Ecology of Tuber Melanosporum
Research on the ecology of the highly prized Black Truffle, Tuber melanosporum, is more extensive and in-depth than that on Tuber magnatum, as it began earlier and a greater number of Italian and French researchers have dedicated themselves to it.
Among the Italian works, worthy of mention is the entire series of "Studies on the ecology of Tuber melanosporum" by the CNR Soil Mycology Center of Turin, published in the journal Allionia. Among the main French works, those by Delmas, Poitou, and Chevalier are notable.
The highly prized Black Truffle has a very wide distribution area, which includes many European nations, from Portugal to Bulgaria, even if the most important for production are France, Spain and Italy. In Italy it is found mainly in Umbria, Marche, Abruzzo, Lazio, but also in Veneto, Trentino, Piedmont, Liguria, Lombardy, Emilia Romagna, Tuscany and Molise.
Tuber Melanosporum develops in Italy on calcareous soils from the Secondary Era (Cretaceous, Jurassic, Lias) and only exceptionally on marly limestones from the Tertiary Era. These are sedimentary soils (hard limestones, sandy limestones, etc.) generally well drained due to the fissuring and porosity of the bedrock and the high content of skeleton material, which makes them appear very brecciated, even if beneath this layer of breccia, the fine earth is relatively abundant. The profile, more or less deep, of these soils is mostly rendzina or brown-calcareous type. The presence of bedrock debris along the profile represents a continuous source of calcium, which becomes very important for maintaining favorable development conditions for the truffle, especially in those soils where lime is leached.
The soils of natural truffle grounds that are good producers of Tuber Melanosporum contain sand, silt, and clay distributed in good proportion, with a texture oscillating between silty loam and clay loam, where clay plays an important role in fixing exchangeable ions (calcium, potassium, and magnesium) and maintains soil freshness for a long time. The presence of a balanced texture, together with the high lime content and the qualities of humic substances, provides these soils with good structural characteristics. In fact, this stable structure, characterized by small aggregates, ensures optimal porosity and water permeability such as to avoid dangerous stagnation for the roots and mycelium.
From a chemical point of view, Tuber Melanosporum soils generally have a medium-high endowment of organic matter (from 1.5 to 8%) with a mull-calcic type of humus quality. It has also been observed that organic matter evolves in quantity and quality according to the age and production status of the truffle ground.
When the truffle has invaded the environment, in correlation with the phenomenon of the "burnt area" or "pianello," a decrease in the organic matter content and a modification in the proportion of humus constituents are observed. The carbon/nitrogen ratio (C/N), which indicates the state of organic matter evolution, is between 8 and 12. The soil of Tuber Melanosporum truffle grounds has an alkaline character with a pH generally above 8.
According to French experience (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 content.
Total limestone (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 range 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 nutrition is an indispensable factor for carpophore production.
Lower limits for calcium and magnesium content are respectively 0.5-0.6% (CaO) and 0.01% (MgO), while upper limits are highly variable depending on soil characteristics. In the case of dolomitic soils (double carbonate of calcium and magnesium), the magnesium content can reach 0.08 (whereas in ordinary soils it ranges from 0.01 to 0.03); in soils with a total calcium carbonate content of 30-40%, the calcium content is between 0.7 and 1.0%.
Important in the soils of these truffle grounds are also trace elements, particularly iron and copper. Iron oxides, which color the soil dark red, seem to give the black truffle a more delicate aroma, while copper, in the form of sulfate, seems to have a selective effect favoring some fungal species in the truffle ground (Poitou and Olivier, 1988).
The development of the prized black truffle requires a climate that, during the different seasons, presents an alternation of various factors, especially concerning the quantity and annual distribution of precipitation and the amount of heat supplied to the soil during the maturation period of the fruiting bodies. Annual precipitation, the amount of which in the most suitable locations for Tuber Melanosporum is around 600-900 mm, must be well distributed so as to allow for a prompt resumption of mycorrhizal activity, good mycelial growth (spring rains), abundant fruiting (intermittent summer storms), and finally ensure the enlargement and maturation of the carpophores (autumn-winter rains).
It has indeed been found that years with poor harvests can be caused both by a prolonged drought in July-August and by an excess of rain during the mycelial growth phase. Excessive and prolonged winter cold (more than 10 days at -10°C) and spring frosts are also very detrimental to the production of fruiting bodies. Excessively oceanic climates (uniform and not very contrasting), excessively continental climates (summers too hot, winters too cold, intermediate seasons not very marked), excessively Mediterranean climates (summers too arid) or high-altitude climates with long cold periods and too weak insolation conditions are not favorable to the prized black truffle.
The Tuber Melanosporum truffle grounds are mainly found in an altitude range that goes from 100 meters above sea level in the northernmost areas to 1000 meters above sea level in the southernmost areas. Slopes and exposure can vary from one environment to another, although those facing south or west are prevalent. The topographic location of Tuber Melanosporum sites is generally found on plateaus, on hilltops, or on the slopes of hillsides, and almost never in valley bottoms or along ditches.
The truffle ground can be represented by large isolated trees, especially oaks in agricultural land or former cultivated areas; other times, the truffle ground is present in clearings of rather sparse woods where the soil, poor in shrubs and devoid of grassy turf, is well lit and warmed by the sun; this demonstrates the Tuber Melanosporum's need for direct sunlight. A characteristic feature in sites where the prized Black Truffle grows is the appearance of areas without adventitious vegetation at the base of the symbiotic plant, called "bruciate" or "cave" in Umbria, "pianello" in Marche, and "terre brulé" 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 hypogeal and epigeal parts of grasses and shrubs with which it comes into contact. The aggressive action of the truffle mycelium occurs mainly during the period of active thallus growth and the beginning of fruiting, i.e., from May to August, so some plants (for example, those of the genus Myosotis and the genus Brachipodium) whose vegetative cycle takes place outside this period, can persist 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 some arboreal, shrubby, and herbaceous species (Bencivenga et al., 1988).
The symbiotic tree species with the highest frequency in truffle grounds (80%) was 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, for the areas of the central Apennines, speak of a peculiar plant association they named Citysosessilifolii-Quercerurn pubesceentis. In addition to the downy oak, typical symbiotic plants of the prized black truffle are, depending on the area, the Holm Oak (Quercus ilex), the Turkey Oak (Quercus cerris), the European Hop-hornbeam (Ostrya carpinifolia) and the Hazelnut (Corylus avellana), but also the Pedunculate Oak (Quercus peduncolata), the Common Oak (Quercus robur) and the Lime trees (Tilia cordata, Tilia platiphyllos, Tilia x vulgaris).
Among the shrubs, there are some plants called "companion plants" because they live near 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 procumhens) and Cistus (Cistus albidus, Cistus incanus, Cistus salvifolius, Cistus monspeliensis).
Among the herbaceous species, some can be considered typical of the "pianelli" of Tuber Melanosporum: bladder campion (Silene vulgaris), salad burnet (Sanguisorba minor), scarlet pimpernel (Anagallis foemina), hemp-nettle (Galeopsis angustifotia), golden marguerite (Anthemis tincroria), and hard fern (Catapodiuni rigidum). Quite common are red bedstraw (Galium puerperum), lady's bedstraw (Galium verum), sainfoin (Onobrychis viciaefolia), helichrysum (Helicrisum italicum), and other calciphilous species. In particular, some herbaceous species are very interesting because their presence is symptomatic of mycelial activity; these are mouse-ear hawkweed (Hieraciurn pilosella), a composite with a leaf blade covered with hairs, and some species of the genus Sedum (Sedum acre, Sedum reflexuni, Sedum sexangulare, Sedum hispanicum), commonly called stonecrops. The disappearance of mouse-ear hawkweed in favor of stonecrops indicates intense mycorrhizal activity and precedes the appearance of truffle fruiting bodies by a few years.
In Tuber melanosporum truffle grounds, there is an entire microflora selected by the truffle mycelium and the plant. Luppi (1970, 1972) reports in his research on the rhizosphere of truffle grounds Penicillium sp.p., Humicola fuscoatra and Mortierella alpina, and a mycorrhizal fungus that seems to compete with truffles, Cenococcum graniforme.
The fruiting cycle of Tuber Melanosporum does not require high humidity and needs moderate temperatures for its development: it is considered a meso-hygrophilic-mesothermal entity (Zambonelli, 1984). In the prized Black Truffle, fruiting begins around July, when profound changes occur in the soil's climatic conditions: increased temperature, decreased humidity, and oxygenation. The first productions of Tuber melanosporum yield superficial truffles, with a reddish peridium and a weak aroma, which do not keep 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 size of those of the prized white truffle.