Biodeterioration Vs Bioprotection: Analysis of micro and macroflora

Abstract

Within the science of restoration and conservation monuments, the colonization of stoneworks by the organisms has been studied with great attention in the 20th century. It is a complex process that involve several time-spatial elements, due to the interrelationship among organisms, substrate and atmosphere. Each monument or artwork must be considered as a habitat where different parts can be considered as microniches and several type of organisms (lichens, fungi, algae, ferns, mosses, higher plants) can find good conditions for development. For a very long time the presence of organisms on stone monuments, in outdoor environment, has been associated to the substrate biodeterioration. However, in the last decades, a new idea regarding the possible positive role performed by the organisms on stonework is coming up. Indeed, several authors state that in some cases the biodeterioration processes are lower than the physical-chemical agents (as weathering process), so they sustain that organisms, in such conditions must not be removed from the stone surfaces because they bioprotect the surface. There is a need for more crossover research and for existing knowledge in a multidisciplinary context. For all these reasons, a biological approach is important to understand the ecological factors and the interrelations that control the colonization, the development and the ecological succession of the organisms on stonework. Hence, my research concerns the analysis of biodeterioration ability of two different situations, where the environmental agents and the specific species have a significant role in the international debate concerning on biodeterioration versus bioprotection processes. Then, the focus of my PhD project is to understand the role in deteriorating or protecting the underlying substrate and some ecological aspects of microflora in tropical environment, and of macroflora, Hedera helix L., as greening species. The colonizer microflora of Angkor temples, UNESCO World Heritage Site, is an interesting case study because the tropical climate acts deeply with monsoon rain and strong sunlight in the dry season, leading to wet-dry cycle that form break and cracks in the sandstone. Then it was interesting to analyse the relation of the strong weathering with colonising organisms’ activities, in order to define the degree of biodeterioration. Subsequently, it is well known, even if not entirely accepted, the deteriorating action of higher plant roots on monuments, but as regards climbing plants, the studies are still few. In particular, the ivy is considered one of the best for green facades as evergreen and in relation to its microclimate effects on the surface and its absorption of airborne particulates. In the same time, its biodeteriogen activity was recognized. Disagreements on the effects of ivy may be due to growth conditions in different geographical areas, but could also be a result of different methodologies adopted in evaluation of plant interaction with surfaces. Therefore, Hedera helix L., influenced by the Mediterranean climate, grows spontaneously in the archaeological sites. It becomes abundant and vigorous in the ground near or inside the crack of stone and climbs up covering totally the ruins. In this regard, it was interesting to analyse the degree of biodeterioration induced by the vigorous Hedera helix L. growth, in a favourable climate. Therefore, at the first time I considered the microflora growing on the Angkor temples, (Cambodia) and in the second time the Hedera helix growing in archaeological sites of Rome (case study Villa of Massentius). In the 2001, the Authority for the Protection and Management of Angkor and region of Siem Reap (APSARA National Authority) and the National Research Institute for Cultural Properties, Tokyo (NRICPT), launched a joint research project for the conservation of Angkor Temple. From a conservation viewpoint, they set a search to find the best micro-climatic conditions for the maintenance of temples, because the forest surrounding area protects the ruins by atmospheric weathering, but contemporary creates a favourable habitat for the biological growth. In this way, the biological patterns, observed on four temples (Ta Prhom, Ta Nei, Bayon and Ta Keo) exposed to different microclimatic conditions was investigated. At first, we examined the biological patterns observed on two temples (Ta Nei and Ta Keo) exposed to different microclimatic conditions, in order to assess the damage caused by the communities present on the stone. We analysed the penetration (depth and spread) into the stone, and the degree of decohesion of seven communities (green algae, cyanobacteria, lichens and mosses). The microscopic analyses was performed by SEM and optical microscope observation and measurements of fracture samples and polished cross section stained by PAS and methylene blue. These highlighted a clear interaction between organism and stone, displaying a trend of increasing harmfulness from the community of the green algae (Trentepohlia) up to the moss communities. All the lichen communities showed biodeterioration abilities: the Pyxine community seems more aggressive than the Lepraria and Cryptothecia communities and more also than the cyanobacterial communities. Then, the positive effects of the lichen cover in reducing dangerous evaporation processes cannot outweigh the negative effects of their hyphal penetration. Light forest cover seems beneficial for the conservation of the Angkor monuments since it reduces evaporation processes, but further studies should be carried out to find an optimal balance between contrasting factors. Hence, In relation to the increasing trend of harmfulness found among the investigated communities, the changing in response to different environmental conditions was analysed. Indeed, the aim of this study is to quantify their frequency and ecological characteristics according to a forest canopy gradient. The descriptive and multivariate statistical analysis applied to data collected from the four temples (Ta Prohm, Ta Nei, Bayon and Ta Keo) in the study identifies various biological communities along with a temple-specific ecological succession. The initial pioneer community is primarily composed of a reddish biofilm of the green alga Trentepohlia sp., and it occurs in xeric and shady environmental conditions, becoming dominant in forested areas. Cyanobacteria biofilm, consisting of species belonging to the genera Scytonema and Gloeocapsa, sometimes in combination with the lichen Endocarpon sp., prevails in xeric and sunny conditions. With the progressive increase of the availability of edaphic water, typical of forested areas, various lichen communities are able to establish themselves (dominated by Lepraria, Pyxine coralligera and Cryptothecia subnidulans respectively), followed by moss and higher plant communities. Understanding these relationships appears to be a very useful way of identifying the best microclimatic conditions for stone conservation. Consequently, we are providing a new and original perspective to understand the general organization of these successions and their structural variability at the local scale. We are integrating the results of expert opinion assessment with those obtained using various statistical approaches, in order to create a synthetic ecological scheme of the ecological successions of biodeterioration patterns recognized. At the same time, during a recent archaeological survey, an unusual blackish patina, covering both the exterior and interior walls of the Neang Khmau temple (of Archaeological park), has been observed. The black alteration of the stone surface is a widespread issue and their nature is a debate extensively discussed in conservation science. This unusual blackish patina is present over all the four facades of the temple, where it covers a relevant part of the vertical surfaces protected from the rainfall; it is also homogeneously distributed on the interior of the monument. The temple was built by using laterite bricks, a natural material rich in aluminium, iron and manganese, with subordinate titanium and silica, formed by weathering of pre-existing rocks in hot and wet tropical areas. In this work, we have investigated the physical-chemical origin of this unusual patina by identifying its mineralogical and chemical composition. Raman and FTIR spectroscopies, SEM-EDS, micro-XRF and XRDP have been applied to characterize the alteration products. Analyses have been performed both on single fragments and on polished rock sections of fragments collected from all the four temple facades, in order to check for any variation due to the different exposition, and from an area in the temple interior. Optical observations of the polished sections have evidenced irregularities in the thickness of the patina. Raman and FTIR spectra revealed that the black layers are mainly composed of manganese minerals (hollandite, romanechite, and manganite). This peculiar alteration pattern is interpreted as due to the tropical climatic conditions of Cambodia. Indeed the variation of wet and dry seasons favors the migration of iron and manganese ions and their successive immobilization in regions not directly subject to leaching. Regards the macroflora, as starting point the hypothesis that wall plants species come mainly from rocky habitats, taking into account that ancient walls show similar characteristics to rocks, was analysed. In this study, we supported the hypothesis that wall plant species come mainly from rocky habitats, taking into account that ancient walls show similar characteristics to rocks. The occurrence of wall species in natural habitats was investigated. The main literature concerning wall and natural vegetation of central-southern Italy was viewed. Vegetation synoptic tables were analysed through statistical procedures for comparing occurrence and behaviour of wall species in ruderal habitats as archaeological sites and natural ones.The study pointed out that wall species show similar ecological and coenological features in both habitats. This confirms that the main natural habitat from which wall species come from are rocks, but secondarily also ephemeral Mediterranean meadows and garrigues. Particularly, some wall species belonging to Parietarietea judaicae class are participating in the formation of natural rocky communities of Asplenietea trichomanis. Species occurring on emerging ruins form ephemeral meadows belonging to Helianthemetea guttati, also found in natural Mediterranean environment. On wide tops of ruins, where community dynamism is higher, some perennial herbaceous and shrubby species participate in the formation of impoverished communities matching to natural aspects of Mediterranean maquis. The knowledge of the natural habitat of wall species can be useful for a better management of archaeological sites. In relation to this general work, the specific case of H. helix growing in the Roman archaeological sites is under analysis in order to establish the ecological pattern of this species. At the end, an evaluation of the aggressiveness of ivy on stone monuments, in favourable growth conditions (study case of Villa of Massentius), was made. Additionally, a quantitative method to measure its dangerousness was provided. We studied the ivy colonization effects within the archaeological site of Villa of Massentius. We applied a quantitative method derived from Monument Mapping Method by Fitzner et al. 1997, to evaluate deterioration due to ivy colonization on vertical surfaces. Instead, the capping ivy effects was investigated through a multi-parameter evaluation. We recognized three different damage categories: the superficial loss of surface (W), ivy residues on wall after the removal event (I), and the loss of compact stone fragments (P/O). Mean W corresponded to 58% and P/O to 14%. The total damage rate was positively correlated to the amount of ivy cover and to favourable shady conditions. For the capping ivy, we calculated an average loss of 4.34 cm3/m2 and a high level of soil deposit, which led to a secondary ivy colonization process. This paper proposes a contribution to the identification and quantification of ivy deteriogen activity. The application of the proposed method also provides a useful tool for planning restoration activity. In this way, analysing some ecological aspects and the biodeteriogens activity of these two different situations (microflora in tropical environment and H. helix in favourable environmental growth condition), several aspects of the organisms’ role on the stone monuments are clarified.