Vegetation in Archaeological Areas: Risks, Opportunities, and Guidelines to Preserve or Remove: An Italian Case Study

Abstract

In the context of research on climate, microclimate, and heritage buildings or museums, archaeological sites represent a unique type of cultural environment. In these settings, the outdoor microclimate is one of the primary factors to consider and manage, both for the preservation of the heritage assets and for the well-being of the visitors. As is well-known, archaeological sites are generally spaces of vegetation colonisation and significant reservoirs of biodiversity. Given that the longevity of a monument is closely tied to its surrounding environment, it is evident that its conservation is significantly influenced by the presence of spontaneous vegetation that colonises it or the designed vegetative structures that surround it. Furthermore, studies have documented that this vegetation is an important factor to consider for the thermal comfort of visitors. In this article, a research methodology is proposed, applied to an Italian case study, in which choices regarding the conservation/removal of the vegetation (roots, leafs, etc.) (Vegetation Hazard Impact Index—VHII) at an archaeological site are examined, along with their impacts on the outdoor microclimate and the thermal comfort of visitors. The findings demonstrate that an approach exclusively focused on removing vegetation deemed invasive risks exacerbating thermal discomfort and, consequently, diminishing the usability of the archaeological site.

1. Introduction

The effects of climate change [1,2] also impact cultural heritage and the heritage sector, encompassing both tangible and intangible aspects [3]. In 2003, UNESCO approved Resolution 24GA8, entitled “Policy Document on Climate Action for World Heritage” [4] during the General Assembly of States Parties at its 24th session. Despite the fact that the phenomenon of climate change has been the subject of international debate since the 1990s, it was only in 2006 that UNESCO officially recognised it as a threat to the world’s heritage, both cultural and natural, tangible and intangible.

In recent years, the scientific community has observed an exponential increase in the effects of climate change, which has exacerbated the well-known risks faced by archaeological heritage. This heritage has long been recognised as a “fragile and non-renewable heritage” [5,6].

The heritage disciplines of restoration and conservation are confronted with climatic phenomena that threaten the material consistency of heritage artefacts.

These include, for example, the risks due to physical, chemical, climatic, and environmental factors given by the context and the use and enjoyment of the assets themselves.

The impacts of climate change on cultural heritage encompass a range of issues related to the preservation and protection of heritage buildings, manufact, and artefacts. These include the need to preserve heritage building (including archaeological sites) from the effects of pollution, particularly facades, and changes in the indoor microclimates in museums and heritage buildings [7,8,9], which can promote damage to artefacts and documents. Additionally, there is a crucial responsibility to protect landscape assets and archaeological sites from the adverse affects of climate change [10].

In the climate and microclimate research field applied to heritage, the management and conservation of heritage sites and archaeological sites is complicated as they are particularly complex entities due to a number of factors, including their physical structure and state of preservation, as well as the interactions between artefacts and their surrounding environment. The protection of heritage sites and the well-being of visitors are both dependent on the ability to control the outdoor microclimate and ensure thermal comfort. The protection of a site is closely related to its use, which in this case, is to preserve heritage [11].

It is widely acknowledged that an open-air archaeological site acquires value and significance from the surrounding vegetation, which often marks it and renders it a unicum. The study of ruins and archaeological sites has a long history, including being research subjects of scholarly investigation from a range of perspectives and viewpoints particularly with regard to their conservation and valorisation. In this context, the interactions between ruins and vegetation will be subjected to close scrutiny, with particular attention paid to the potential hazards of vegetation component and its impact on the outdoor microclimate [11].

In accordance with our previous research, by the team of the DA University of Bologna, on the topic of archaeological site and outdoor microclimate [12,13], the research described in this study concerns the relationship between the outdoor thermal comfort of visitors and the role of vegetation. The objective is to ensure that visitors’ tours and activities in an archaeological site are sustainable and pleasant. In particular, our research focused on the following:

(a) The outdoor thermal comfort (OTC) of visitors, defined as the relationship between the thermal comfort perceived by people and the presence of trees and vegetation on site.

(b) The replacement and/or integration of vegetation in relation to its hazards and effects on the local microclimate.

From a conservation point of view, the spontaneous flora that settles in monumental areas, especially in archaeological sites, represents a significant direct and indirect driver of accelerated deterioration processes [14].

Vegetation management at archaeological sites not only affects site protection but also has a significant impact on tourists’ thermal comfort, and there are significant differences in the impact of different vegetation management strategies. The relationship between vegetation and archaeological sites is a result of the interaction between living (vegetation) and non-living matter (bricks, stones, etc.) that changes over time.

At least since 1970s, a wide scientific literature examining the risks associated with the uncontrolled presence of vegetation in archaeological areas has taken place. The physical processes that result in the micro–macro-decohesion of artefacts due to the action of organisms and microorganisms, as well as to the damage caused by the growth of ruderal plants in archaeological areas, particularly in relation to the characteristics of the root system, have been extensively documented. Similarly, the chemical alteration mechanisms induced by the metabolic processes produced by plant organisms present in these areas, which result in the production of organic and inorganic acids, CO₂, enzymes, alkalis, and pigments, are well documented [15,16,17]. Furthermore, studies have been conducted on the positive effects vegetation can have on built heritage. This includes the use of vegetation as a protective element of ancient city walls [18,19], of the management of roots as a risk and degradation factor [15,16], the presence of mould growth [20], as well as conservation factor management [21], risk indices, and biodeterioration risk [22,23,24,25,26].

In addition, the actual plant structure can contribute to promoting the conservation of archaeological elements by reducing solar radiation, wind, atmospheric precipitation and pollutants [27], or marine aerosol, which are significant factors in degradation [27]. In an archaeological site, vegetation is initially regarded as a component that requires management and care. However, it can also play a role in the conservation of the archaeological heritage itself [28]. This is evidenced by the case of Giacomo Boni, who proposed the use of lawns in the early 20th century as a means of protecting ancient surfaces. This approach has been the subject of recent investigation in the Anglo-Saxon context [29,30].

In this regard, in the studies of Maria Adele Signorini, which were conducted as early as 1995 [31], proposed a classification of plant species for the purpose of determining the risk for archaeological structures. This classification is based on a number of the biological category, determined by the duration of the life cycle and the habit of the plants, as well as its invasiveness and vigour, referring to their capacity for vegetative propagation and the type of growth and finally to the type of root system of the plant.

The presence of plants in an archaeological site can play a protective role against environmental factors that might otherwise contribute to the alteration and decay of ruins. Vegetation can help preserve archaeological elements by reducing solar radiation, wind, rainfall, pollution, and marine aerosols, all of which are important degradation factors [27]. Among the autotrophic biodeteriogens responsible for biological degradation, algae, mosses, and lichens are often mentioned. The impact of these organisms varies depending on the environment and the specific lithotype, leading to material losses on a micron scale and different timeframes. Therefore, the potential harm of each species needs to be assessed on a case-by-case basis, especially in light of surface damage caused by aggressive cleaning methods of stone surfaces (which can exceed the micron level) when not carefully executed. Some researchers, however, have long pointed out that lichens, in particular, can protect stone by reducing the intensity of water exchanges with the environment and, in the case of highly porous materials, offer some defence against weathering agents such as water, wind, marine aerosols, and certain pollutants [17]. The presence of grass carpets, made from carefully selected species, can also protect surfaces from the harmful effects of atmospheric pollutants or airborne particles. For example, lead concentrations on the leaves of trees in urban parks and gardens are ten times lower than on street trees, confirming the filtering effectiveness of grass cover compared to other mineral surfaces like roadways [24].

Additionally, grass cover can provide valuable protection against erosion and landslides due to the stabilising effect of root systems. Similarly, selected shrub species can be used for bio-consolidation of slopes, as was conducted by planting broom in a live piling system to support a landslide front at the archaeological site of the Pieve di San Giovanni in Galilea (FC) [32]. Ground vegetation can also aid in regulating soil water through evapotranspiration, the process by which water is transferred from the soil to the atmosphere in the form of vapour through plants. In the 1990s, at the archaeological park of Mohenjo-daro in Pakistan, the spontaneous growth of plants was proposed as a natural pump to control the water table in areas near the site [33].

In 2002, English Heritage published the results of research on using grass coverings to protect wall crests at Hailes Abbey [34]. This study was conducted by Oxford University in collaboration with Western Kentucky University. Laboratory tests measured the thermal protection capacity of the coverings in an environmental chamber, their resistance to penetration, and water retention. Two years after these results were presented, the Archaeological Superintendency of Rome decided to protect the tuff layers supporting the remains of the Domus Tiberiana on the western slope of the Palatine Hill using a covering of sedum acre—a hardy, perennial, evergreen plant. The goals were to retain moisture in the layer to prevent corrosion, create a protective layer without harming the geological structure, establish an easily maintainable surface, reduce water runoff typically found in standard mineral surfaces, and inhibit the growth of invasive plants. Moreover, this method was considered more respectful of the historical and aesthetic value of the ruins compared to conventional treatments for wall crests [35].

The usual archaeological restoration approach identifies the presence of vegetations as a degradation factor of the relic. The conservation of an archaeological site is ensured by the planning of maintenance and conservation activities, including those pertaining to the plant component of which the Vegetation Hazard Impact Index (VHI index) must be evaluated.

The Vegetation Hazard Impact Index (VHI index), which was studied and codified in Italy by M.A. Signorini [31], provides a summary of the potential hazards for architectural artefacts of a plant species. Each plant species (tree) is evaluated according to three parameters: (i) growth form; (ii) invasiveness and vigour; and (iii) characteristics of the root system.

The relationship between the presence of people and the archaeological site is linked to the duration of peoples’ visits. The majority of archaeological sites are located in an open setting, sometimes with some part partially or fully covered by canopies, more rarely inside a building.

Archaeological sites may be visited by the general public in an outdoor environment, provided that meteorological conditions are favourable. This typically occurs during the spring and summer seasons, and when the temperature is relatively cool. The presence of visitors depends on thermal comfort as a determining factor that depends on the outdoor microclimate and outdoor thermal comfort. The latter is expressed in terms of thermal stress with Physiological Equivalent Temperature (PET) [36,37,38]. Physiological Equivalent Temperature (PET) is a human biometeorological parameter that describes the thermal perception of an individual. It is defined as the air temperature at which, in a typical indoor setting (without wind and solar radiation), the heat budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed [39].

The evaluation of the microclimate model is carried out with Envimet software [40], which is able to analyse the performance of the vegetation [41] and the impact of different ground surfaces on thermal behaviour [42].

The scientific literature about microclimate is also of interest to the heritage sector, particularly in the context of individual case studies rather than as a specific research methodology. The case studies refer to gardens, e.g., House Jelinek in Trieste (Italy) [43] and Jiangna traditional village (China) [44]; archaeological parks [45]; urban parks [46]; and the case of study in Damasco, Syria [47], China [48], with a particular focus on the renowned prestigious case of Emperor Qin’s Mausoleum Museum [49,50] and Mogao Caves [51].

Archaeological restoration offers a range of tools to investigate the role of vegetation in archaeological sites, with the potential to enhance visitor thermal comfort and facilitate their use and exploitation. Indeed, the presence of vegetation, thanks to shade and evapotranspiration, allows for a reduction in the average radiant temperature and increases the relative humidity, improving the perceived thermal comfort. In other words, the presence of vegetation represents an opportunity for a positive interaction with the archaeological site experience.

The present study introduces a novel methodology for the management of vegetation in archaeological sites, which is directed towards professionals engaged in the conservative and those studying microclimatic comfort and outdoor thermal well-being. This methodology is based on the analysis of the Vegetation Hazard Index (VHI) of the site’s plant structure and its impact on thermal comfort.

The goal is to provide tools for selecting and integrating plant species to preserve archaeological remains while ensuring visitor comfort. The objective of this study is to present a methodology, illustrated by a case study, for the assessment of plant and vegetation systems in archaeological conservation and restoration, as well as in studies on microclimate in outdoor thermal comfort. This methodology is designed to facilitate the selection and integration of plant species in a manner that preserves archaeological remains while ensuring visitor comfort. The vegetation that surrounds archaeological artefacts is typically regarded as a potential risk to their preservation. The present research believes that plant species also can confer benefits and opportunities with regard to conservation, utilisation, and enhancement of the archaeological site (Figure 1).

In particular, it is believed that vegetation forms part of the architecture itself, having been modified over the course of years or centuries. The role of vegetation in ensuring thermal comfort and, therefore, the use of an archaeological relic is demonstrated by microclimatic assessments.

2. Case Study

The case study is located in the central Italian region of Marche, east coast, specifically in the municipality of Falerone (GIS 43.1021, 13.5000) (Figure 2). The Roman Theatre of Piane di Falerone (Figure 3) was built in the first century A.D. and is located within the Archaeological Park of Falerio Picenus. Piane di Falerone is a small hamlet of Roman origin situated within the Municipality of Falerone, located in central Italy. It is classified by the CFA Koppen-Geiger classification [52,53]. The surrounding area is predominantly farming and agricultural.

The climate of Falerone is characterised by warm, clear summers and very cold and partly cloudy winters. The temperature typically ranges from 2 °C to 28 °C throughout the year, with rare exceptions of below −2 °C or above 33 °C. The cool season lasts 4 months, from 19th November to 13th March, with an average daily maximum temperature of less than 12 °C. The coldest month of the year in Falerone is January, with an average high temperature of 2 °C and a minimum of 8 °C. The hot season lasts 3 months, from 11th June to 9th September, with a maximum daily temperature over 24 °C. The month with the highest average temperature in Falerone is July, with an average high temperature of 28 °C and a low of 18 °C (weather data sourced from the website Weatherspark [54]).

The presence of tourists, visitors, or spectators (the theatre is also used for the staging of theatrical and musical plays) during the summer season, from June to September, depends on climatic conditions, in particular, the local microclimatic conditions at the site. These conditions are dependent on both the general climatic context, including factors such as air temperature and relative humidity, and on the architectural characteristics of the archaeological site. The reflectance and albedo of the archaeological materials, as well as the evapotranspiration on the surrounding vegetation and trees, contribute to the overall microclimate.

The Roman Theatre of Piane di Falerone was built in the first century AD for the purpose of hosting performances for the inhabitants of the Roman city. Subsequently, in the second century AD, it was abandoned and subsequently used as a quarry from which to extract valuable materials. The structure has undergone many restorations over time, particularly in the 1970s.

Presently, the majority of “cavea” and entrance “vomitoria” (the corridors leading to the bleachers/seating area) remain discernible and visible (Figure 1). The “front scaenae”, or front stage comprising both the stage and backstage, was entirely destroyed; only the bases of the walls that constituted it remain. The theatre is situated in a location isolated from the surrounding road and the residential areas. It is surrounded by dense vegetation comprising a variety of plant species, including a centuries-old oak tree that has grown leaning on the structure of the theatre and artefact.

3. Methodology

The research methodology is to undertake a comparative analysis of two indexes.

(a) The VHI index (in Italian language, Indice di Pericolosità) [31];

(b) The outdoor thermal comfort (OTC) [12,37,38,55].

The VHI index refers to the protection of the archaeological asset and considers vegetation and trees as a potential source of risk and damage to the archaeological site. In contrast, the evaluation of the outdoor microclimate, such as air temperature, relative humidity etc., and thermal comfort indices enable the assessment of the thermal comfort conditions for the use of the artefact. The approach adopted considers both aspects in order to provide guidance to those who will be responsible for decision-making, including archaeologists, restorers, site managers, cultural enhancement projects leaders, administrators, managers, promoters, and others. It offers insights into the preservation of the asset, including recommendations on the retention or removal of plant elements, with a focus on their impact on thermal comfort (Figure 4).

The research method is divided into three main phases.

1—The modelling and simulation of the outdoor microclimate of the study area in the state-of-the-art archaeological site.

(a).

Interpretation of outdoor data: Physiological Equivalent Temperature—PET [56], air temperature (T_a, in °C), relative humidity (RH, in %), and mean radiant temperature (T_mrt, °C).

(b).

Evaluation of the plant species that can be eliminated and cannot be eliminated for comfort/microclimate.

2—The identification of plant species to be eliminated is conducted using the VHI index. The VHI index is then elevated based on a number of factors by tabulated values, allergenicity index, height, and so forth, as well as the definition of intervention scenarios.

3—Microclimatic modelling of intervention scenarios.

(c).

A scenario in which some plant species, with high VHI index (IP > 7), are eliminated.

(d).

A scenario in which sedum (a genus of succulent plants belonging to the Crassulaceae family) is added to damaged portions of the bleachers.

4—Evaluation of the VHI index and OTC, with indications for the restoration, management, and enhancement project.

3.1. Outdoor Microclimate Modelling and Simulation

The microclimatic evaluations of the Roman Theatre of Falerone were modelled and simulated with the ENVI-met v.5.6.1 software [40] utilising 3D modelling. The area selected for the creation of the 3D model has dimensions of 100 m × 100 m, with a grid of 100 × 100 cells with a size of 1m x 1m (please refer to Figure 5 and Figure 6). In consideration of the actual elevation of theatre, which is approximately 6 m, we adopt a number of 6 cells of 1 m × 1 m size. This is because the software Envimet does not allow us to simulate 0.5 m for the cells. The material adopted for the cells in the amphitheatre model is wall-burned brick (reflection 0.3 emissivity ε = 0.93), which corresponds to the thermophysical characteristics of the materials and real terracotta of the artefact. The stage is modelled with the material Wood Burned (albedo a = 0.35 emissivity ε = 0.80), the grass (grass burned (albedo a= 0.2 emissivity ε = 0.97)) that surrounds the artefact, and the portion of the wall remaining of the front scenae is always wall-burned brick. The species of the trees that are actually present on site and included in the Envimet database if they were present were entered into the model, otherwise, the data regarding the plant species absent in manual mode (Acer campestre, Cupressus sempervires, Fraxinus ornus, Platanus occidentalis, Pinus pinea, Quercus petraea, Quercus pubescens, Magnolia grandiflora, Thuja, Vitis vinifera) were entered.

Data collection on the archaeological area of Falerone was provided by field measurements and surveys. In addition, climate data were obtained from the Marche Regional Environmental Protection Agency. Scenarios based on previous research experience to the same case studies on outdoor microclimate were also consulted.

The simulations refer to 4 July 2023, which was the hottest day of the year in 2023. The simulation started at 6:00 a.m. and continued for 24 h of the calculation. The weather data were obtained from the nearby data stations of the Civil Protection Service of the Marche Region of Servigliano (FM) and Montemonaco (FM).

The duration of the simulation required approximately 50 h of the calculation, to which we added the simulation of the PET and Universal Thermal Comfort Index (UTCI) thermal comfort indices [57,58,59] with the software “BIO-met” (https://envi-met.com/).

The results were represented graphically using Outdoor Microclimate Maps (OMM [56]) referring to the variables air temperature (°C), relative humidity (%), mean radiant temperature (°C), and PET (°C).

The PET results are of particular significance, as they provide insight into the thermal comfort and thermal stress of people. In this instance, we hypothesised to evaluate the thermal comfort of an adult male (height 1.75 m weigh 80 kg) with summer clothing (0.4 clo). In this way, the World Meteorological Organization (WMO) will make it possible to verify how much and where the thermal stress is greater and the role of trees.

The current state (Scenario 0) was modelled considering the materials, vegetation, and trees described above.

3.2. Identification of Plant Species to Be Eliminated with High VHI Index

The study and evaluation of the plant species present in the area and their relationship with the artefacts and archaeological remains is a fundamental prerequisite for the subsequent phase of analysis, which includes the maintenance, aesthetic, economic, and ecological management choices of these sites. In order to preserve and protect the monuments, which represent a primary objective of these projects, it is necessary to intervene on the plant structure to contain, maintain, and manage it in the most optimal manner possible.

Once the phase of study and cataloguing of the plant structure around the Roman theatre was completed, the calculation of the VHI index was carried out. This index expresses an intrinsic characteristic of the species and is calculated independently of its position in relation to archaeological emergence.

First, we took into account additional information about the allergenic potential of the vegetation species present. The allergenic risk of each species was therefore investigated in order to ascertain which species might cause the greatest irritation or annoyance to a hypothetical user of the area. This resulted in the creation of a Dangerousness Index, which ranges from 0 to 3, then consequently evaluating the possible removal of the species deemed to pose the greatest risk. The intrinsic hazard of each species increases with the following:

• The biological form of a plant: The final size of the plant after growth and habit (herbaceous, shrubby, liana, arboreal plants). The species included in the model starting from Envimet’s “Tree” database, which includes the data canopy tree, leaf type and leaf area, foliage shortwave albedo, foliage shortwave transmittance, and emissivity of leaves.
• The invasiveness and vigour, referring to the mode of growth (plants with limited or invasive growth) and tendency to spread by vegetative propagation (the ability of many plants to regenerate missing parts or entire individuals starting from portions).
• The root system, which can be more or less extensive, deep and/or intrusive. The species were included in the model from Envimet’s “Tree” database, which includes Root Zone, Root diameter, and Root depth data.

The VHI index for each tree in the case study is calculated in accordance with tabulated values (

Table 1. VHI index by type of vegetation present in archaeological site.

Biological Shape	Invasive and Strength	Root	Dangerousness Index
0—Annual tree	0.0 Crawling and not crawling	0.0.0 without taproot	0
		0.0.1 slightly taproot	1
		0.0.2 robust taproot	2
	0.1 Reptants	0.1.0 without taproot	1
		0.1.1 slightly taproot	2
		0.1.2 robust taproot	3
	0.2 A Vigorous growth	0.2.0 without taproot	2
		0.2.1 slightly taproot	3
		0.2.2 robust taproot	4
1—Biennial tree	1.0 Crawling and not crawling	1.0.0 without taproot	1
		1.0.1 slightly taproot	2
		1.0.2 robust taproot	3
2—Never-ending Weed	2.0 Moss and lichen	2.0.0 without taproot	2
	2.1 Not intrusive grass	2.1.0 without taproot	3
		2.1.1 slightly taproot	4
		2.1.2 robust taproot	5
	2.2 Intrusive grass or vigorous grass	2.2.0 without taproot	4
		2.2.1 slightly taproot	5
		2.2.2 robust taproot	6
3–4—Shrub	3.0 Suffruticose	3.0.0 not intrusive	3
		3.0.1 intrusive	4
		3.0.2 very intrusive	5
	4.0 Small shrub or not suckering shrub	4.0.0 not intrusive	4
		4.0.1 intrusive	5
		4.0.2 very intrusive	6
	4.1 Suckering shrub	4.1.0 not intrusive	5
		4.1.1 intrusive	6
		4.1.2 very intrusive	7
	4.2 Shrub with rooting suckers	4.2.0 not intrusive	6
		4.2.1 intrusive	7
		4.2.2 very intrusive	8
5—Liana	5.0 Not Sucker	5.0.0 not intrusive	5
		5.0.1 intrusive	6
		5.0.2 very intrusive	7
	5.1 Sucker	5.1.0 not intrusive	6
		5.1.1 intrusive	7
		5.1.2 very intrusive	8
6—Tree	6.0 Small tree or not sucker	6.0.0 not intrusive	6
		6.0.1 intrusive	7
		6.0.2 very intrusive	8
	6.1 With stump sprouts	6.1.0 not intrusive	7
		6.1.1 intrusive	8
		6.1.2 very intrusive	9
	6.2 Also radical sucker	6.2.0 not intrusive	8
		6.2.1 intrusive	9
		6.2.2 very intrusive	10

) derived from Maria Adele Signorini’s research [31], wherein a numerical value is associated with each of the categories indicated. The VHI index range is measured on a scale of 0 to 10.

• The VHI index values from VHI = 0 to VHI = 3 indicate plants that can be considered not particularly hazardous to buildings and are therefore generally negligible in vegetation control operations.
• From VHI = 4 to VHI = 6, the plants are moderately dangerous, and the necessity of intervention must be evaluated on a case-by-case basis. Factors such as abundance and any aesthetic and/or cultural value may be taken into account in this evaluation.
• From VHI = 7 to VHI = 10, the plants are highly dangerous, and intervention is generally necessary.

The VHI index plays a role in determining the final decision regarding whether or not to intervene and, if so, in what manner for a specific plant. Consequently, the designer’s subjective assessment also carries significant weight. This assessment may include the removal of a plant due mechanical or chemical damage to the artefact incurred during weeding operations.

Explanation of scenarios

The intervention scenarios were determined on the basis of the VHI index.

In the first scenario (Scenario 1), the decision was taken to eliminate vegetation and trees with a VHI greater than 7. The resulting outdoor microclimate and outdoor thermal comfort were then evaluated.

In the second scenario (Scenario 2), the addition of supplementary vegetation in comparison to Scenario 0 was considered, specifically the addition of sedum.

This approach allows for an examination of how outdoor thermal comfort is influenced by the presence of different plant species, including those that may pose a risk to the artefact, while also considering the integration of species aligned with the principles of archaeological conservation. (See Figure 7).

3.3. Microclimatic Modelling of Intervention Scenarios

Following the selection of intervention scenarios for the simulations, the Envimet model was modified, the simulations were launched, and the OMMs of the scenarios were extrapolated. Two further simulations were therefore conducted. The second eliminated plant species with high I.P., while the third involved the insertion of sedum in the damaged section of the bleachers so that these three scenarios can be understood and associated with the effects on thermal comfort.

3.4. Evaluation

The final phase of the methodology consists of the analysis and interpretation of the results obtained from each scenario, with the objective of proving valuable insights for the restoration, management, and enhancement project of the archaeological area. It is evident that the methodology allows for the reiteration of phases 2 and 3 and related simulations should the results not meet the requisite standards with respect to the VHI index or the design needs or outdoor thermal comfort of visitors. The analyses and design choices do not refer only to the conservation of the building (dangerousness risk) but also to its use (comfort).

4. Results

The results of the simulations, in particular at the times of 10:00, 15:00, and 21:00, are presented below. This is because the theatre is used for both daily visits and evening shows. The variables reported are the most significant for the objective of our research, namely air temperature and PET, measured in °C.

The results of the simulations were compared: on the left, we find Scenario 0, i.e., the current state with all the trees present; in the middle is Scenario 1, i.e., the state in which the trees with a high VHI index were eliminated; and on the right is Scenario 3, the situation with the insertion of the sedum. Indicated at the top is the cutting plane and the time and day of the simulation, while on the right is the respective thermal variable and the scale of the temperature bands.

The results of the simulations conducted at 10:00, 15:00, and 21:00 are presented below. This is due to the fact that the theatre is utilised for both daily visits and evening shows. The thermal variables reported are the most significant for the context of our research. During the morning, at 10:00 a.m. (Figure 8), the removal of trees (Scenario 1) leads to a worsening of thermal comfort (PET). Furthermore, the mean radiant temperature (Tmr) worsens considerably due to the elimination of the trees that were a source of shading in Scenario 0; this confirms the role of the vegetation in the archaeological site because it plays a double role: shading and evapotranspiration. In Scenario 0, the areas where the TMR is below 25 °C are much larger and more frequent than in Scenario 1 where they decrease significantly.

Similarly, the same difference can be seen for the air temperature (Ta) after the elimination of the trees the band around the theatre ranging from 27 to 27.50 °C extends further. In Scenario 2, compared with Scenario 0, the addition of sedum does not result in an improvement in outdoor thermal comfort in the area surrounding the theatre; however, it does significantly improvement thermal comfort for those sitting on the terraces, as we can see from Figure 9.

A comparison of the simulations conducted in the morning and early afternoon (Figure 8, Figure 9 and Figure 10) reveals that the decision to follow only the criteria related to tree safety, specifically the elimination of trees with a VHI index greater than 7, results in a deterioration of the microclimate. This is evident in the decline of the mean radiant temperature (due to the absence of shade of the foliage on the steps of the theatre) and the reduction in the PET values. The deterioration of outdoor comfort perception increases a lower usability of the archaeological site by tourists and visitors.

5. Discussion

The results demonstrate the efficacy of the methodology. It is able to provide the tools to decide which trees and plant components to remove or add in the case of intervention in an archaeological site. The sole criterion related to the potential danger of the trees and the conservation of the site may result in an unacceptable alteration of the microclimate. In other words, the reduction in vegetation may result in an increase in thermal discomfort, which could subsequently lead to a decline in visitor numbers to the archaeological site. Therefore, although until now the practices of “sterilization” of archaeological sites have been historically privileged, it has become evident that a crucial initial step is to identify and categorise the species and associations that colonise an archaeological site. This is essential in order to evaluate the potential interferences (positive or negative) on the conservation and enhancement processes. The VHI index is a new index based on the vegetation dangerous index (Table 1), and outdoor thermal comfort and the VHI index should be applied in any environments and archaeological sites.

The study therefore highlights that, based on this knowledge, the manager of these locations can make scientifically informed decisions regarding the removal of specific trees and plant components. However, this approach is not enough to guarantee the protection of a site or to facilitate its utilisation. Instead, it may potentially lead to an unacceptable alteration of the microclimate. In other words, the reduction in vegetation in order to mitigate the risk of the archaeological site, may, in fact, increase thermal discomfort and, therefore, limit its use by favouring the processes of abandonment and degradation.

In an empirical way, it is feasible to express the role of the plant component in relation to the risk posed by the VHI index and thermal discomfort, as illustrated in Figure 11. As the number of trees should be determined in accordance with hazard index rise, so too is the thermal discomfort while the number of visitors decreases.

The purpose of the restoration of archaeological sites, however, is not only a matter of conservation, it is also regarded as a means of ensuring the enjoyment of people in these places that would otherwise remain forgotten. Therefore, it is essential to consider the well-being of visitors or spectators with regard to the theatre in particular. It is in our opinion that the VHI index described above cannot be taken as the only basis for determining the fate of trees and the plant components (lawns, shrubs, sedums, etc.). Rather, it must be corrected in consideration of the outdoor thermal comfort of visitors. Usually, the approach to the relationship between vegetation and archaeological area suggests that all vegetation should be removed because it can cause degradation. Our criterion for deciding whether to retain or remove vegetation is based on the results of the PET and the Dangerousness Index risk (Table 1). It is a fact that the long term impact of vegetation management on the surrounding environment needs to consider climate change, the ecosystem, and the surrounding environment. In our research, we simply propose to consider thermal comfort and the VHI index as part of vegetation management. For vegetation with high VHI values, it is necessary to consider gradually removing and introducing drought-resistant, non-invasive vegetation to achieve a balance between site protection and tourist comfort.

The recent news about the decision to close the Acropolis to protect tourists from blistering heat [60] highlights the necessity to consider outdoor thermal comfort and the well-being of visitors in archaeological areas in both research on the built environment and the discourse surrounding the restoration and conservation interventions of archaeological sites. Like static evaluations, botanical and material evaluations must be integrated into the conditions of the outdoor microclimate and the thermal comfort of visitors.

The Limits of the Research

The ENVI-met software is typically employed to model larger areas than those under consideration here, as well as buildings. Consequently, the model of our theatre was approximated in terms of both materials and dimensions. In addition, each terrace of the stalls was considered 1 m high instead of 0.5 m (the real height) as the software does not permit the input of quantities lower than 1 m. In any case, the overall height of the Roman theatre (6 m) was maintained. The trees were inserted in the correct position and with the correct plant species; however, it is evident that the geometric characteristics of the trees present in the DB Envimet do not correspond to the actual condition of the existing trees. It is assumed that these approximations due to the model do not affect the interpretation of the WMO and the final evaluations provided to the designer. Method and VHI index can be applied in other countries and under other climatic conditions.

In the same way, some strategies as only mentioned, and we have not mentioned difficulties in financing, political support, technological means, etc., as these difficulties are actually out of the topic of this study. Moreover, this is a first step of our research where, at this stage, we are proposing a new method and a new VHI index. We hope to continue along this path to refine our choices.

6. Conclusions

In conclusion, the proposed methodology facilitates the identification of a useful path for determining the optimal criteria for the conservation or removal of vegetation from archaeological sites. Future research can explore guidelines for vegetation management, such as the long-term monitoring of vegetation management, or how new technologies, such as remote sensing and artificial intelligence, can be used to optimise vegetation management and the protection of archaeological sites.

The results presented above highlight the value to continue research in this area, in relation to the following future studies:

• The relationship between the presence of visitors and the microclimatic conditions of the archaeological site.
• The role of vegetation in defining microclimatic conditions in archaeological sites.
• Evaluations on how to balance the beneficial effect of trees with a high VHI index with other vegetation that guarantees the same OTC but is more suitable for archaeological sites (e.g., adopt 3 m2 of sedum instead of pine).
• The relationship between conservation needs and the variables given by the microclimatic context.
• Define a specific risk index for visitors to archaeological sites, based not only on climate data.