The archaeological sounding carried out on the outer surface of Trajanic galleries 20A and 20B entailed, as a preliminary phase, the removal of all the weeds that had taken root in the layer of earth above the screed connecting and covering the vaults.
The types of plants which presented the greatest problems for the wall structure and that were most difficult to remove and eradicate were an Ailanthus altissima (ailanthus or tree of heaven) and a large Laurus nobilis (laurel) bush.
Referring back to the concept, already expressed previously, that defines all those plant species that grow where they are not desired as weeds since they create an imbalance, we can say that both these are high risk species given their negative impact on the architectural structures.
The HI, an index of the hazard posed by plant species, was developed in 1996 by M.A. Signorini (in M.A. Signorini, 1996: l’Indice di Pericolosità: un contributo del botanico al controllo della vegetazione infestante nelle aree monumentali. Inform. Bot. Ital., 28: 7-14). The Hazard Index is a botanical publication of considerable importance for identifying the capacity of individual species to cause damage to structures, as well as their potential to spread and overcome other species, and the greater or lesser difficulty of removing them by weeding (cutting) or using weed killer. The system identifies each plant species in a numerical index on the basis of three parameters: biological form, invasiveness and vigour, type of root system.
- The biological form specifies the habit of the species and the duration of the biological cycle, in other words if the plant is a tree, liana, shrub or herbaceous plant and how long its life cycle lasts: perennial, biennial or annual.
- Invasiveness and vigour refer to the plant’s capacity for propagation and the type of growth: some plants are propagated by pollens and can release these even if felled at the base of the trunk (Laurus nobilis) or release them from their roots (Ailanthus altissima). These are the types of plants which cause the most damage;
- Type of root system. The root system may sometimes reach a greater size than the aerial part (crown). There are two types of root system: a fibrous root system consisting of many ramified roots with a usually superficial development; a tap root system consisting of a single main deep root. Both pose dangers to archaeological structures although under some circumstances fibrous roots may help to preserve the soil structure.
Since for each parameter the value of the hazard index increases as the danger to monuments grows, the sum of these three values gives the Hazard Index (H.I.) of the species.
Identifying the hazard index is fundamental because it measures the evolution of the problem: it is obvious that the root system of a tree or shrub will be more invasive than that of a herbaceous plant and that a short life cycle is less damaging than a perennial one in which the plant will gradually grow and extend its root system to cause increasing damage.
The H.I. may range from a minimum of 0 to a maximum of 10: species with an H.I of 0 to 3 are considered of low risk, those with an H.I of 4 to 6 of medium risk and from 7 to 10 of high risk.
The plant cover present in the layer of earth weighing on the outer surface of vault 20B presented a range of H.I. values from very low (grass, annual herbaceous plants) up to the maximum of 7-10 represented by highly invasive perennial woody plants with a vigorous root system such as Hedera elix (ivy), Ficus carica (fig), Ailanthus altissima (ailanthus or tree of heaven) and Laurus nobilis (laurel).
From the point of view of the interaction between plant organisms and archaeological remains, the woody species present behaved as calciophiles, plants that require or tolerate large amounts of calcium in the soil, here present in excess due to the presence of bricks and the lime in the mortars. The ailanthus is also incredibly ecologically flexible, adapting to any type of soil and exposure and its enormous regenerative capacities makes this the most feared weed.
Periodic weeding operations in the area known as the “Little Barracks” have prevented the development and spread of the ailanthus and other pioneering species that had taken root in the almost bare substrates of the masonry.
The situation is different for the presence of the laurel bush; work to prune and cut it have controlled the development of the crown, preventing the plant from becoming a tree, and were aimed at devitalizing the roots only when an immediate intervention to secure and consolidate the structures of the vault where into which they had grown had been planned.
Devitalization leads to a progressive change in the thickness and resistance of the root system, which gradually detaches from the monument; as it dries out and retracts, it creates preferential routes for the percolation of water, encouraging the soil and mortars to crumble.
Devitalization and the removal of root systems from masonry must take place alongside work to secure and consolidate the archaeological structures.
The sounding evidenced the minimal thickness of vault 20B; the plant had installed itself at the thickest point, the contact wall between the two vaults, surrounding some bricks from the structure.
Removal entailed cutting the deepest roots and some smaller roots that had inevitably remained in place to avoid disintegrating the wall structure further by tearing them out.
The whole park area of the Trajanic terrace presents the same problems identified by test pit 20A and 20B. The large Quercus ilex trees (holm oaks), the Robinia pseudoacacia trees (locust trees) and the Pinus roxburghii (Chir pine) stand on the Neronian or Trajanic structures, surrounding them with their roots. Roots, with numerous branches, emerge from the inside of the ancient vaults like those found in Room 41 where we collected the samples that allowed us to determine that they belonged to the root structure of the large chir pine and of broad-leaved trees.