It is of real importance for all those who, like us, are concerned for the Domus Aurea and its vegetation, respectively a unique and irreplaceable archaeological heritage and a magnificent natural heritage, to discuss the difficult relationship between these two features in the park on the Oppian Hill.
As already mentioned in our earlier article of 14.12.2012, we have collected samples from parts of the root systems emerging from the wall surfaces inside Room 42; identical samples were later collected in Room 41.
These two rooms, from then onwards, were identified as Station 1 and Station 2.
The decision about where to collect the samples was dictated by the ability to effect the removal in a non-invasive way with respect to the structural and pictorial components of the monument.
After detailed inspection, four sampling areas were chosen, based both on their location and on the typological diversity of the root systems, visible even to the naked eye.
We selected samples on the basis of the potential to remove homogeneous pieces with a mean diameter of under a centimetre, between 5.00 and 8.00 mm. We collected a significant quantity of the roots emerging from the surface by cutting them and preserving them in an alcohol solution.
An initial visual examination immediately indicated morphological differences that allowed us to attribute the samples from Station 1 prevalently to conifers given their colour and the presence of resin, a characteristic of the division Pinophyta. The conifers present on the Trajanic Terrace are essentially the large pine trees along the avenue, which are therefore in a marginal position, and the large Himalayan pine in the centre of the upper garden, closer to the underground area where the sample was taken.
The samples from Station 2 belong exclusively to broad-leaved trees given their dark brown external colour, shape and woody consistency.
Transverse and longitudinal microscopic sections were then taken from the fresh samples collected using two technical procedures: immersion in alcohol and colouring with methylene blue.
The sections thus obtained were examined under a microscope to identify the species based on the distinctive features of the composition and arrangement of the root cells of each species sampled.
The analysis of the samples from Station 1 confirmed that they belonged to conifers of the genus Pinus.
In the microscope slides we saw the typical cellular organization of resin ducts specific to the genus Pinus: Pinus roxburghii Sarg. present on the Trajanic Terrace, the typical thickness of resin-secreting cells is immediately obvious in the fresh slide without colouration.
The samples from Station 2 were prepared in the same way for microscope analysis. Observation indicated the well-defined organization of the sap cells of broad-leaved trees, belonging to the genus Quercus, more evident in the slides with colouration.
In this first phase of recording and collecting representative portions of the root systems emerging from the monumental structures, we noted the complexity of their development in terms of both size and growth, constantly evolving and developing.
The extension of the root systems is notable given the current position of the trees on the Trajanic Terrace and their linear distance from the samples taken inside the rooms of the Domus Aurea. The linear distance between the Pinus roxburghii Sarg. trunk and the samples taken in Station 1 – Room 42 is over 25 metres. The samples taken in Station 2 – Room 41 are also an average linear distance of 25 metres from the trunks of the Quercus ilex L. trees.
One of the observations made in the various underground rooms is that the roots were found in a specific place: the roots become closer together and grow in a continuous way between the wall structures and the plaster.
The chemical composition of the mortars presents a complex mixture of mineral salts of varying nature and geolithological origin: this mixture seems to provide the fundamental nutrients for deep roots. Whenever these emerge from the mortars and become exposed, such as outside the plasterwork, the root ends generally protrude for a few tens of centimetres and then die.
By contrast, we find new side shoots from the roots in areas of wall that are still partially intact, where these continue to grow, preferably between the load-bearing structures and the plasterwork.
Additionally, the climate conditions (temperature, humidity, percolation of rainwater, etc.) constantly present in the Domus Aurea, which as we know is almost entirely underground, favour and guarantee a perfect environment for constant growth even during periods when dry conditions might cause problems outside.
The laboratory analyses thus confirmed that the roots emerging from the vaults and walls of rooms 41 and 42 are those of woody plants that, as already described in our earlier article, belong to the category of species presenting the highest risk in terms of their ability to cause damage not only to the decorations but also to the stability of the wall structures. This is evident in both of the rooms studied, where the surface affected by the roots is eroded to a greater depth than other walls in the same room.