Ecology of Tuber Melanosporum

Research into the ecology of the Black Winter Truffle, Tuber melanosporum, is more extensive and in-depth than that of Tuber magnatum, as it began earlier and has involved a greater number of researchers, both Italian and French.

Among the Italian works, the entire series of “Studies on the Ecology of Tuber melanosporum” by the CNR Center for Soil Mycology in Turin, published in the journal Allionia, is noteworthy. Among the main French works, those by Delmas, Poitou, and Chevalier are notable.

The Black Winter Truffle has a very wide distribution area, which includes numerous European nations, from Portugal to Bulgaria, even though the most important for production are France, Spain, and Italy. In Italy, it is found primarily in Umbria, Marche, Abruzzo, and Lazio, but also in Veneto, Trentino, Piedmont, Liguria, Lombardy, Emilia Romagna, Tuscany, and Molise.

Tuber melanosporum develops in Italy in calcareous soils from the Mesozoic Era (Cretaceous, Jurassic, Lias) and only exceptionally on marly limestones of the Tertiary Era. These are sedimentary soils (hard limestones, sandy limestones, etc.) generally well-drained due to the fissuring and porosity of the parent rock and the high content of skeletal fragments, which make them appear very stony, even if fine soil is relatively abundant beneath this layer of debris. The profile of these soils is mostly of the rendzina or brown calcareous type, with varying depths. The presence of parent rock debris throughout the profile represents a continuous source of calcium, which becomes crucial for maintaining favorable developmental conditions for the truffle, especially in soils where lime is leached.

The soils of natural truffle grounds that are good producers of Tuber melanosporum feature sand, silt, and clay distributed in good proportions, with texture oscillating between silt-loam and clay-loam, where clay plays an important role in fixing exchangeable ions (calcium, potassium, and magnesium) and keeps the soil fresh for a long time. The presence of a balanced texture, combined with a high lime content and the quality of humic substances, gives these soils good structural characteristics. Indeed, this stable structure, characterized by small aggregates, ensures optimal porosity and water permeability, enough to prevent stagnation that is dangerous for roots and mycelium.

From a chemical standpoint, Tuber melanosporum soils generally present a medium-high endowment of organic matter (from 1.5 to 8%) with a mull-calcic type of humus. It has also been observed that organic matter evolves in quantity and quality according to the age and production status of the truffle ground.

Once the truffle has invaded the environment, one observes, in correlation with the phenomenon of the “burnt area” or “pianello”, a lowering of the organic matter rate and a modification in the proportions of the humus constituents. The carbon/nitrogen (C/N) ratio, which indicates the evolution status of the organic matter, is between 8 and 12. The soil of Tuber melanosporum truffle grounds exhibits an alkaline character with a pH generally above 8.

According to the experience of the French (Poitou et al., 1983), the pH should not exceed 8.5 because beyond this value mycelial growth decreases. It is important that the pH corresponds to the presence of sufficient calcium and is not due to other cations, as sometimes happens in the presence of a high magnesium rate.

Total lime (CaCO3) is always very high (30-40%), although it can vary from 1 to 75%. The contents of nitrogen, phosphorus, and potassium, expressed as a percentage of dry matter, are normally low (Delmas, 1982) and vary from 0.1 to 0.3% (N), from 0.1 to 0.3% (P), and from 0.01 to 0.03% (K). Some research (Dupré et al., 1982) has shown that an excess of nitrogen and phosphorus leads to a reduction in mycorrhization, even if sufficient phosphorus supply is an essential factor for the production of sporocarps.

Lower limits for calcium and magnesium content are 0.5-0.6% (CaO) and 0.01 (MgO), respectively, while upper limits are highly variable according to soil characteristics. In the case of dolomitic soils (double carbonate of calcium and magnesium), the magnesium rate can reach 0.08 (while in ordinary soils it ranges from 0.01 to 0.03); in soils with a total lime content of 30-40%, the calcium rate is between 0.7 and 1.0%.

Also important in the soils of these truffle grounds are trace elements, particularly iron and copper. Iron oxides, which color the soil dark red, seem to impart a more delicate aroma to the black truffle; copper, in the form of sulfate, seems to have a selective effect, favoring certain fungal species within the truffle ground (Poitou and Olivier, 1988).

The development of the Black Winter Truffle requires a climate that features an alternation of various factors across different seasons, especially regarding the quantity and annual distribution of precipitation and the amount of heat provided to the soil during the maturation period of the fruiting bodies. Annual precipitation, the amount of which in the most suitable stations for Tuber melanosporum is around 600-900 mm, must be well-distributed so as to allow for a prompt resumption of mycorrhizal activity, good growth of the mycelium (spring rains), abundant fruiting (intermittent summer thunderstorms), and finally to ensure the swelling and ripening of the carpophores (autumn-winter rains).

It has indeed been observed that years with poor harvests can be caused either by prolonged drought in the months of July-August or by excessive rain during the mycelium growth phase. Also very detrimental to the production of fruiting bodies are excessive and prolonged winter colds (more than 10 days at -10°C) and spring frosts. Not favorable to the Black Winter Truffle are excessively oceanic climates (uniform and lacking contrast), excessively continental ones (summers too hot, winters too cold, indistinct intermediate seasons), excessively Mediterranean ones (summer too arid), or high-altitude climates with long cold periods and weak insolation conditions.

Tuber melanosporum truffle grounds are predominantly located within an altitude range extending from 100 meters above sea level in the northernmost areas to 1000 meters above sea level in the southernmost zones. Slopes and exposure can vary from one environment to another, although south or west-facing ones are prevalent. The topographic location of Tuber melanosporum stations is generally found on plateaus, hilltop flats, or hillsides and almost never in valley bottoms or along ditches.

A truffle ground can be represented by large isolated plants, especially oaks in agricultural land or former cropland; at other times, the truffle ground is present in clearings of rather sparse woods where the ground, poor in shrubs and devoid of grass cover, is well-lit and heated by the sun; this proves the need of Tuber melanosporum for direct sunlight. Characteristic of stations where the Black Winter Truffle grows is the appearance of areas devoid of adventitious vegetation at the base of the symbiont plant, called “bruciate” or “cave” in Umbria, “pianello” in the Marche, and “terre brûlée” in France. This is a specific phenomenon, well-known and studied by numerous researchers (Fasolo Bonfante et al., 1971, 1972; Montacchini et al., 1972, 1977; Papa and Porraro, 1978). The disappearance of adventitious vegetation is caused by competition for water and mineral substances, but above all by the phytotoxic and antibiotic action exerted by the mycelium, which secretes a substance (marasmin) that inhibits the germination of seeds and the development of the underground and above-ground parts of the herbs and shrubs it comes into contact with. The aggressive action of the truffle mycelium occurs mainly during the active growth period of the thallus and the beginning of fruiting, i.e., from May to August; thus, some plants (for example, those of the genus Myosotis and the genus Brachypodium), whose vegetative cycle takes place outside this period, can survive in areas colonized by the truffle mycelium. The vegetation of natural Tuber melanosporum truffle grounds in Central Italy is characterized by the almost constant presence of certain tree, shrub, and herbaceous species (Bencivenga et al., 1988).

The symbiont tree species with the highest frequency in truffle grounds (80%) was found to be the Downy Oak (Quercus pubescens). It is no coincidence that the French often state that the development area of Tuber melanosporum corresponds significantly to that of the Downy Oak, and this is in agreement with what was also identified by Avena et al. (1981), who describe a peculiar plant association for the Central Apennine areas to which they gave the name Cytiso sessilifolii-Quercetum pubescentis. In addition to the Downy Oak, typical symbiont plants of the Black Winter Truffle are, depending on the area, the Holm Oak (Quercus ilex), the Turkey Oak (Quercus cerris), the Hop Hornbeam (Ostrya carpinifolia), and the Hazelnut (Corylus avellana), but also the Pedunculate Oak (Quercus pedunculata), the Sessile Oak (Quercus robur), and the Lindens (Tilia cordata, Tilia platyphyllos, Tilia x vulgaris).

Among the shrubs, there are some plants called “companion plants” because they live in proximity to the typical symbiont; these are: Junipers (Juniperus communis, Juniperus oxycedrus), Mahaleb Cherry (Prunus mahaleb), Spanish Broom (Spartium junceum), Blackthorn (Prunus spinosa), Hawthorn (Crataegus monogyna, Crataegus oxyacantha), Cornelian Cherry (Cornus mas), and some subshrubs such as Fumana (Fumana procumbens) and Rockroses (Cistus albidus, Cistus incanus, Cistus salvifolius, Cistus monspeliensis).

Among the herbaceous species, some can be considered typical of Tuber melanosporum “pianelli”: Bladder Campion (Silene vulgaris), Salad Burnet (Sanguisorba minor), Blue Pimpernel (Anagallis foemina), Hemp-nettle (Galeopsis angustifolia), Yellow Chamomile (Anthemis tinctoria), and Rigid Fescue (Catapodium rigidum). Quite common are Lady's Bedstraw (Galium purpureum), Yellow Bedstraw (Galium verum), Sainfoin (Onobrychis viciifolia), Helichrysum (Helichrysum italicum), and other calciphile species. In particular, some herbaceous species are very interesting because their presence is symptomatic of mycelial activity; these include Mouse-ear Hawkweed (Hieracium pilosella), a composite with leaves covered in hair, and some species of the genus Sedum (Sedum acre, Sedum reflexum, Sedum sexangulare, Sedum hispanicum), commonly called stonecrops. The disappearance of the hawkweed in favor of stonecrops announces intense mycorrhizal activity and precedes the appearance of truffle fruiting bodies by a few years.

In Tuber melanosporum truffle grounds, there exists an entire microflora selected by the truffle mycelium and the plant. In her research on the rhizosphere of truffle grounds, Luppi (1970, 1972) reports Penicillium spp., Humicola fuscoatra, Mortierella alpina, and a mycorrhizal fungus that appears to be competitive with the truffle, Cenococcum graniformis.

The fruiting cycle of Tuber melanosporum does not require high humidity and requires moderate temperatures for its development: it is referred to as a meso-hygrophilic-mesothermic entity (Zambonelli, 1984). In the Black Winter Truffle, the start of fruiting takes place around July, at the moment when profound changes occur in the climatic conditions of the soil: rise in temperature, lowering of the humidity rate, and oxygenation. The first productions of Tuber melanosporum yield superficial truffles with a reddish skin and weak perfume, which do not preserve well. The subsequent production, which runs from December to February and can continue until March, provides the best carpophores. The fruiting bodies of Tuber melanosporum, which are generally always isolated, do not reach the maximum dimensions of those of the white truffle.