Biodiversity Hotspots in Peri-Urban Areas: The Case of the Old-Growth Forest Kouri, Thessaloniki, Northern Greece

Abstract

In the context of the ongoing climate crisis, the health and sustainability of forest ecosystems in peri-urban areas play a crucial role in alleviating the adverse impacts of climate change on urban populations, particularly in cities with limited green spaces. This study explores the biodiversity and ecological values of an old-growth forest in the peri-urban area, Thessaloniki, northern Greece, the Kouri Forest. These types of forest ecosystems, except for their high ecological values, provide a lot of benefits to the city residents and the surrounding areas, and to achieve that they should have appropriate composition, structure and function to be able to provide high-level ecosystem services. The research was based on collecting analytical field data, including field sampling plots, and a series of tree cores for tree age determination and tree growth analysis. Data analysis demonstrates the unique characteristics of this forest, which was found to be an old-growth forest dominated by deciduous oak species, aged over 180 years. The high biodiversity of the forest and the rich composition and the multistorey stand structure, in combination with the long age of the trees, suggests that the forest is an old-growth (ancient) forest, and set the forest as an important biogenetic reserve, despite its small area, proximity to the city of Thessaloniki, and the pressures subjected. Accordingly, special management measures are suggested to aim at the sustainable use of peri-urban natural resources.

1. Introduction

Under the current climate crisis, urban forests and forest ecosystems of a favorable ecological status in the peri-urban areas consist of an important element in mitigating the negative effects of climate change on citizens [1], especially for cities of low green space, such as Thessaloniki city in northern Greece. This study explores the ecological values of a unique old-growth forest (Kouri Forest) in the peri-urban area, Thessaloniki, northern Greece, a city that is characterized by very low urban green space.

Urban and peri-urban forest ecosystems have a high ecological and environmental value for the adjacent cities; they provide a lot of regulation, cultural and provisioning services, and they also contribute to biodiversity of urban areas [2]. The main regulations concern climate regulation (e.g., cooling), carbon storage, air pollution removal and flood regulation [3]. Cultural services mainly concern recreation, aesthetics, natural heritage and knowledge transfer [3]. Provisioning services include products such as wood, food, fuelwood, clean water, medicines, etc. [4]. Furthermore, urban and peri-urban forests improve cultural diversity and increase urban resilience to environmental stress [5]. However, their function greatly depends on their naturalness, the level of biodiversity [6], species composition, community structure and effective nutrient cycling and complex food chains [7]. It is widely accepted that forests characterized by a rich composition, and high heterogeneity of vertical structure are more able to provide high-level ecosystem services, especially in the context of current climate change [8]. Accordingly, they should be managed in an appropriate and sustainable way to conserve all their structural characteristics and functional components [9].

Nowadays, climate change is considered a great environmental problem, especially global warming. Europe emits one of the largest carbon amounts in the world [10], while forests act as important carbon sinks. Thus, forest managers face great challenges in increasing carbon sequestration in forest pools, resulting in mitigation of the negative effects of climate change. Especially, peri-urban forests can play a great role, due to the proximity to large cities and the great demands for urban expansion [11,12]. Forest management in peri-urban areas have the responsibility to conserve structure and function of forest ecosystems in order to provide ecosystem services of a high level. Peri-urban forests provide, among others, recreation, and important protection functions, such as clean water, and soil protection. Thus, forest managers have a crucial role in protecting forest functions and biodiversity values in forest areas close to cities. Natural resources around the cities, and especially forest ecosystems, are subjected to great pressure for urban land expansion and the great number of visitors [13,14]. Especially, urban sprawling has large impacts and, in many cases, loss of forest areas [15]. While urbanization near forested areas increases in many areas around the world, this affects people who live in transition, as forests act as a home for social and ecological services that improve human health and well-being [16]. Finally, land management should secure all the services and benefits provided by forests, which human societies need, according to the cultures and the live patterns the communities follow [17].

Well-structured forests can also mitigate natural hazards, such as erosion, floods or rock [18]. These natural hazards can be eliminated by appropriate forest composition and structure. Forest managers can succeed in this by taking appropriate silvicultural measures to mitigate or prevent such events [19,20,21]. Therefore, forest composition and stand structure are a critical characteristic [22,23] regarding forest resilience and for providing high ecosystemic services by the forests.

The current research concerns a unique peri-urban forest near the city of Thessaloniki, northern Greece, the Kouri forest. Cartographic data from the year 1945 proves the existence of the forest since that time (thus the forest is characterized as an old-growth forest). Due to its special characteristics and the proximity to the city of Thessaloniki, there is great public interest combined with a high scientific interest in conserving its natural gene pool and other ecosystems elements with special value, and for including in strategic plan for the sustainable development of the metropolitan area of Thessaloniki. Considering the new flyover of the ring-road of Thessaloniki, which is being constructed and destroys a significant part of the peri-urban forest area of Thessaloniki, it is very crucial to gain detailed knowledge for the forest resources of the area, requested for conservation and sustainable management of all forest resources around the city, especially those of high ecological values. Furthermore, no scientific data is available for the forest.

The aim of this study was to estimate the ecological value and highlight the importance of peri-urban forests and their role for ensuring long-term sustainability of society by effective management of nearby natural resources and sustainable urban development. The specific objectives of this research were (i) to determine the composition, structure and age of the unique peri-urban Kouri forest; (ii) to analyze the composition and stand structure of the forest and evaluate its biodiversity value; (iii) to evaluate the resilience of this forest ecosystem; and (iv) to contribute to sustainability of the city of Thessaloniki, offering important knowledge for an effective land management in the Metropolitan area of the city.

2. Materials and Methods

This research was conducted in Kouri peri-urban forest of Thessaloniki, northern Greece (Figure 1 and Figure 2), located at a distance of 8 km far from the city of Thessaloniki (2565888 E, 4931507 N in WGS_1984_World_Mercator—EPSG: 3395), northern Greece, the second largest city in Greece. The studied forest is located in the peri-urban area of the city at an altitude of 150–500 m asl, in slopes of north-northwest aspect. The forest is dominated by two oak species, Quercus frainetto and Quercus pubescens, and it has not been under systematic management at all, even though it suffers from high human pressures such as many recreation activities, walking, trailing, bicycling, picnics, which cause disturbances and degradation on forest function and services. However, no tree cutting was applied in the forest for at least the last five decades. Finally, no scientific data is available for the forest.

The dominate geological structure of the area is gneiss and the most common soil types are red and black clays [24,25]. The climate is shaped and influenced by its geographical location and altitude, with the Thermaic Gulf to the west and the Hortiatis mountain range to the east exerting a significant influence. Winds play a major role in regulating weather phenomena, with the most important being Vardar, a strong northwesterly wind, which acts as a cloud breaker. At the same time, the easterly wind from Hortiatis, although local, is strong and is often associated with rainfall, snowfall and general bad weather. Overall, the study area enjoys good climatic conditions, with the climate characterized as Mediterranean with mild winters and short, hot and dry summers. Bioclimatically, the area under study is classified as having a strong Mediterranean-Mediterranean character with a number of biologically dry days, X, during the warm and dry period: 75 < X < 100, while it falls into the humid with cold winter with an average minimum temperature of the coldest month m: 0 °C < m < 3 °C.

A meteorological station with reliable and continuous data in the wider area is the Loutra Thermi Meteorological Station, which operates under the auspices of the Forest Research Institute (ELGO-DEMETRA) at an altitude of 30 m, φ = 40°30′27, λ = 23°04′58 and a distance of about 14.5 km. From the data of the Meteorological Station for the period 1978–2014, the following climatic characteristics emerge:

• Average annual precipitation 468 mm;
• Average annual air temperature: 15.5 °C;
• Warmest month: July;
• Coldest month: January.

According to the Kὅppen Climatic Classification, the area belongs to the Csb climate type, and the climate. The studied forest stands have not been under systematic management up to now. In fact, no systematic management was ever applied.

2.1. Field Data Sampling

In order to conduct this study, twenty sampling plots of 500 m² each (20 m × 25 m) were established in 2023, based on a systematic sampling, covering all the area of the forest. Each sampling plot was taken at approximately 100 m from each other, and at least 50 m from the forest borders to avoid edge effect. The aim was to cover all spatial heterogeneity of the studied forest, in terms of altitude, land aspect, slope inclination and soil fertility. Each sampling plot was used for measuring the forest stand characteristics, and for inventory of all plant species. In each plot, we measured all the tree species with diameter at breast height (dbh) over 4 cm; measurements included tree diameter at breast height; and the total tree height. Furthermore, the vitality of each tree was assessed according to IUFRO classification in 3 vitality classes (10, 20 and 30) [26]. For the estimation of plant diversity, all the plant species within each plot were recorded at the species level.

Stand age and growth pattern of the dominant trees was determined by taking one tree core per plot. We selected one of the dominant trees, belonging to overstorey, freely developed, for sampling. The selection of sampled individuals was based on their phenotype, estimating they had normal growth, and with no strong competition from the neighbors. Taking into consideration that cutting was not performed in the forest at least for the last five decades, the estimation of tree free growth—based on the distance of the neighbors and the presence of a well-developed tree crown—can be regarded as an effective indicator of normal tree growth and dynamics [27]. The sampling aimed to analyze species growth pattern of adult individuals to produce optimal results. A Haglöf increment borer (400 mm/16″) was used to obtain the tree cores, and in order not to pose a phytosanitary risk to the individuals after the trypanid extraction, the holes were smeared with artificial inoculation resin (Novatrim). To measure the width of the annual rings of each tree core, a special microstereoscope (Parker Instruments’ Electronic Machine for Measuring Annual Growth Rings Mod 3) was used [28], with an accuracy of hundredths of a millimeter, in the Laboratory of Silviculture, AUTH (Figure 3). Before measurement, the samples were properly prepared by grinding the upper surface, so that the annual rings could be clearly distinguished, and the annual ring will be measured with great accuracy (Figure 3). The measurements were recorded starting from the innermost ring and ending at the bark.

2.2. Statistical Analysis

According to the collected field data, the studied forest was distinguished in three forest types, based on the dominant tree species in each plot area. Stands dominated by Quercus frainetto (the main part of the forest), stands dominated by Quercus pubescens (less abundant), and mixed stands dominated by the two oak species, either without other tree species in stand composition or with abundant presence of Carpinus orientalis (locally appeared). To evaluate the forest stand structure and provide efficient tools for applying climate adaptive forest management to the concerns and needs of society, an analysis of the optimal stand structures was performed, testing the most appropriate theoretical functions to estimate the structure of the stands. This was undertaken using the two most common theoretical functions, Normal [29] and Weibull [30], since these are characterized by high flexibility and accuracy in describing even aged forest stands [31]. The goodness of fit was tested using the chi-square criterion and the Kolmogorov–Smirnov [32] test. Comparison of tree morphological values was performed by R using library “fitdistrplus” to fit Normal and Weibull distributions and “ggplot2” for visualizing the graphs. To test the goodness of fit, Kolmogorov–Smirnov test was conducted and AIC (Akaike Information Criterion) was calculated to evaluate which distribution better fits the dataset for each type of forest studied.

3. Results

3.1. Ecosystem Composition

Even in the small area of the studied forest, 21 woody (trees and shrubs) plant species were recorded (

Table 1. Flora species of the studied forest.

Woody Species	Herb Species
Quercus frainetto Quercus pubescens Quercus cerris Quercus coccifera Acer monspessulanum Carpinus orientalis Cornus mas Ostria carpinifolia Phillyrea latifolia Fraxinus ornus Fraxinus angustifolia Juniperus communis Juniperus oxycedrus Paliurus spina-christi Pirus amygdalοformis Pyrus communis Pyrus spinosa Platanus orientalis Prunus domestica Prunus spinosa Ligustrum vulgare Punica granatum Ulmus minor	Achillea millefolium Agrostis alba Alopecurus pratensis Arimonia agrimonides Asparagus acutifolius Avena fatua Brachypodium sylvaticum Cardus pycnocephalus Clematis vitalba Convolvulus arvensis Dactylis glomerata Doronicum orientale Festuca heterophylla Filipendula vulgaris Fragaria vesca Hedera hélix Helleborus cyclophyllus Hieracium bauhini Hieracium murorum Hordeum murinum Galium aparine Galium verum subsp. Verum Geranium lucidum Jasminum fruticans Koeferia cristata Lamium maculatum Lapsana communis Lathyrus laxiflorus Lathyrus venetus Leontodon cichoraceus Leontodon cripus Lonicera etrusca Malva sylvestris Medicago mínima Mentha pulegium Moehringia trinervia Muscari comosum Muscari neglectum Mycelis muralis Myosotis ramossisima Phalaris tuberosa Plantago lanceolata Plantago major Poa nemoralis Poa pratensis	Primula veris Pteridium aquilinum Ranunculus neapolitanus Ranunculus repens Rosa arvensis Rubus canescens Rumex acetosella subsp. Acetoselloides Rumex pulcher subsp. pulcher Ruscus aculeatus Salvia sclarea Sanguisorba minor Satureja (clinopodium) suaveolens Sedum amplexicaule subsp. Tenuifolium Silene italica Silene latifolia Symphytum bulbosum Potentilla micrantha Tanacetum corymbosum Taraxacum officinale Teucrium chamaedrys Teucrium polium subsp. capitatum Thymus sibthorpii Thymus vulgaris Trifolium alpestre Trifolium campestre Trifolium pignantii Trifolium pratense Trifolium repens Trisetum flavescens Umbilicus ruprestris Verbascum nigrum Verbascum sinuatum Veronica chamaedrys Veronica serpylifolia subsp. serpylifolia Vicia villosa Vincetoxicum hirundinaria Viola alba Viola kitaibeliana Viola reichinbahiana Viola riviniana

) within the sampled plots. Also, the herb understorey was quite rich, where 85 plant species were found, most of them typical of deciduous oak forests of the Quercetalia pubescentis floristic zone. This indicates a well ecological conservation status of the forest, and the great importance of the forest for conserving plant species biodiversity.

As it is previously mentioned, based on the dominant tree species, the forest was distinguished in three types: stands dominated by Quercus frainetto (the main part of the forest), stands dominated by Quercus pubescens (less abundant and in worst site qualities), and mixed stands dominated by the two oak species (locally appeared), either without presence of other tree species, or with abundance of the species Carpinus orientalis. Stands dominated by Q. frainetto present a high number of tree species participating in stand composition, in a percentage of 27.1% of the total stem number (Figure 4a), indicating a quite diverse stand composition. On the contrary, in the stands dominated by Q. pubescens, only three tree species were recorded (Q. pubescens, Carpinus orientalis and Fraxinus ornus) (Figure 4b). Mixed stands are dominated by the two oak species (Q. frainetto and Q. pubescens) and high presence of Carpinus orientalis, in a percentage of tree number of 50% were found locally in small areas, as a degraded stage of oak forest (Figure 4c).

3.2. Ecosystem Structure

The studied forest, in all the three types of stands, based on diameter and height distribution (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10, presents the structure of even-aged stands. The stand density expressed as the total number of trees per hectare of all tree species varies from 1200 tree per ha (in Q. pubsecens stands) to 1720 (in Q. frainetto stands). Mixed stands with Carpinus orientalis present intermediate values with an average of 1380 trees per ha (

Table 2. Descriptive statistics (mean and standard errors of means, in parenthesis) of tree diameter and height in the three types of forest ecosystems. Values of the same parameter followed by different letters are significantly different between the three forest types.

Forest Type	Maximum Tree Age Years	Total stand Density N/ha	Mean Tree Diameter of Dominant Storey (cm)	Mean Tree Height of Dominant Storey (m)	Mean Tree Crown (m)
Q. frainetto stands	180	1720 (300.4) a	32.8 (4.1) a	16.1 (2.6) a	16.7 (2.8) ab
Q. pubescens stands	131	1200 (152.2) b	31.1 (3.2) b	12.6 (2.0) b	18.5 (2.5) a
Mixed oak stands with C. orientalis	120	1380 (160.8) b	27.2 (3.4) c	12.4 (1.9) b	14.8 (2.1) b

). On all sampled plots, the stands are quite dense, containing in all cases more than 1000 trees per hectare, a tree density which is quite high for a mature oak forest. Both mean tree diameter and tree height was found to be significantly higher in stands dominated by Q. frainetto, compared to the other two forest types (Table 2).

The statistical analysis reveals that in all twenty plots, and in the three different forest types, there is a relatively large variation in tree morphological characteristics, evidenced by the values of standard deviation (Table 2), and diameter and height frequency distribution (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10).

Stand Structure of the Different Forest Types

The first type dominated by Q. frainetto presents a form of two-tree-storey stand structure (Figure 5 and Figure 6), the dominant upper story consisted of oak individuals of high dimensions, and a secondary (middle) storey of many individuals of the other tree species participated in the stand composition. These tree species are species of relatively low size (usually of height less than 10 m), and they commonly appear as understorey (secondary stand) in mature oak forests, under the dominant oak storey, contributing to enhancing forest biodiversity. This storey consisted of abundant individuals of deciduous trees, mainly Carpinus orientalis, Fraxinus ornus, Acer monspensulanum (Figure 4a), which are all considered trees of small size at mature age. These stands comprise the main part of the studied forest, and are the oldest stands in the area, with trees being up to 180 years in age. The total stand density is quite high, considering the tree age, presenting an average value of 1720 trees per hectare with dbh over 4 cm. The dominant storey counts the largest part of the trees (1195), while the secondary storey counts 530 trees with a height range from 3 to 12 m, and diameter from 4 to 32 cm. Trees of dominant storey present a height ranging from 12 to 24 m, and diameter from 24 to 52 cm. Most trees of the dominant oak species Q. frainetto were found to be concentrated in the height class of 12–15 m.

The second type dominated by Q. pubescens also presents a two-storey tree stand structure (Figure 7 and Figure 8), the upper story dominated by oak individuals, and a middle storey consisted of many individuals of the other tree species. These stands are a little younger, with the age of the older trees up to 131 years. The total stand density is quite high, presenting an average value of 1200 trees per hectare with dbh over 4 cm. The dominant storey counts the largest part of the trees (970), while the secondary storey counts an average of 230 trees, with a height ranging from 3 to 12 m, and diameter from 4 to 24 cm. Trees of dominant storey present a height ranging from 12 to 18 m, and diameter from 24 to 52 cm. Most trees of the dominant oak species Q. pubescens were found to be concentrated in the height class of 9–12 m, while quite a high number of dominant trees were found in the height class of 6–9 m.

The third forest type, that of mixed oak stands with high contribution of Carpinus orientalis, which was found locally in small areas, represents a degraded stage of the other two forest types. This type of forest consisted of mixed stands of the two oak species (Q. frainetto and Q. pubescens) and Carpinus orientalis, and is possibly resulted from degradation of oak stands at local scale, and the gradually replacement of some oak individuals by C. orientalis (e.g., by illegal cutting of oak trees, considering the hard wood of oaks, that was usually used in the past not only for fuelwood but also, and commonly, for house construction and other uses). The structure of this type consisted of an upper storey dominated by trees of the two oak species (Figure 9) and a very small percentage of Acer monspensulanum, and a middle storey consisted only of individuals of Carpinus orientalis. This tree distribution is evidence considering mainly the tree height distribution (Figure 10) but also the tree diameter distribution (Figure 9). Individuals of oak species are all over 16 cm in diameter and over 9 m in height, while individuals of Carpinus orientalis are under 16 cm in diameter and 9 m in height, indicating a clear distinction of the two-storey structure.

Goodness of fit tests showed that in most cases of the forest types, the Normal distribution is followed (

Table 3. Goodness of fit of Normal and Weibull distributions.

Forest Type	Distribution	KS Test		AIC		Preferred Model
		D-Statistic	p-Value	Normal	Weibull
Q. frainetto	Normal	0.177	0.552	5015.3	5012.7	Weibull
	Weibull	0.174	0.575
Q. pubescens	Normal	0.184	0.490	4195.1	4193.0	Weibull
	Weibull	0.179	0.536
Quercus spp. (mixed stands)	Normal	0.290	0.076	2985.9	2970.4	Weibull
	Weibull	0.252	0.176
C. orientalis	Normal	0.444	<0.001	4687.2	4683.2	Weibull
	Weibull	0.399	0.004

). In mixed Quercus forest, this hypothesis is weak, and we can certainly confirm that Normal distribution is not followed in the C. orientalis. Weibull distribution performs better in all cases. For Q. frainetto and Q. pubescens (which appear more symmetrical), the differences between the models of Normal and Weibull distribution are small. In Quercus spp. and C. orientalis forest type (which are highly skewed), the Weibull model shows a noticeably better fit, especially for Quercus spp., as the AIC values indicate.

3.3. Forest Age and Tree Growth Pattern

Figure 11 and Figure 12 show the age and pattern of tree annual radial growth of the two dominant Quercus species (Q. frainetto and Q. pubescens) in the studied forest, while Figure 13 shows the tree ring details of the oak species as they are shown under the view of a stereomicroscope. Initially, both species show a similar pattern, with a low increment during the early tree life (10–20 years). Then, there is a differentiation of tree growth between the two species; the growth of the trees of Q. frainetto slightly increases for a period of the next 40–50 years, up to the age of 80 years, and it continues for the whole tree life. This pattern probably indicates that the stands are of seed-origin, while they have been grown without pressure and strong competition, since this pattern is common in free-grown trees. On the other hand, trees of Q. pubescens grow at a low growth rate during their life, which results in a low tree size, even at its high age. The ages of the older trees are 180 years and 173 for the species Q. frainetto (Figure 11), and 113 and 131 years for the species Q. pubescens (Figure 12). The dimensions of those trees were diameter (dbh) 26.7 cm and 28 cm, respectively, and height 18.7 m and 13 m for the species Q. frainetto, and diameter 22 cm for both trees and height 12.1 and 9 m for the species Q. pubescens. These low values of tree size are probably due to the harsh site conditions that exist in the area, and the high human pressure on the ecosystem (walking, trailing, picnic, gaming, cycling, mushroom collection, etc.).

However, taking into consideration the large age of the dominant trees of both oak species (180 and 131 years, respectively), in combination with the analysis of plant community composition, and stand structure, it can be concluded that the studied forest comprises the climax forest community in the area. This conclusion is confirmed by (i) the fact that the studied forest geographically belongs to Quercetalia pubescentis vegetation zone [33], (ii) most of the plant species recorded (Table 1) belong, from a phytosociological point of view, to the typical or characteristic species of the alliance of Quercetalia pubescentis, (iii) the physiognomy of the forest in 1945 (based on the air photo of 1945) indicates the existence of oak forest. This conclusion clearly indicates the importance and the high value of the studied forest, which can be finally characterized, based on all the above-mentioned data, as an important old-growth peri-urban forest of high priority for conservation. It is also very important that the forest lies just in the border of the city of Thessaloniki, the second biggest city in Greece, with approximately one million inhabitants.

4. Discussion

4.1. Biodiversity and Ecological Value: A Sanctuary Amid Urbanization

Among the most notable revelations throughout the current research is the high level of phyto- and structural diversity of the studied forest. According to the data collected in Kouri Forest, Thessaloniki northern Greece, a number of 106 plant species (21 woody species and 85 herbaceous) were recorded within the sampled plots, all native with no exotic species. Contrary to the results of our study, exotic species spontaneous occurrence is a common phenomenon in forest habitats in and around the cities [34]. The species recorded in our study are the typical plant species of deciduous oak forests of Quercetalia pubescentis that made up the forest flora, and thus, Kouri Forest can be considered a well-preserved ecological state. A similar number (109) and types of plant species were reported [35] for a natural deciduous oak forest belonging to the same vegetation zone (Quercetalia pubescentis) of a NATURA 2000 site (GR1240002-ORI TZENA) in northern Greece, confirming the well-preserved ecological status of the studied forest, with similar plant composition to natural Greek oak forests. Also, most plant species recorded belong to the category of the species “1.1 = largely restricted to closed forests” [35]. It is worth mentioning that this high number of woody and herbaceous plant species largely restricted to closed forest was recorded in the restricted area of the sampled plots of the current study, even the small area of the forest, and the proximity and the great pressures of citizens. Even though no quantitative data for visitors and human pressure are available, it is an undoubtful fact that the forest is subjected to great human pressures by a high number of visitors every year.

The studied forest is characterized by even-aged structure consisting of trees of medium-sized diameter. This forest structure tends to provide enough resources for a balanced community between higher vegetation (trees and shrubs) and herbaceous species, which supports species richness and species evenness [36]. Similarly, ref. [36] reported that forests with a small DBH between 14 and 38 cm had the highest phytodiversity in urban forest ecosystems of the Ruhr Metropolitan Region, in Northrhine-Westphalia, Germany. However, not every ancient woodland is by default a biodiversity hotspot supporting a high species richness. A large variation in alpha diversity is often observed due to different land-use history, the silvicultural interventions, tree species composition, and site quality [37,38,39]. Following the ‘‘hotspot strategy’’ of Meyer et al. [40], woodland patches with a high diversity of ancient woodland indicator plants should be identified and protected or managed for nature conservation [41].

This diverse composition reflects not only the forest’s age but also its ecological stability, which is a characteristic of mature ecosystems. Such biodiversity becomes less common in peri-urban areas where urban sprawl and habitat fragmentation have led in most cases to a standardization of species composition [42,43]. It also seems that the lack of any systematic management measures did not have negative impacts on forest composition and structure. Probably, people traditionally respected the forest, conserving in a way forest area, structure and composition. This biodiversity is even more crucial, as it must deal with the challenges of urban expansion. The multi-storey structure presence, whose dominant upper canopy layer is composed of oaks (mainly Q. frainetto and Q. pubescens), with a secondary layer of smaller tree species like C. orientalis and F. ornus, is a complex ecological structure. Both vertical and horizontal diversity are supported by this architectural form which, in turn, is reinforced by the forest’s ability to offer a broad range of ecosystem services such as carbon fixation, water regulation and habitat provisioning for urban wildlife [44,45].

Interestingly, the smaller trees comprising a second-storey of Q. frainetto-dominated stands might be highly useful from the point of view of forest resilience [45,46,47,48]. In contrast to most urban forests that have simple canopy structures, Kouri Forest’s layered composition raises its structural complexity, thus more species can find their ecological niches here and, consequently, the forest becomes more stable. This is an indispensable finding in the context of climate change adaptation whereby the mentioned biodiversity and the associated structural complexity can be instrumental in increasing the ability of the forest to withstand droughts, pests and extreme weather [45,49].

4.2. Stand Structure and Dynamics: Climax Forest Features in a Changing Environment

The analysis of the main statistical indicators used in this study revealed that the studied forest consists of stands with even-aged structure. Thus, both tree height and diameter distribution of the dominant tree (oak) species follows the general pattern of Normal (Gauss) distribution, with small declines from the typical distribution. The distribution in mixed Q. frainetto with Q. pubescents forest stands and C. orientalis forest stands deviate statistically significantly from normal curve according to KS test.

The stand structure of Kouri Forest tells a detailed story of the forest’s changing nature. At first glance, the forest shows a clear even-aged structure in all three types of stands—those dominated by Q. frainetto, Q. pubescens, and C. orientalis mixed stands—but it also has dynamic change signs. The two-storey structure existing in the stands of both Q. frainetto and Q. pubescens as well as the high tree density (from 1200 to 1720 trees per hectare) are strong indications of a mature forest with long growth and history of competition phenomena. Such a double-layered structure is typical of climax communities that have, over time, developed a stable, multi-layered canopy which provides habitats for numerous plant and animal species [50]. This conclusion is confirmed by the cartographic data of the area presented in Figure 1, where the existence of dense forest in 1945 (80 years ago) is clearly shown, contrary to the rest of the nearby areas that were bare land (there were no urban pressures that time).

The Q. frainetto-dominated stands can boast of their mature age (up to 180 years) and are impressive with the trees that may attain 60 cm in diameter and 12–24 m in height. These stands’ structural complexity resulting from the secondary canopy of smaller tree species questions only ecological diversity and resistance, which are higher than those of even-aged stands. On the other hand, the Q. pubescens-dominated stands that are a little younger and less varied still make a valuable contribution to the forest’s biodiversity but are at a slightly different ecological phase. Degraded areas boasting mixed oak stands with a significant proportion of C. orientalis are an important signal for the history of human disturbance in that place. These areas are located at the smallest distance from the local settlements. These mixed stands symbolize a transitionary phase, probably the outcome of strong human influences such as illegal logging and over-exploitation of oak trees. Although regeneration dynamics may be inferred from the presence of C. orientalis in the secondary storey, it is equally important as an ecological indicator of the factors that have impacted on the structure of the forest. The occurrence of such stands in small, localized areas only points to the fragile equilibrium that must be maintained between conservation and human activity in peri-urban ecosystems.

4.3. Growth Patterns and the Role of Human Pressure

The radial growth analysis of the two dominant oak species (Q. frainetto and Q. pubescens) is an innovative aspect of this research. Essentially, the study unveils major differences in the growth patterns of these two species, which helps to better understand the influence of local site conditions and human disturbances on forest dynamics. It is worth mentioning that the rather moderate growth rate of Q. pubescens, notably in comparison with Q. frainetto, may be pointing out that the forest ecosystem is exposed to harsh site conditions and high human pressure resulting from activities such as recreational activities and mushroom collection. Even though no quantitative data for visitors and human pressure is available, based on unofficial data (e.g., personal observations during the field data collection, photos in social sites, articles in internet), the forest is subjected to great human pressures by a high number of visitors every year. Also, in this slow growth trend, the factor of tree size is playing an important role since the size of trees, even the old ones, remains low and it may therefore be interpreted as stress caused by abiotic factors. It is commonly accepted that even though both oak species (Q. frainetto and Q. pubescens) appear in the same areas, the first species dominates in the better sites while the latest occupies the worst ones [33].

The age of the older trees reaches 180 years for the species Q. frainetto, and 131 for the species Q. pubescens. Mature trees of both oak species were found to be in good wood health condition, with wood appearing a clear ring-form pore distribution (Figure 13). Based on the analysis of tree rings, the tree annual increment is generally low, ca 1 mm, with a range between 3.6 mm (maximum recorded value) to 0.48 mm (minimum recorded value) for the species Q. frainetto, slightly lower (2.8 mm–0.42 mm) for the species Q. pubescens. The maximum value was observed earlier in tree life for the first species compared to the latest.

The study’s age analysis—with trees reaching 180 years old—is a strong indication that Kouri Forest is a near-to-climax community, thus probably approaching the final stage of natural succession in this area. The fact that such forests have been able to retain their resilience even with the impact of the city proves the resilience of the ecosystem and the climate change mitigation role these forests are playing. An important factor that contributed to maintaining this ecological forest hotspot was probably the reforestation activities which took place in the area among the city of Thessaloniki and Kouri Forest, after the decades of 50 s and 60 s, and the creation of the peri-urban forest of Thessaloniki Seich-Sou. This new planted forest covers an area of 3000 ha, borders the city, the city residents can reach it on foot, and thus, it probably significantly contributed to the protection of the studied old forest by creating a buffer zone of a few (8) kilometers, protecting it from human activities, and helped dramatically this rare forest ecosystem reservation after all those years, although it is so near to the second biggest city center of Greece. The findings could become a source of strategies for the forest managers who are willing to come up with plans to facilitate the growth and regeneration of forests bordering the cities that are under threat of the expanding cities [15].

4.4. Implications for Conservation and Urban Green Space Management

The innovative results of this study indicate that the Kouri Forest, Thessaloniki, is a model case of how old-growth forests in peri-urban areas can maintain ecological integrity in contrast to urban sprawling. The biodiversity, structural complexity and long growth history of this forest make it a key biogenetic reserve, despite all the difficulties it faces due to urban encroachment and human disturbance. As this forest lies on the very edge of Thessaloniki, Greece’s second-largest city, its conservation is not only a priority for biodiversity protection but also for enhancing quality of life among urban residents. The major challenge of forest management in these cases is to balance citizens’ high demands for high-level ecosystem services without harming the structural and functional characteristics of the natural ecosystem [51]. For example, for the Grünewald forest, Berlin, according to estimations of both the forest administration and a scientific analysis [51], an annual visit number of ca 100 million visitors are estimated, a very intensive use, and it denotes a wide range of conflicts and management challenges in order to maintain the stability of the ecosystem and visitor demands. Even though there is no visitor’s data for the studied peri-urban forest, the estimation for citizen demands is expected to be extremely high due to the high ecological and aesthetic value of the forest, and the proximity to the city of Thessaloniki, which is characterized by an extremely low green space.

This study calls for a paradigm shift in the way peri-urban forests are regarded within the overall perspective of urban planning. Instead of treating them purely as bits of greenery, old-growth forests such as Kouri should be recognized as vital urban infrastructure providing a wide array of ecosystem services—from regulating the climate to public health benefits. It is about this premise that any future management strategies should ensure sustainable interaction with these forests in their ability to act as ecological buffers between cities and the natural environment. This paradigm also shows how reforestation actions can effectively protect biodiversity hotspots, creating buffers of protection, even though they are so near to city centers. These interactions should also be considered for future reforestation activities between mature and newly established forests and all the cumulative benefits of these actions.

To sum up, it is of great importance that a special management plan should be immediately developed for these types of important old-growth (ancient) forests, determining the required management measures in details, such as forest zonation with specific uses and restrictions for each zone, determination of the current visitor capacity, locations characterized as hotspot biodiversity needing special protection measures, silvicultural treatments for enhancing forest resilience, and stimulating forest regeneration by seeds where it is necessary, etc. Also, at regional, national and even at the European level, for nature conservation practice, there is a great need for precise study of such peri-urban woodland and forests that can be categorized as ancient woodland, based on their ecological characteristics and the existence of old trees or stands. The value of such forest islands for nature conservation should be considered extremely high, while their value depends on historical ecological continuity [52]. Under such circumstances, these types of deciduous ancient woodland sites can be valuable hotspots of forest plant biodiversity and forest genetic resources conservation and gene flow. Additionally, they can act as important propagule sources for the expansion of ancient woodland species into adjacent recent woodlands, regardless of the long time the expansion may need [53].

For the studied forest, a special management plan should suggest, among others, zonation with specific uses and restrictions for each zone, spatial arrangement of visitor’s activities, determination of the current visitor capacity, development of specific silvicultural treatments for enhancing forest resilience, and enhancement forest regeneration by seeds, as well as design of applying selective thinning of low intensity in locations with high tree density to enhance tree resistance to global warming [54], spatial restrictions of visitors to specific parts of the forest, prohibition of cutting plant species, exclusion of grazing, placing signs to inform visitors for the high value of the forest, etc.

5. Conclusions

This study confirms the exceptional ecological status of the Kouri Forest in the peri-urban landscape of Thessaloniki, northern Greece. The investigation, based on analytical field data and tree core analyses, classifies this area unequivocally as a rare example of old-growth forest in the region, with trees surpassing 180 years of age, high biodiversity, and complex multi-storey stand structure. The fact that those characteristics persist immediately adjacent to a major metropolitan area underlines the critical function of the Kouri Forest as an essential biogenetic reserve and a vital asset in terms of urban climate resilience. Such a mature, well-structured ecosystem greatly enhances the quality and quantity of ecosystem services available to citizens in Thessaloniki and directly addresses the challenge of low green space within the city and city’s long-term sustainability goals. Despite its small area and the evident pressures, it faces from urban expansion, the results call for a management paradigm that is emphatic and proactive in the frame of sustainable development. This forest should be vigorously conserved to protect its unique constituent ecosystems and maintain its structural integrity. The results strongly argue for the formal recognition of Kouri Forest as a protected area or biogenetic reserve, where sustainable management should ensure the protection of its old-growth features in respect of its long-term ecological function and the continued welfare of the urban community growing around it. The proposed forest recognition and management emphasized that the high ecological value of the forest and prioritizing the important role of such peri-urban forests for ensuring long-term sustainability of society could be considered a key element to sustainable urban development and expansion towards the peri-urban areas. Under current human pressures and climate change demands, effective management of nearby natural resources is a cornerstone for securing cities sustainability.