Assessing the Impact of Lichens on Saint Simeon Church, Paşabağ Valley (Cappadocia, Turkey): Potential Damaging Effects versus Protection from Rainfall and Winds

Abstract

The impact of lichens on the conservation of monuments, such as the World Heritage Site (WHS) of Cappadocian churches, presents a multifaceted challenge for conservators. Previous studies have shown that lichens can both induce deterioration processes of stone through their penetration into the substrate and chemical interactions as well as provide bioprotection, forming encrustations including calcium oxalate layers, which help mitigating the effects of weathering, reducing water penetration and eolian erosion. Evaluating the impact of lichens requires a comprehensive understanding of various factors, which include the type of rock substrate, the colonizing lichen species, the monument architecture, and the prevailing physic-chemical weathering processes. This study aims to provide a comprehensive analysis of the impact of lichen colonization on Saint Simeon Church in the Paşabağı Valley (Turkey) with a multidisciplinary approach to investigate the interplay between lichens, microclimatic conditions, and the degradation of stone. Specifically, this study examines the influence of wind-driven rain (WDR) occurrences on lichen distribution and stone weathering to develop comprehensive conservation strategies. The results confirmed the previous observations and showed a prevalence of the protective role of lichens over their deterioration. The northwest side of the church, despite being heavily impacted by environmental factors such as WDR and freezing–thawing cycles, showed reduced deterioration due to extensive lichen coverage. In contrast, the northeast side, with lower lichen colonization, demonstrated more severe deterioration. These findings suggest that integrating the protective aspects of lichen colonization into conservation strategies can enhance their preservation.

1. Introduction

The assessment of lichen biodeterioration compared with bioprotection is complex and depends on various factors, such as the specific lichen species, the type of rock, the architecture of the stone monument, surfaces orientation, and the microclimatic conditions [1,2,3]. Endolithic lichens (e.g., Verrucaria nigrescens, Petractis clausa, Strigula endolithea) [2], in particular are known to significantly impact the weathering process of rocks, making them undesirable in the context of monument protection. Endolithic lichens grow within a rock’s pores and cracks, causing both physical and chemical weathering that can lead to the substantial deterioration of stone monuments [1,2]. Managing their growth involves several challenges because endolithic lichens are difficult to detect. Despite the evident induced deterioration processes, lichens can also offer bioprotection to stone surfaces by retaining moisture, enhancing waterproofing, and reducing thermal stress and erosion [2,4,5,6,7,8]. Properly identifying lichen species and understanding the ecological factors that promote their growth makes it possible to evaluate their impact on stone monuments and assess their bioprotective effects [2,8].

The unique Cappadocia monuments, known as “fairy chimneys”, in Göreme National Park are located in Central Anatolia, Turkey, and belong to the World Heritage Sites (WHS) of UNESCO; they consist mainly of rock-hewn churches with significant wall paintings, attracting tourists from around the world. Unfortunately, these monuments are undergoing significant weathering processes due to the effects of shrinkage caused by temperature variations between night and day and during the seasons, as well as the various effects of driving rain and water penetration and also wind erosion, which lead to the wearing away of the tuff substrate [9,10,11,12]. Several studies have been conducted to examine the physical and chemical weathering rates of these rock monuments to address future conservation projects, focusing on in situ and in lab tests, such as wetting–drying and freezing–thawing, static rock, and slake durability [9,10,11,12,13,14,15].

Moreover, the biodeterioration aspect plays an important role, since these monuments are highly colonized by different epilithic and endolithic species [16,17]. Cyanobacteria, meristematic fungi, and mosses have been detected, but the rock surfaces appear mainly covered by lichens, which varies depending on the monument surface exposure [17]. Their occurrence could significantly affect the monument conservation, since lichens can contribute to both physical and chemical weathering: their hyphae can penetrate rock surfaces, causing mechanical disintegration, while their metabolic activities can produce acids that chemically alter the rock [1,2,18,19,20,21,22,23,24]. However, their impact on the conservation of the Cappadocia monument is not clear or easy to assess. Previous studies have emphasized enhanced deterioration processes due to their penetration in the tuff substrate, but on the other hand, some studies have also indicated that lichen-covered tuff reduces water penetration compared with non-colonized surfaces [2,16]. Additionally, the production of calcium oxalate by certain lichen species creates hard and insoluble encrustations on stone surfaces, useful for reducing the action of further weathering agents [17]. Evaluating the balance between lichen-induced biodeterioration and bioprotection on the “fairy chimneys” involves understanding the interplay of various factors, considering the type of rock, specific lichen species, monument architecture, microclimatic conditions, and the weathering processes on the non-colonized stone. Among the weathering process, moisture is a significant agent responsible for the deterioration of stone monuments due to the activation of various physical, chemical, and biological processes, including freeze–thaw damage, leaching, efflorescence, and biological deterioration [1,2,3]. The amount of water deposited on above-grade building envelopes by driving rain is generally larger than any other source (e.g., condensation, rising dump), in almost all cases [25]. Such a phenomenon also heavily influences biological colonization [25,26,27,28] and needs to be evaluated when considering the trends of climatic changes [29]. Furthermore, despite the importance of driving rain in both biodeterioration and chemical/physical deterioration processes, there is a lack of quantitative data relating to the magnitude, duration, and frequency of rain deposition in relation to wall exposures.

Within a wider project of international cooperation for the conservation of the WHS of Cappadocia, this study aims to provide a comprehensive analysis of the impact of lichen colonization on St. Simeon Church, situated within the Paşabağı Valley, Turkey. The church, within the fairy chimney formations, stands as a testament to the unique monastic traditions of Cappadocia [30]. The integration of natural rock formations with religious architecture, the significant iconography of the apse, and the historical importance of St. Simeon highlight the site’s cultural and religious value. A multidisciplinary approach to investigate the intricate interplay between lichens, microclimatic conditions, and the degradation of stone monuments has been adopted. Our research integrated field surveys, laboratory analysis, ecological studies, and long-term weather data to provide a comprehensive understanding of these interactions. Specifically, we examined the frequency of wind-driven rain occurrences and correlated them with wall orientations to understand their influence on lichen distribution and stone weathering [28]. By combining findings from studies on physical and chemical weathering with those on biodeterioration, we can plan a comprehensive conservation strategy.

2. Materials and Methods

2.1. Site Description: Historical, Geological, and General Climatic Features

Cappadocia, a UNESCO World Heritage Site since 1985, is listed under the name “Göreme National Park and Rock Sites of Cappadocia”. This area is renowned for its unique geological, historical, and cultural significance, reflecting a rich interplay between natural and human-made elements. Early settlers took advantage of the soft tufa rock to create dwellings and settlements. Initially, they focused on the steep hillsides of the valleys, but as space became scarce, they began to hollow out the insides of the rock formations. This lifestyle, characterized by living in rock-cut dwellings, persisted for centuries due to the insulating properties of the rock, which kept the interiors warm in winter and cool in summer [30]. In the 4th century AD, Cappadocia emerged as a significant religious center for hermits and monastic communities. This led to the creation of numerous rock-cut churches and monasteries. The Göreme Open Air Museum is a prime example of these settlements. The St. Simeon Church is situated within the unique “fairy chimney” geological formations of the Paşabağ Valley in the Avanos district of Nevşehir (38°40′37.5″ N 34°51′16.9″ E). The site features three pillar hermitages; these are vertical rock columns where hermit monks lived in seclusion. The central hermitage, named after St. Simeon, highlights his importance and the influence of stylite monasticism in the region. The apse of the church in question features significant religious iconography typical of the late 9th and early 10th centuries in Cappadocia [31].

The monuments in Cappadocia are mainly made of dirty white to pink tuff rock [11,12]. These tuff materials, resulting from intense volcanic activity, are widespread across Central Anatolia, particularly in the Cappadocia region which formed during the Miocene–Pliocene epochs (thick and extensive layer of volcano-sedimentary sequences) [9]. The Cappadocia tuff mainly consists of lapilli-sized (2–64 mm) phenocrysts and rock fragments embedded within a tuffaceous matrix. It contains minerals such as plagioclase, quartz, and biotite. The rock is characterized by a highly porosity, with an average of 38% [11,12].

The Central Anatolia Region is predominantly influenced by continental semi-arid steppe and cold climate types. Summer is hot, with cool and dry nights, while the winter season is cold and rainy [32,33]. The average temperature is 22 °C in summer and −1 °C in winter with recurrent days of freezing values (56 frost days during the winter seasons 2014–2016) [33]; between 1995 and 2007, the average freezing–thawing cycle was 37 [10], and 68, 38, and 62 freezing–thawing cycles occurred, respectively, during 1990, 1991, and 1993 [11].

2.2. Biological Colonization Assessments and Mapping of Deterioration Patterns: In Situ Observations and Sampling Procedure

In order to provide a quali-quantitative evaluation of the lichen vegetation coverage on the church, and their variation combining exposure and inclination, we selected five test areas (Figure 1). Exposure mainly followed the sub-cardinal axes but this number was reduced due to the characteristic geometry (Figure 1). For each test area, a measurement of the surface inclination was recorded by using a smartphone application with a digital compass–clinometer module.

The observations were made on surfaces of 1 m², allowing for comparisons across different exposures. The coverage value of each lichen species, observed in the field, was visually observed using the Braun–Blanquet’s scale of values employed in phytosociological studies (+ = sporadic species, 1 = < 5%, 2 = 5–25%, 3 = 25–50%, 4 = 50–75%, 5 = > 75%) [34].

A total of 28 samples, referred to in the text as “Ril” (Relief), consisted of fragments of stone containing lichen structures. These samples were collected using a scalpel from various church exposures (Figure 2), and a comprehensive report detailing them is presented here. It is noted that “Ril 2” was conducted for separate purposes, and its findings are not discussed in this paper.

• South/East Exposure: 5 samples (Ril1a, Ril1b, Ril1c, Ril1d, Ril1e) (Figure 2a);
• North/East Exposure: 6 samples (Ril3a, Ril3b, Ril3c, Ril3d, Ril3e, Ril3f) (Figure 2b);
• North/West Exposure: 14 samples (Ril4a, Ril4b, Ril4c, Ril4d, Ril4e, Ril4f, Ril4g, Ril4h, Ril4i, Ril4l, Ril4m, Ril4n, Ril4o, Ril4p) (Figure 2c);
• West exposure: 2 samples (Ril5a, Ril5b) (Figure 2d);
• South/West exposure: 1 sample (Ril6a) (Figure 2d).

Preliminary investigations have focused on mapping deterioration patterns through on-site visual inspection (Figure 2), the areas I–VIII correspond to visual observation locations within the church exposures. An area was selected to compare visual observations of the deterioration pattern across different facades with the biological findings. The process involved a qualitative inspection to document existing deterioration, following the classification criteria of the main weathering groups outlined in the International Council on Monuments and Sites (ICOMOS) glossary [35]. The main deterioration patterns observed on the surface, such as delamination, exfoliation, and erosion, were evaluated, also considering their exposure to the various environmental factors, such as wind-driven rainfall and prevailing winds.

A semiquantitative scale for rating the entity of damage was conducted using a hazard scale to categorize these deterioration phenomena into five classes: (1) Very Low (<5%), (2) Low (5–25%), (3) Medium (25–50%), (4) High (50–70%), and (5) Very High (>75%) [36]. This approach allows for a systematic evaluation of the severity and extent of weathering effects on the monument surfaces.

2.3. Lichen Identification

The identification of lichens was conducted through a combination of field observations and laboratory analysis. Lichens were initially identified directly in situ based on visual characteristics (e.g., thallus morphology and color); for later identification, lichen samples were examined in the laboratory.

A binocular microscope with up to 50× magnification was used for initial examination.

A microscope with 5×, 10×, 20×, and 100× objective lenses with the possibility of oil immersion (1000× magnification) was used for sections with thalli and fruiting bodies. Chemical spot tests K, C, KC, and PD were performed for most samples, as necessary. Cluzade and Roux’s [37] identification key was used as a reference. Nomenclature strictly follows that adopted in the ITALIC 7.0 version of the Information System on Italian Lichens [38] and the Consortium of North American Lichen Herbaria [39].

2.4. Penetration Assessment

Stereomicroscope and Scanning Electron Microscope (SEM) observations were carried out in order to evaluate the penetration of lichens into the rocks. All fragments were photographed using a Zeiss Axio Zoom V16. Digital pictures were taken with a CCD Zeiss Axiocam 503 using a 0.25 NA objective lens. The fractured rock samples were placed on conventional Al stubs and sputter-coated with gold (Emitech Technologies Ltd., Kent, UK). The samples were then studied using a Field Emission Scanning Electron Microscope (FE-SEM, Sigma 300, Carl Zeiss SMT). Samples were placed in a vacuum, and secondary electron (SE) images were taken at an accelerating voltage of 5 kV and a working distance of 11 mm to obtain morphological information about the lichen colonization within the rock.

2.5. Climatic Elaborations and Wind-Driven Rain (WDR) Calculation

Microenvironmental monitoring for St. Simeon church, such as T/RH, solar radiation, and wind direction/speed, as well as soil–water measurement, have been started since September 2019. However, due to some gaps in data collection caused by technical problems and the COVID-19 situation, this study utilized hourly weather data from nearby weather stations data from the weather API of Visual Crossing (www.visualcrossing.com) (8 September 2023). The Visual Crossing weather API is a comprehensive weather data collection, leveraging multiple sources and models to provide accurate weather predictions [40]. The collected weather data are created by aggregating information from public weather platforms and privately owned weather stations. In this study, the weather stations were located in Kayseri (30 km away) and Gülşehir district (55 km away).

The hourly weather datasets were retrieved from the Visual Crossing platform for two specific time periods: the year from 1 October 2019 to 30 September 2020 (hygrothermal calendar that begins on October 1st and ends on 30 September in the next year) [41], as well as a broader range covering 20 years from 2000 to 2020. From the long-term measured datasets, we specifically calculated the total and monthly amount of rainfall, the wind speed and direction, and the temperature trends.

To determine the amount of rainwater likely to impact a wall, the semi-empirical method described in ASHRAE Standard 160 was used [41,42]. This method uses hourly weather data, specifically rainfall horizontal amount, wind speed, and wind direction.

Considering the site’s open landscape and the church’s geometric morphology, associable to a cone, a simplified “free field” model has been applied, disregarding certain factors that could affect the amount of water reaching the walls, such as the surrounding topography (buildings nearby), the building’s height, and rain protection measures. The hourly wind-driven rain (WDR) on a simplification model ”free field”, indicated as rbv, [kg/(m²·h)], was calculated by the following formula [41]:

$$\mathit{r}\mathit{b}\mathit{v}\mathbf{=}\mathit{F}\mathit{L}\mathbf{\times }\mathit{U}\mathbf{\times }\mathbf{c}\mathbf{o}\mathbf{s}\mathbf{\left(}\mathit{\theta }\mathbf{\right)}\mathbf{\times }\mathit{r}\mathit{h}$$

where

• FL: an empirical constant, typically 0.2 kg s/(m³·mm);
• U: the hourly mean wind speed [m/s];
• rh: the hourly rainfall [mm/h];
• θ: the angle between the wind direction and the normal to the wall surface [°].

For calculating the annual mean wind-driven rain (WDR) based on long-term weather data, the following formula was applied [41]:

$$\mathbf{r}\mathbf{b}\mathbf{v}\mathbf{M}\mathbf{Y}\mathbf{=}\mathsf{\Sigma }\mathbf{(}\mathbf{0.2}\mathbf{\times }\mathbf{U}\mathbf{\times }\mathbf{c}\mathbf{o}\mathbf{s}\mathsf{\theta }\mathbf{\times }\mathbf{r}\mathbf{h}\mathbf{)}\mathbf{/}\mathbf{N}$$

where

• rbvMY: annual mean WDR [kg/(m²·a)];
• Σ: summation over all hours where cosθ is positive;
• U: hourly mean wind speed in m/s;
• θ: the angle between the wind direction and the normal to the wall surface [°];
• rh: hourly rainfall intensity in mm/h;
• N: number of years over which the data are collected.

In order to consider the directional aspect of WDR, calculations were performed for the different façade orientations (South; North–East; North–West; South–West).

For each church exposure, the WDR calculation was evaluated with respect to the real orientation of the wall expositions, considering the inclination of the single surface.

3. Results

The Saint Simeon Church features a body with a conical shape and a cap rock at the top. Extensive lichen colonization was observed across the façades, except for the area beneath the cap rock. From the in situ survey, evident deterioration processes such as diffuse erosion were identified in the bottom sections (up to 0.80 m) on all exposure sides (

Table 1. Lichen communities and deterioration pattern occurring in the St Simeon Church in relation to esposition Legend: + = sporadic species, 1 = < 5%. 2 = 5–25%, 3 = 25–50%, 4 = 50–75%, 5 = > 75%. Hazard scale for deterioration pattern: Very Low (1), Low (2), Medium (3), High (4), and Very High (5).

Survey	Ril1	Ril3	Ril4	Ril5	Ril6
Exposure	120° SE	20° NE	326° NW	280° W	229 °SW
Slope (°)	80°	90°	80°	70°	75°
Species Identified
Acarospora versicolor	2
Placynthium sp.	1
Lecanora sp.	4
Candelariella aurella	2
Acarospora impressula	1	1	3	4
Lecidella stigmatea		2
Lecanora pannonica		1	2
Lecidea gruppo auriculata		1
Rhizocarpon geographicum			+
Xanthoparmelia verruculifera			1
Tephromela atra			1	2
Physconia grisea			+
Blastenia crenularia			+
Lecidea lapicida			1
Acarospora sulphurata					2
Lichen not identified (a)		2
Lichen not identified (b)		1
Lichen not identified (c)			+
Lichen not identified (d)					4
Bryophytes			3
Total coverage following the Braun-Blanquet scale	90%	50%	95–100%	80–90%	80–90%
Deterioration pattern
Exfoliation	Very Low	Very High	Very Low	Low	Very Low
Erosion	Very High	Very High	High	High	High

) (Figure 2a-II,b-IV,c-V,d-VIII), likely due to rising dump action [14].

Moreover, the eastern and northeastern sides exhibited the highest deterioration (Figure 2b-III,IV), with evident material detachment mainly in the form of exfoliation (Table 1). These areas also showed lower lichen colonization, as depicted in Figure 3. Conversely, the northwest side appeared in better condition, with no significant loss of rock layers.

3.1. Lichen Identification and Distribution

Lichen species occurring on St. Simeon Church in relation to esposition, and their coverage on the rock surfaces, are shown in Table 1 and Figure 4.

The lichen cover on the different exposures resulted as very high, almost always exceeding 90%, with the exception of the North–East (NE) side. The occurring lichen species, chlorolichens, have very similar ecological requirements, are adaptable to xeric conditions, and are capable of tolerating high direct solar radiation and living in a relatively wide pH range of the substrate, and in conditions of low to moderate eutrophication [38,39].

The southern exposures (SE and SW) were dominated by squamulose species, such as Acarospora versicolor, A. impressula, and A. sulphurata, which are often found on steeply sloping rock surfaces.

The North–West (NW) exposure, together with some bryophytes, also hosted some foliose lichen species, such as Physconia grisea and Xanthoparmelia verrucigera. These species have a more complex thallus structure with respect to that of the crustose lichens, and are more demanding of water availability. Their presence could indicate the persistence of more pronounced moisture conditions throughout the day and the year, which in turn could lead to greater physiological activity of the species.

On the NE exposure, the coverage of lichen colonization was significantly lower than on the other exposures and it was mainly constituted by crustose and more generalist lichen species with a wider range of ecological requirements, such as Lecidella stigmatea.

From a biodeterioration perspective, the percentage of coverage of fungal hyphae inside the samples was generally very high, typically ranging between 25% and over 75% (see Figure 5).

3.2. Penetration Assesment

3.2.1. South/East Exposure

SEM observations of all five samples (Acarospora versicolor (RIL 1a), Placynthium sp. (RIL 1b), Lecanora sp. (RIL 1c), Candelariella aurella (RIL 1d), and Acarospora impressula (RIL 1e) highlighted the presence of a network of fungal hyphae (which are part of the lichen structure) throughout the entire sample (~2–3 mm), where the hyphae density decreases with depth, with only a few present at the bottom (see Figure 6a). The images also indicate that a relatively large portion of the rock shows evidence of fragmentation and crumbling. As shown in Figure 6, mineral grains are completely embedded within the hyphae networks.

Notably, the density of the fungal hyphae networks varied across samples. As shown in Figure 5, Acarospora versicolor, Placynthium sp., and Acarospora impressula have a deeper spread, with a coverage value ranging from 50 to 75%

3.2.2. North/East Exposure

SEM observations of all four samples (Acarospora impressula (RIL 3a), Lecidella stigmatea (RIL 3c), Lecanora pannonica (RIL 3d), and Lecidea auriculata (RIL 3f)) showed a relevant lichen penetration throughout the entire sample (~2–3 mm) with a striking density and penetration of the fungal hyphae networks. Figure 6b highlights the mineral grains completely embedded in the hyphae networks. However, the percentage of coverage of fungal hyphae inside the samples was lower than in the other expositions, with Lecidella stigmatea and Lecidea auriculata having 50–75% coverage, and Acarospora impressula and Lecanora pannonica having 25–50% coverage (see Figure 5).

3.2.3. North/West Exposure

SEM observations of the fragments (Lecanora pannonica (RIL 4a), Rhizocarpon geographicum (RIL 4b), Xanthoparmelia verruculifera (RIL 4e), Tephromela atra (RIL 4f), Xanthoparmelia verruculifera (RIL 4g), Lecanora sp. (RIL 4h), Acarospora impressula (RIL 4i), Acarospora impressula, (RIL 4l), Physconia grisea (RIL 4n), Blastenia crenularia, (RIL 4o), and Lecidea lapicida (RIL 4p)) showed a significant loss in the rock matrix and dense fungal hyphae networks that penetrate the substrate to a depth of several millimetres (~2–3 mm). Only in the case of the species Physconia grisea was colonization not observed in depth, but only at the interface between the rock and lichen thallus. Specifically, Figure 6c underscores that mineral grains are completely embedded in the interior of the hyphae networks. Considerable coverage of fungal hyphae exists inside the samples with a value of 50–75%, except for Physconia grisea which had a low value between 1 and 5%, Blastenia crenularia with a value of 5–25%, and Acarospora impressula with a value between 5 and 50% (see Figure 5).

3.2.4. West Exposure

SEM observations of the two fractured samples (Acarospora impressula (RIL 5a) and Tephromela atra (RIL5b) highlighted the heavy presence of fungal hyphae networks throughout the entire sample (~3–4 mm) (see Figure 7). The images indicate that mineral grains are completely embedded in the interior of the hyphae networks, and the rock’s structure is superposed by the fungal network. The percentage of coverage of fungal hyphae inside the samples were remarkable for both, with Acarospora impressula reaching a value of 75–100% (see Figure 5).

3.2.5. South/West Exposure

SEM observations of the sample Acarospora sulphurata (RIL 6a) underscored the presence of fungal hyphae networks throughout the entire sample (~1–2 mm) (see Figure 6d), where the hyphae decreased with depth and only a few hyphae can be seen at the bottom. The images also indicate that a greater portion of the rock evidences fragmentation and crumbling.

3.3. Wind-Driven Rain (WDR) and Other Weather Data

3.3.1. Wind-Driven Rain (WDR) and Dominant Winds

The annual amount of wind-driven rain (WDR) varies significantly based on wall orientation, as shown in

Table 2. Wind-Driven Rain (WDR) values in the main exposition of the church, as resulting from the data of weather station in Kayseri and Gülşehir area.

MONTH	South/East	North/East	North/West	West	South/West	Total WDR for Period
October 2019	3.75	1.53	0.58	0.47	0.36	6.68
November 2019	4.98	3.61	7.30	7.80	4.78	28.47
December 2019	29.89	31.12	39.08	27.23	12.77	140.09
January 2020	0.50	31.32	57.90	56.04	33.30	179.05
February 2020	4.52	6.03	47.93	71.94	55.56	185.97
March 2020	5.47	97.64	79.33	39.19	30.70	252.33
April 2020	4.40	14.05	12.32	8.56	10.86	50.19
May 2020	22.17	8.56	9.33	25.74	50.56	116.36
June 2020	47.08	10.97	28.93	26.27	7.97	121.23
July 2020	0.00	26.62	32.84	0.08	0.08	59.62
August 2020	0.00	0.00	0.00	0.00	0.00	0.00
September 2020	1.09	0.03	0.05	0.04	0.32	1.53
Total 2019–2020	157.99	231.47	315.60	229.20	207.24
Average 2000–2020	162.82	315.92	379.57	297.72	279.66
SEASON	South/East	North/East	North/West	West	South/West
Winter	34.90	68.46	144.91	155.20	35.83	439.31
Spring	32.04	120.26	100.99	73.48	101.6	428.38
Summer	47.08	37.59	61.78	26.35	92.1	264.91
Autumn	9.83	5.17	7.93	8.31	8.1	39.28

and Figure 8. The North–West side is the most affected by WDR, with a value of 315.60 kg/m²·a. On the contrary, the South–East side is the least affected, with a value of 157.99 kg/m²·a. This trend is consistent over a 20-year period, with the North–West side always being the most affected by WDR (Figure 8c).

Monthly data for 2019–2020 (see Figure 9) indicate that in March, the North–East side receives the maximum amount of WDR compared with other months and sides. To under-line the trend in correspondence to the seasonal differences, the year has been divided into Winter (December, January, February), Spring (March, April, May), Summer (June, July, August), and Autumn (September, October, November). Figure 8b (blue line) shows that during Winter, the West and North–West sides are the most affected by WDR, while during the Spring (yellow line), the North–East side is most affected due to the abundance of WDR values in the March month.

Figure 9 and Figure 10 illustrate the number of hours with WDR events and the hourly amounts of WDR on different walls. The West and North–West sides are the most exposed with about 150 h of WDR events. Additionally, Figure 10 shows two extreme WDR events on the North–East side, with approximately 35 kg/m² and 28 kg/m², constituting about 40% of the total WDR amount, while the West and North–West sides have the highest number of WDR hours and the highest number of extreme WDR (>20 kg/m²).

The results regarding the dominant wind direction over the long period from 2000 to 2020 are depicted in Figure 11, which shows the wind rose with the frequency of wind directions. According to the data, the prevailing wind direction is from the North, indicating that winds from this direction were most frequent during the analysed period. This consistent wind pattern plays a significant role in the distribution and impact of wind-driven rain (WDR) on the different sides of the St. Simeon Church, influencing both biological colonization and weathering processes.

3.3.2. Temperatures

To individuate seasonal cycles of physical weathering processes, especially focusing on freeze–thaw cycles, it is crucial to detect and analyze trends of temperature variations around and below 0 °C. Figure 12 shows the months during 2019–2020 that experienced temperature variations below 0 °C, highlighting the already mentioned freezing and thawing cycles [43]. The number of freezing events has been examined by the daily temperature drops below 0 °C (dotted line in Figure 12) between one day and the next [43]. These graphs underline the maximum and minimum temperature variations in relation to the months that received the most WDR events: November, December, January, February, March, and April (see Table 2). January appears to be the month with the most “freezing–thawing” cycles. The minimum temperature values sometimes reach as low as −10 °C in February.

4. Discussion

By combining findings from studies on physical and chemical weathering with those on biodeterioration, effective conservation strategies could be planned that balance the preservation and protection of these unique monuments. This study of lichen identification and distribution on St. Simeon Church provides valuable insights into the combined effects of lichen colonization and weather factors in the resulting stone weathering.

4.1. Lichen Colonization and Penetration in the Rocks

The extensive lichen colonization observed on St. Simeon Church was distributed with high coverage values, ranging from 50% in the North–East to 80–100% on the remaining sides, but with a vicariant role of certain species. These findings show how lichen colonization varies across different sides in terms of coverage and species diversity, being influenced overall by exposure and inclination [20,21,24].

In fact, the consequent variations in solar lighting and the effects of the prevailing winds can differently favor evaporation and influence water content, arising from the wind-driven rain (WDR) amounts. This dynamic interplay between solar exposure, wind patterns, and WDR contributes to the observed differences in lichen colonization and deterioration processes across various sides of the Church. For instance, in North-facing exposures, which receive less direct sunlight, the evaporation is lower, and once wet, these surfaces remain damp for much longer than other sides, but a difference exists among NE and NW in the results of prevailng winds and WDR. This slow-drying condition could be responsible for the higher and diversified lichen colonization on northern faces, compared with the rapid wetting and drying cycles on the South surfaces [3]. In particular, the north–west (NW) exposure is particularly favourable to the lichens’ growths, along with some bryophytes, and hosted foliose lichen species such as Physconia grisea and Xanthoparmelia verrucigera. These cryptogams, due to their more complex thallus structure compared with crustose lichens, have higher demands regarding water availability dynamics [38,39]. Their presence indicates the persistence of more pronounced moisture conditions throughout the day and year, potentially leading to greater physiological activity for these species [38]. Within general xerophylous conditions and with tolerance to the high direct solar radiation in a relatively wide pH range of low to moderate eutrophication, the specific composition and growth forms of lichen colonization showed some clear differences, with an almost complete turnover rate between exposures [38].

In all the collected samples, the SEM analyses confirmed some previous data [17], revealing a severe deterioration of the internal part of the rock surfaces, characterized by dense networks of fungal hyphae. These observations indicate an extensive penetration of lichens into the rock matrix, leading to the embedding of mineral grains and subsequent fragmentation and crumbling. The variation in hyphal density across different lichen species suggests varying capabilities in terms of their spread and penetration into the substrate, influenced by both intrinsic factors related to the species and extrinsic environmental factors.

4.2. Wind-Driven Rain (WDR) and Other Weather Data

This paper showed an application of a simplified method of WDR calculation using hourly weather record data of two nearest weather stations, both starting from rain deposition on buildings for one year and both with a long period of 20 year. This method seems very useful in absence of weather stations directly established in a site.

In our case, the South–East side resulted the least affected by WDR, with 157.99 kg/m²·a, and the highest values are in North facing ones. The role of WDR, which significantly favors the biological colonization of vertical surfaces, as previously shown for several cases [1], reveals significant differences in the amount of precipitation received by the different sides of the structure. In detail, the North–West side is the most affected by WDR, receiving 315.60 kg/m²·a, particularly during Winter, with a maximum of 150 h of rain and several single events of higher WDR amount. However, in parallel, such exposure is also ventilated (see Figure 12) and this can favor a certain vapor evaporation. The North–East side receives a lower quantity (231.47 kg/m²·a of WDR), and overall, a significant concentration in March, accounting for 40% of the total annual WDR and a lower number of hours of WDR (100 h) compared with the other sides but a higher amount of WDR in single events, i.e., more intense WDR events. Such factors can explain the diffuse exfoliation in this exposure that might reduce the growth of lichens [16].

The meteorological data regarding the predominant wind direction over an extended period are comparable with previous research conducted in a similar context at Üzümlü Church (September 2014–May 2015), recorded by a set of environmental monitoring stations in situ [33]. However, there are notable differences in the daily wind patterns between Üzümlü Church and Saint Simeon Church. At the considered weather stations, the data indicate that the prevailing wind direction is generally from the North. In contrast, at Üzümlü Church, the wind direction was reported to be from the North during the nighttime and from the South during the daytime. The difference in daily wind patterns between these two sites can likely be attributed to the setting where Üzümlü Church is located. Specifically, the surface temperature changing on the slope behind (North of) the church plays a crucial role in influencing wind patterns. During the day, the solar heating of the slope creates a thermal updraft [33]. This updraft near the back slope can generate a southward wind flow, affecting the wind direction observed at the church site. Due to the microclimatic data not being acquired in situ, we cannot definitively exclude the possibility of partial similar behavior in wind patterns at St. Simeon Church.

4.3. Stone Weathering and Biodeterioration vs. Bioprotection

It was established that freezing–thawing processes are the main weathering events causing deterioration of the tuff in Cappadocia monuments [10,11]. The 9% volumetric expansion upon the phase change of water seems to be a major cause of rock deterioration [10]. This process leads to significant physical stress within the rock, contributing to the formation of micro-fractures and subsequent material loss. Topal and Doyuran [12] determined that after 52 freezing–thawing test cycles, tuff experiences substantial physical degradation: weight loss of about 27%, an increase in porosity by 8.57%, and a reduction in dry unit weight by 2.78%. When the weathering depth reaches a significant level at the bottom of fairy chimneys, micro-fractures are generated [12]. These fractures propagate and conjoin over time, accelerating the deterioration process. The cumulative effect of these cycles contributes to the gradual but persistent weakening of the rock structure, ultimately leading to its breakdown [10].

Understanding the correlation between temperature variations, particularly around the 0 °C value, and precipitation, including wind-driven rain (WDR), is crucial when pointing out the risk of volumetric expansion deterioration phenomena. Among the months with the highest WDR amounts (Winter and Spring), the Winter poses the greatest risk for freezing–thawing cycles (see Figure 12). During these months, the North–West side is the most affected by WDR, making it particularly vulnerable to weathering due to the combination of high moisture and freezing temperatures. However, this side, which is completely covered by lichens, did not show relevant surface detachment. In contrast, during March, the month when the North–East side is most affected by WDR, significant freezing–thawing cycles do not occur. Probably tuff surface on this side exhibits the most severe deterioration phenomena, including the exfoliation and disintegration of the surface and the least lichen coverage, due the combined effect of high physical weathering, high ventilation, and usually low WDR.

The biodeterioration process seems lower compared with physical and chemical weathering, depending on environmental conditions and substrate characteristics [44,45,46,47], probably due to a combined bioprotection effect. In fact, while lichens are traditionally considered agents of biodeterioration due to their penetration into rock surfaces and contribution to surface fragmentation, they can also provide significant protective benefits. Lichens can shield stone surfaces from temperature variations, thermal stress, and water penetration. Acting as a barrier, they protect against wind, rain, pollutants, and marine aerosols [2,4]. This is corroborated by the presence of widespread exfoliation, salt efflorescence, flaking, powdering, and honeycombing on non-colonized surfaces [2]. Several studies [48,49,50] highlighted that the lichen-covered surfaces experience slower water penetration compared with non-colonized surfaces, supporting the idea of lichens’ protective effects. Previous studies on Cappadocian monuments revealed that lichen-covered tuff had higher vapor diffusion resistance and slower water penetration than non-colonized surfaces, ultimately leading to a decreased weathering rate [16].

In the specific case study of St. Simeon Church, lichens seem play a protective role. Tuff surfaces without lichen cover show higher rates of deterioration compared with those colonized by lichens. Surprisingly, despite being the most affected side by environmental factors, such as wind-driven rain events (WDR) and potential freezing–thawing cycles, the North–West side does not exhibit severe detachment of the surface, suggesting a reduced water penetration due to the protective effect of the lichen coverage. In fact, the Cappadocian tuff, as noted by Topal et al. [12], is characterized by very high porosity and high deformability and, in such porous substrates, the presence of lichens can retard rainwater absorption, partially preventing dissolution and precipitation processes. Additionally, previous studies on other Cappadocian monuments [17] have revealed the production of calcium oxalate by lichens, which can further contribute to protective effects by forming a protective layer on the surface and inhibiting further weathering processes.

Topal et al. [12] emphasized that the deterioration of tuff can be mitigated by strategies aimed at minimizing moisture infiltration and sealing discontinuities in the substrate. Consequently, in this context, the protective role of lichens can contribute to the preservation of historical monuments like St. Simeon Church by acting as a barrier against moisture, wind, and other environmental factors that accelerate weathering processes.

5. Conclusions

This study of lichen distribution and weathering on St. Simeon Church revealed the complex interactions between biological colonization and environmental factors. The protective/biodeterioration role of lichens, influenced by wall orientation and exposure to WDR and freezing–thawing cycles, underscored the need for integrated conservation approaches that consider both biological and physical aspects of weathering. In this case, the prevalence of the protective role of lichens over their deterioration process has been underlined. Therefore, conservation efforts should consider both physical interventions, such as a reduction in discontinuities, as well as the avoidance of lichen remotion to continue the protective mechanisms against weathering. Integrating these strategies can contribute to the long-term preservation of this important cultural heritage site. Future research will focus on applying this approach using field data obtained directly in correlation with the morphology of the monuments.