregation of maternal individuals sharing the same mating type has been repeatedly observed in planted orchards (Rubini et al. 2011a; Murat et al. 2013) and may limit mating between neighbors. In T. melanosporum populations, the distribution and the life-style of paternal individuals is still poorly understood, and raises the question of the conditions that favor reproduction, and thus, truffle production.

10The Black Truffle’s pedo-climatic conditions are quite broad, but favorable soils are generally porous and calcareous with alkaline trends (Jaillard et al. 2014), reduced organic matter content and high biological activities (microfauna; Callot, 1999). As an endemic Mediterranean species, the truffle requires summer drought and high summer temperatures (Bonet et al. 2011). Rainfall distribution patterns drive truffle production (Le Tacon et al. 2014) and ongoing climatic changes may favor the expansion of T. melanosporum to higher latitudes (Büntgen et al. 2015). In planted orchards and in spontaneous truffle grounds, irrigation offers valuable supports to truffle growers (Olivera et al. 2014).

11T. melanosporum is well adapted to highly disturbed ecosystems; this early-successional fungal species establishes during secondary successions in vegetation mosaics where scattered oaks (Quercus ilex, Q. pubescens and Q. coccifera) dominate a matrix of shrubs and herbs. Multiple ectomycorrhizal shrub species transitorily coexist before the arrival of Mediterranean oaks, and progressively decline after complete canopy closure. In this typical habitat, T. melanosporum is a member of highly diverse fungal communities (Taschen et al. 2015). In this vegetation, the Black Truffle establishes on oak roots only before canopy closure, and shows poor affinities with co-occurring ectomycorrhizal (ECM) shrubs (i.e. Arbutus, Cistus, Helianthemum; Taschen et al. 2015).

12In brûlés, some plant species, named ‘companion species’ by truffle growers, resist the deleterious effect of T. melanosporum, while others disappear. When considered altogether, biotic interactions in truffle grounds are diverse, and nested from the direct host-symbiont physical link (ectomycorrhiza), to the indirect and truffle-mediated plant-plant interaction within the brûlé, and including interaction among fungal species within the species rich ECM communities. From this perspective, truffle grounds are made of functionally interconnected but spatially disjointed sources of fungal inoculum and sexual partners for established truffle mycelia.

Social-ecological resilience: the truffle economy, market structure and institutions

13The ancient Greeks and the Romans held the Black Truffle in high esteem. In medieval times truffles were used in rural areas as a food resource by farmers and they became a delicacy for rich tables only during the Renaissance. Chatin (1892), who amassed extensive production data from 54 French regions, estimated that 2,000 tons were collected annually in France for national markets and for export. The market structure was based on truffle brokers who collected truffles for sale in renowned truffle markets such as Carpentras in France. The existence of specialized markets is an indication that farmers were probably not merely collecting truffles haphazardly. Production was at its peak at the end of the 19th century and declined progressively reaching an average amount of 20 tons in 1996 (Olivier et al. 2012). This crisis led to a radical re-organization in knowledge exchange networks between truffle growers and between the latter and scientists. The market structure also changed radically. New marketing approaches were based upon direct selling by truffle growers, with a parallel collapse of the “broker” system. The latter favored secrecy and speculations rather than information exchange. During the last decades, a much larger diversity of market places, ranging from large well-known markets to village-level truffle markets, flourished in the form of small fairs. A high quality control was established at the national level and was enforced by the Fédération Française des Trufficulteurs as well as regional federations which developed in the 1970s in response to the truffle production collapse.

From knowing the “places” to co-constructing with researchers

14At the end of the 19th century, farmers knew of “places” that were productive brûlés. Secrecy was the only way of “controlling” the places. Historical texts indicate that farmers used a small hoe to slightly till the soil of the brûlés, thus provoking small disturbances. Other techniques including that of leaving small pieces of truffle after harvesting aimed at “sowing” the truffle in analogy with the action of planting seeds.

15Truffle growers made diverse attempts to propagate truffle trees based on empirical experience. In the 1860s, the Phylloxera crisis affected large areas of vineyards. Natural recolonization of agricultural landscapes by holm-oak allowed the production of large quantities of truffles. The observation of this phenomenon led farmers to start planting acorns from tree highly productive in truffles, creating what was known as “Plants planteurs” (Chatin 1892) including attempts to inoculate with small pieces of truffle. In the 1970s, the French Agronomic Research Institute (INRA) developed an inoculation process that led to large plantations in France and elsewhere in the world. However, production hardly increased and reached its peak in 2005 with 40 tons.

16The inoculated system recently developed by INRA induced the development of specialized large commercial nurseries. Subsidized inoculated plants became the norm as well as plantations of inoculated trees. In Languedoc, our previous studies show that truffle growers’ response to these incentives has been much more complex than simply adopting the agricultural package proposed. Indeed, truffle growers, especially those with old family plantations and truffle woods, recovered elements of this truffle landscape. They maintained the old plantations of their forefathers and natural truffle woods and inserted new plantation trials following the techniques proposed by INRA (Aumeeruddy-Thomas et al. 2012). While agricultural chambers were designed for orchards of even-aged inoculated truffle trees, some truffle growers were experimenting with the creation of these complex mosaics at landscape level. According to their empirical knowledge this mosaic is favorable to truffle production (Aumeeruddy-Thomas op. cit). Sets of practices developed by truffle-growers included techniques that maintain the habitat open (canopy-pruning) and disturbed (soil tillage) as in its typical vegetation.

17The necessity to try and recover truffle production led to a reorganization of knowledge systems. The previously secretive system shared vertically (father to son) within family and intimate circles was progressively transformed into knowledge horizontally shared between truffle growers within regions, at national and international favored by the creation of federations of truffle growers at regional and national levels. Furthermore, experimental stations that were established to test and follow truffle productions under the supervision of the Agricultural Chambers favored interactions between truffle growers and technicians. The shift from a vertical to a horizontal system of exchange based on larger exchange networks led to the wide sharing of empirical techniques such as tree pruning, watering and mulching. The most recent technique invented by truffle growers is that of the truffle trap. It is spreading very widely, and many examples are posted and explained thoroughly on the internet. The approach consists generally of excavating the soil in a part of the brûlé, filling it with a mix of soil and peat with a few grams of mature truffle. The truffle grower may use a driller to access the roots which are hurt purposefully to force the latter to develop new roots. This truffle trap technique is at the center of much debate, exchanges among growers and new designs, and is the object of attentive research protocols (Richard et al. forthcoming).

18Large research projects such as the ANR SYSTRUF or the European MYCOSYLVA have proved that progress and innovation can now be based on an exchange of ideas between local and scientific knowledge.

Synergies between local and scientific knowledge

19A literature review of species which persist on the brûlé shows lists of plants that can grow on the brûlé naturally, despite the toxic compounds that eliminate most plants (Martegoute & Cordeau 2002). Scientists for their part had identified many plants from several lineages (from monocots such as orchids to eudicots such as Cistus spp) that could host Black Truffle mycelium without leading to the production of the fruit body (Gonzales Armada et al. 2010). The results of a large survey among truffle growers in Languedoc shows that truffle growers selectively manage companion plant species on the brûlé. Some plants are systematically eradicated while others are protected, if not favored through sowing. These plants are perceived as having an enabling effect on truffle production–as stepping stones for providing suitable fruiting habitat for truffles or through producing disturbances with their roots that may initiate fruiting. Truffle growers manage the plant communities of the brûlé and also take into account plants outside the brûlé as potential stepping stones for enabling truffle production within the brûlé. Indeed, truffle growers provide lists of plants which they consider favorable to truffle production outside the brûlé – a vision which integrates the nested ecology of the disjointed tree-host-brûlé patterns within the overall ecosystem. Both initial scientific findings and grower’s practices helped develop a hypothesis to test the effect of companion species through bringing together local knowledge and scientific knowledge (Photo 1). Experimental set-ups within mesocosms (controlled plantations) with different companion plant species, including some designated by truffle growers, have been attempted and are likely to offer promising insights into the mechanisms involved in these complex interactions primarily detected by the accurate observation and ingenious empiricism of truffle-growers (Taschen et al. forthcoming) (Photo 2).

Photo 1
Assistant researcher working with truffle growers accompanied by their dog, for recording truffle production on a brûlé where the latter had placed small signs to follow their growth. © Elisa Taschen, 2014

Photo 2
Mesocosm experiment with inoculated oak trees with selected companion plants at the experimental station of the Center for Functional and Evolutionary Ecology, Montpellier. © Elisa Taschen, 2015

Discussion and conclusion

20The crisis in truffle production is linked to the historical transformation of the garrigue ecosystem, largely related to the decline of human-based activities. The ecological niche of the Black Truffle coincided with ancient regimes of anthropogenic disturbances in these ecosystems. The closure of forest canopies and the collapse of pastoralism-mediated disturbances probably governed truffle decline in these Mediterranean landscapes. The current synergy between technical innovation by truffle growers and scientific progress aims at recovering truffle productivity. While scientists have primarily focused their research on the brûlé, truffle growers still consider the interstices, the frontier areas, in between two brûlé as an integrated part of a larger system, for instance for companion plants. This is a possible area for further research and experimentation.

21Technical inventions by truffle growers such as the truffle trap now clearly demonstrate the existence of a large exchange network between truffle growers who constantly discuss their on-going experimentations. This major shift from vertical knowledge transmission (father to son) to a large horizontal system of sharing knowledge – including with researchers – is a major sign of an emerging body of shared knowledge which is a key trait of cultural and technical development. This is a prerequisite for the emergence of a domestication process. The restructuring of the truffle market based on quality and regional origins is also an important situation that drives techniques in source production areas.

22However, the domestication paradigm which has prevailed regarding man-plant or man-animal interactions faces a major challenge regarding complex interactions that imply not only mutualistic linkages between two organisms forming a chimeric body, but also interactions with a large set of other ECM fungi and companion plants linked belowground by invisible mycelial networks.

23Large-scale plantation of inoculated trees grown in nurseries has not yet succeeded in recovering half of the amounts produced at the dawn of the 19th century, although we hope that time will favor these plantations, especially if they are progressively re-configured. It seems, however, that the strictly agronomic approach with even-aged plants may need to reconsider an ecological engineering approach at the landscape level.

24The taming or domestication of the truffle, as an element of human Mediterranean food webs, will probably also require the taming of complex truffle ecological webs and biotopes. Such an approach may greatly benefit agricultural approaches in general and is indicative of the absolute necessity today to move from high input agricultural techniques towards agroecological approaches.

Bibliographie

References

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Table des illustrations

Légende Photo 1Assistant researcher working with truffle growers accompanied by their dog, for recording truffle production on a brûlé where the latter had placed small signs to follow their growth. © Elisa Taschen, 2014
URL http://books.openedition.org/irdeditions/docannexe/image/23895/img-1.jpg
Fichier image/jpeg, 334k
Légende Photo 2Mesocosm experiment with inoculated oak trees with selected companion plants at the experimental station of the Center for Functional and Evolutionary Ecology, Montpellier. © Elisa Taschen, 2015
URL http://books.openedition.org/irdeditions/docannexe/image/23895/img-2.jpg
Fichier image/jpeg, 338k

Auteurs

CEFE, UM, France
Ecology of plant-fungi interactions, University of Montpellier Centre d’Écologie Fonctionnelle et Évolutive (UMR CEFE, 5175)
elisa.taschen@cefe.cnrs.fr

CEFE, UM, France
Ecology of plant-fungi interactions, CEFE, UMR 5175, CNRS, Montpellier University, EPHE, France
franck.richard@cefe.cnrs.fr

© IRD Éditions, 2016

Conditions d’utilisation : http://www.openedition.org/6540