A Methodological Approach for the Integrated Assessment of the Condition of Field Protective Forest Belts in Southern Dobrudzha, Bulgaria

Abstract

A system of field protective forest belts (FPFBs) was created in the middle of the 20th century in Southern Dobrudzha (Northern Bulgaria) to reduce wind erosion, improve soil moisture storage, and increase agricultural crop yields. Since 2020, prolonged climatic drought during growing seasons and the advanced age of trees have adversely impacted the health status of planted species and resulted in the decline and dieback of the FPFBs. Physiologically stressed trees have become less able to resist pests, such as insects and diseases. In this work, an original new methodology for the integrated assessment of the condition of FPFBs and their protective capacity is presented. The presented methods include the assessment of structural and functional characteristics, as well as the health status of the dominant tree species. Five indicators were identified that, to the greatest extent, present the ability of forest belts to perform their protective functions. Each indicator was evaluated separately, and then an overlay analysis was applied to generate an integrated assessment of the condition of individual forest belts. Three groups of FPFBs were differentiated according to their condition: in good condition, in moderate condition, and in bad condition. The methodology was successfully tested in Southern Dobrudzha, but it could be applied to other regions in Bulgaria where FPFBs were planted, regardless of their location, composition, origin, and age. This methodological approach could be transferred to other countries after adapting to their geo-ecological and agroforest specifics. The methodological approach is an informative and useful tool to support decision-making about FPFB management, as well as the proactive planning of necessary forestry activities for the reconstruction of degraded belts.

1. Introduction

Southern Dobrudzha, situated in the north-eastern part of the Danubian Plain, is the driest region of Bulgaria, resembling a steppe ecosystem [1]. In this region, a system of field protective forest belts (FPFBs) was created in the 1950s to reduce the wind erosion and moisture evaporation leading to soil degradation; to distribute the snow cover evenly across the fields; and to improve the microclimate and increase the agricultural crop yields of the adjacent agricultural lands [2,3,4,5,6]. The system of FPFBs has increased forest cover, enhanced landscape diversity, and improved the habitat quality and connectivity for multiple species. In the regional context, protective afforestation is designed to solve the problems of regulating microclimatic parameters on agricultural land [7,8,9].

The FPFBs are linear forest stands composed of dominant (canopy-defining) tree species, intermediate tree species, and shrubs. The dominant tree species create the canopy, ensuring the height of the belts and providing the greatest extent of field-protective functions, reducing the wind speed. The FPFBs in Southern Dobrudzha are mainly Turkey oak (Quercus cerris L.), honey locust (Gleditsia triacanthos L.), black locust (Robinia pseudoacacia L.), European ash (Fraxinus excelsior L.), white ash (Fraxinus americana L.), common oak (Quercus robur L.), northern red oak (Quercus rubra L.), field elm (Ulmus minor Mill.), and, very rarely, other tree species [10]. The intermediate tree species are usually small-sized trees, whose function is to improve the height growth of the dominant species and their self-pruning. Shrubs are also very often afforested—usually as end rows to protect the young forest belts.

The field protective forest belts in Southern Dobrudzha were created and developed under comparable natural and climatic conditions. There was no significant variation in altitude, relief, exposure, soil conditions, or habitat types [10].

According to their exposition (function), the FPFBs are divided into main and auxiliary. The main FPFBs are exposed perpendicularly to the prevailing north-easterly winds, allowing for a 30° deviation. The auxiliary belts are placed perpendicularly to the main ones, creating a net form. The agricultural fields they formed have dimensions of 1000–2000 m (for the main belts) and 400–500 m (for the auxiliary belts).

In 2022, the FPFBs in Bulgaria covered an area of 10,695.5 ha [11]. They were a part of the Bulgarian State Forest Fund, with public state property, and managed by the State Forestry Enterprises (SFEs) and State Hunting Enterprises (SHEs). The FPFBs were created as meliorative facilities, and the main goal of their sustainable management is to ensure efficient and effective protective functions over as long a period as possible in an appropriate structure, as well as in good health conditions [12,13,14,15,16,17].

The health deterioration of the FPFBs in Southern Dobrudzha has significantly increased over the last decade. This led to the progressive decline in some tree species, resulting from various factors, such as the uneven distribution of precipitation, physiological weakening of trees, and attacks by pests and diseases [11,18]. Since 2020, ash dieback and tree mortality, caused by singing cicadas (Hemiptera: Cicadidae), have significantly increased in the region of SFEs Dobrich and General Toshevo and SHE Balchick [18]. Severe damage caused by Cicada orni L. was most likely due to the weakening of ash trees caused by the worsening environmental conditions and the advanced age of trees. Dutch elm disease has led to the almost complete mortality of the field elm (Ulmus minor Mill.) in the FPFBs. In some years, the spongy moth (Lymantria dispar L.) (Lepidoptera: Erebidae) has caused defoliation of Turkey oak (Q. cerris) in the territory of SHE Balchick [18]. The fungal pathogen Biscogniauxia mediterranea (De Notaris) Kuntze, the causal agent of charcoal canker disease, was established for the first time in Bulgaria on the northern red oak (Q. rubra) in the FPFBs, planted in SHE Balchik and SFE Dobrich. The disease has caused the dieback of infected trees and increased their vulnerability to attacks by xylophagous insect pests [18].

Presently, the most serious problem is the progressive drying of the forest belts that are afforested by ash species (Fraxinus spp.), field elm (U. minor), and northern red oak (Q. rubra), as well as the need to reconstruct large areas of deteriorating belts.

A lack of scientific and practical experience in the afforestation and management processes of the FPFBs in the first decades of their creation was the reason for making serious mistakes: the use of tree species that are ecologically incompatible with the natural conditions of the area; the use of biologically incompatible dominant and intermediate tree species; untimely thinnings were not conducted; etc. [6]. Despite the inappropriate management practices made in the past, valuable experience has been gained over the years. Nowadays, there is a need to develop a comprehensive concept for the management of the FPFBs, including a scientifically based methodology for assessing their condition, a differentiated approach for their management, and the provision of a stable financial mechanism for the restoration of degraded belts and their subsequent reforestation.

This study aimed to develop and test a new methodological approach for an integrated assessment of the condition of the field protective forest belts in Bulgaria and their protective capacity. The main objectives of this study were to identify the main structural and functional characteristics of the FPFBs that contribute to the full implementation of their protective functions; to determine the indicators for categorizing the FPFBs according to their condition; and to test the methodology in different FPFBs, regardless of their tree composition, age, and origin.

2. Materials and Methods

A conceptual scheme showing a structured overview of the steps undertaken for the implementation of the methodology is provided below (Figure 1). The methodology included a sequential application of the following: selection of a basic spatial unit for evaluation; development of a criterion system for assessment; integrated assessment of the FPFBs’ condition; testing the methodology; and results analysis.

2.1. Selection of a Basic Spatial Unit for Evaluation

The main source of information was the Forest Inventory Database of Bulgaria. Digital models of the forest inventory of all State Forest Enterprises or State Hunting Enterprises are freely available on the website of the Executive Forest Agency—Ministry of Agriculture and Food of Bulgaria. In the Forest Inventory Database, each forest belt is distinguished as a separate forest division with a specific number. Usually, each forest belt is divided into several forest subdivisions (on average 2–3), most often due to damage (drying, diseases, fires) or anthropogenic (legal or poaching logging) impacts, which have caused their original composition and/or structure to change in different parts of the belt. For this reason, forest subdivisions of the forest inventory categorization in Bulgaria were used as spatial units for evaluation and analysis in the methodology. Such an approach could ensure connectivity between the FPFBs’ assessments, on the one hand, and the forest management plans, on the other, in order to achieve specific spatial results and facilitate the easy planning of necessary sylvicultural activities.

2.2. Development of a Criterion System for Assessment

Specific metrics were used to identify and measure the condition of the FPFBs (Table 1). A selection of five targeted indicators was organized in two thematic sets, including structural and functional characteristics and health status assessment, which most influence the potential of forest belts to perform their protective functions. The selection of specific indicators was based on the following:

(i)

Classical scientific studies on the optimal structural–functional characteristics of the FPFBs [12,13,14].

(ii)

The reports from the National Conference held in 2002 in Dobrich, Bulgaria, that reviewed the long-term experience and outlined the main problems in the FPFB management [3,6].

(iii)

The data on the periodical monitoring of the FPFBs’ condition carried out by the Executive Forest Agency [11].

(iv)

The current research that examined the contemporary characteristics of the FPFBs in Southern Dobrudzha [10], problems in their current management [10], and deterioration in their health status [18].

The proposed methodology included defined and measurable criteria, which allowed for comparability and analysis of the results obtained.

Table 1. Indicators for the integrated assessment of the condition of the field protective forest belts.

Indicator (Assessment Units Are the Forestry Subdivisions, Forest Inventory of Bulgaria)	Metrics
A. Structural and functional characteristics
Preservation of the dominant tree species, %	well-preserved (≥70%)	partly preserved (50%–70%)	poorly preserved (<50%)
Permeability	permeable	open-worked	impermeable
B. Health status assessment
Defoliation and discoloration of crowns, %	slight (<25%)	moderate (25%–60%)	severe (>60%)
Damage on stems, %	slight (<25%)	moderate (25%–60%)	severe (>60%)
Dieback, %	slight (<25%)	moderate (25%–60%)	severe (>60%)

Table 1. Indicators for the integrated assessment of the condition of the field protective forest belts.

Indicator (Assessment Units Are the Forestry Subdivisions, Forest Inventory of Bulgaria)	Metrics
A. Structural and functional characteristics
Preservation of the dominant tree species, %	well-preserved (≥70%)	partly preserved (50%–70%)	poorly preserved (<50%)
Permeability	permeable	open-worked	impermeable
B. Health status assessment
Defoliation and discoloration of crowns, %	slight (<25%)	moderate (25%–60%)	severe (>60%)
Damage on stems, %	slight (<25%)	moderate (25%–60%)	severe (>60%)
Dieback, %	slight (<25%)	moderate (25%–60%)	severe (>60%)

2.2.1. Thematic Set A: Structural–Functional Characteristics

• Indicator 1: Preservation of the dominant tree species.

The dominant tree species in the FPFBs were afforested in rows according to a certain scheme and density (number of rows, inter-row spacing, and distance between trees in the row). Currently, the proportion of the main tree species in the structure of the FPFBs has significantly decreased. The number and integrity of the rows of the dominant tree species have been disrupted by various natural factors (outbreaks of pests and diseases) and human-induced disturbances (fires; felling with subsequent unsuccessful shoot regeneration; afforestation of biologically incompatible tree species together, e.g., R. pseudoacacia with G. triacanthos, Fraxinus spp. with R. pseudoacacia; thickening of the belts with shrubs, some of which, with their rapid growth at a young age, have displaced the dominant tree species; failure to carry out timely cultivation activities; etc.) [6]. In about 4% of the belts, the dominant species have already completely disappeared [10]. Indicator 1 measures the extent to which the participation of the dominant tree species in the belt is preserved. Three grades were adopted—well-preserved (row integrity preserved by at least 70%), partly preserved (row integrity preserved between 50% and 70%), and poorly preserved belts (row integrity preserved below 50%) (Table 1).

• Indicator 2: Permeability of the belt.

The efficiency of the field protective forest belts depends on their permeable structure [7,19,20], which determines their ability to pass part of the airflow through them, to reduce wind speed and evaporation from adjacent agricultural areas, and to distribute the snow cover evenly across a field. According to their permeability, the FPFBs were divided into three groups [12,13,14]:

• Impermeable belts—Impermeable belts are dense, leafy from the ground to the top. Most often, they are three-layered, consisting of dominant tree species forming the canopy, intermediate tree species forming the subcanopy, and shrubs forming the understory. The dominant species have highly branched crowns. The belts’ widths are usually over 15 m. Because of the high foliage rate of the vertical profile, the wind passes through the belts to a negligible extent. During the growing season, they pass a maximum of 15% of the airflow. After the autumn leaf fall, their wind permeability increases by about 20%–25%. The retained airflow passes over impermeable belts like airflow passing over a dense forest massif. The wind begins to slow down even before the belt itself. Inside the belt, the wind decreases sharply, and a certain degree of calm sets in. Behind the calm zone, an air gap is formed, after which the wind speed increases rapidly, and, at a distance of 20–30 times the height of the tree stand, it equals its speed in an open field (Figure 2). Because of the formed air gap, the snow in impermeable belts reaches no further than 10–15 m away from the belt, and its distribution is not allowed on the neighboring fields. Impermeable belts are suitable for snow retention along roads and railways, but not for the protection of agricultural areas, where an even distribution of the snow cover over the area of agricultural fields is necessary.
• Open-worked belts—Open-worked belts are less dense than impermeable belts. The dominant trees have trunks cleared of branches—the height of the self-pruned trunk is from 1/3 to 1/2 of its height. There are intermediate tree species or high shrubs forming the understory. In the leafy state, openwork belts let in from 15% to 35% of the airflow, and the rest of it passes over the belt. Because of their higher air permeability, the wind passes through openwork belts and reduces its speed, but to a lesser extent and not as quickly as the wind passing through impermeable belts. The influence of openwork belts on the wind speed ends at a distance of approximately 30–35 times the height of the belt (Figure 2).
• Permeable belts—In permeable belts, the dominant trees have trunks cleared of branches to at least 1/3 of their height; they are single-story, without understory and shrub species. They often have a small number of rows and a small width. In the leafy state, they let in more than 35% of the airflow. The wind entering permeable belts is divided into approximately two equal parts. One of them passes through the belts and slows down its movement, and the other passes over the crowns and partially through them. Permeable belts do not reduce the wind speed above them. In them, as well as behind them, the wind speed is close to that in an open field. Behind them, the wind speed decreases and then gradually increases, but more slowly compared to the increase in impermeable belts. The influence of permeable belts on the wind speed extends over a greater distance than behind the belts with a denser structure. Permeable belts ensure an even distribution of snow cover, temperature, and evaporation and help increase crop yields. When managing FPFBs, the goal is to achieve and maintain a permeable structure in order to achieve maximum field protection and economic effect. The large participation of intermediate and shrub species and the failure to carry out thinning activities led to the formation of dense belts with an impermeable structure.

2.2.2. Thematic Set B: Health Status

In this thematic group, the defoliation and discoloration of the tree crowns and the damage caused by insect pests, fungal pathogens, and abiotic factors to the stems and branches of the trees in the FPFBs were assessed.

• Indicator 3: Defoliation and discoloration of tree crowns.

Defoliation and discoloration was defined as leaf and color loss in the assessable crown as compared to a reference tree, following the methodology of ICP Forests [22]. The defoliation and discoloration of 40 trees was assessed in 5% steps, ranging from 0% (no defoliation and discoloration) to 100% (dead tree). The average defoliation and discoloration of tree crowns in the FPFBs was calculated as a weighted average of the individual values of the forty surveyed trees. Depending on the mean value of the defoliation and discoloration, the FPFBs were grouped into the following three categories: slight (<25%), moderate (25%–60%), and severe (>60%).

• Indicator 4: Damage to stems.

Damage to stems was assessed according to the signs and symptoms of attacks by insect pests, fungal pathogens, and abiotic factors. Particular attention was paid to damage to non-renewable tissues (bark, phloem, and sapwood)—wounds, cracks, boring holes, dust, galleries of xylophagous insects, mycelium, rhizomorphs, and fruiting bodies of fungal pathogens, etc. [22]. Depending on the severity, the tree trunks were assigned to the following three categories of damage: slight (<25%) (Figure 3A), moderate (25%–60%) (Figure 3B), and severe (>60%) (Figure 3C). An exception was made for healed wounds that did not significantly worsen the health and physiological condition of the trees (e.g., callused frost cracks on the stems of Q. cerris) (Figure 3D).

• Indicator 5: Dieback.

Crown dieback was assessed as the drying (loss) of the tops of stems, branches, and young shoots as compared to a reference tree (Figure 4A) [22]. Depending on the severity, the dieback was assigned to the following three categories: slight (<25%) (Figure 4B), moderate (25%–60%) (Figure 4C), and severe (>60%) (Figure 4D).

2.3. Integrated Assessment of the FPFBs’ Condition

Depending on the above five indicators, the FPFBs were differentiated into three groups by their condition: good, moderate, and poor. According to the integrated assessment of the five indicators, the FPFBs were categorized into the following groups:

• Group 1: Belts in good condition.

This group included the FPFBs in which the dominant tree species were well preserved (Indicator 1 was at least 70%) and were in good health condition (Indicators 3, 4, and 5 were up to 25%).

Subgroup 1.1. included belts in good condition with a permeable or open-worked structure.

Subgroup 1.2. included belts in good condition with an impermeable structure.

• Group 2: Belts in moderate condition.

This group included the FPFBs that met one of the following conditions:

• The dominant tree species were partially preserved (Indicator 1 was between 50% and 70%) and were in good or moderate health condition (Indicators 3, 4, and 5 were up to 60%).
• The dominant tree species were well or partially preserved (Indicator 1 was at least 50%) and were in a moderate health condition (at least one of Indicators 3, 4, and 5 was between 25% and 60%, with none of them exceeding 60%).

Subgroup 2.1. included belts in moderate condition with a permeable or open-worked structure.

Subgroup 2.2. included belts in moderate condition with an impermeable structure.

• Group 3: Belts in poor condition.

This group included the FPFBs that met at least one of the following conditions:

• The dominant tree species were poorly preserved (Indicator 1 was below 50%);
• The dominant tree species were in poor health condition (at least one of Indicators 3, 4, and 5 was above 60%).

The belts in good condition (Group 1) had a well-preserved dominant tree species, which was also in good health. They had the capacity to perform their protective functions well and for a relatively long period. The belts in moderate condition (Group 2) had a reduced participation of the dominant species and/or had some health problems. These belts had reduced resilience, and their condition was expected to deteriorate in the future. The belts in poor condition (Group 3) had too little participation of the dominant species and/or were in poor health. In practice, without dominant tree species, the FPFBs were degraded and did not provide their protective functions because they did not have enough height.

Group 1 and Group 2 were divided into two subgroups depending on their wind permeability (Indicator 2). The aim was to separate the impermeable belts, in which it is necessary to carry out thinning to reduce their density. It was not necessary to divide Group 3 into subgroups according to permeability since it included degrading belts that must be completely reforested.

The research approach aims to define an appropriate scientific basis for identifying the main problems in different kinds of FPFBs, evaluating their protective capacity, analyzing the information, and presenting the results to decision-makers and stakeholders. Another important objective of this approach was to establish an appropriate basis for the systematization of results at different spatial scales and the comparability of assessments at the regional and national levels.

2.4. Testing the Methodology

The proposed methodology was tested in 2023 in 190 field protective forest belts with a total area of 283.6 ha in Southern Dobrudzha (Northeastern Bulgaria). The studied sites were evenly distributed across the territory of three state enterprises: SHE Balchik, SFE General Toshevo, and SFE Dobrich (Figure 5). These three enterprises were selected because 72% of all the FPFBs in the country are located on their territories [11].

Each studied site is one forest subdivision in a different forest belt (see Section 2.1). Since the average area of subdivisions in the FPFBs in Bulgaria is 1.2 ha [10], the sites were selected to have an area between 1 and 2 ha. This research was conducted over the entire area of the subdivisions.

The studied sites were selected to correspond to the relative share of the dominant tree species, origin, and age of the belts in the country [10]. According to the dominant tree species, the number of the studied sites was distributed as follows: ashes (Fraxinus excelsior and F. americana)—99; oaks (Q. cerris, Q. robur, Q. rubra)—52; honey locust (G. triacanthos)—19; black locust (R. pseudoacacia)—12; and others (U. minor, Juglans regia L., Acer pseudoplatanus L., Cedrus atlantica Endl.)—8.

According to the origin of the tree stands, the studied sites were distributed as follows:

• − A total of 130 seed belts (68%), proportionally distributed among all tree species;
• − A total of 60 coppice belts (32%) of ashes, honey locust, black locust, and field elm.

For the seed FPFBs, the distribution of the studied sites by age was as follows: 18 belts (14%) up to 20 years old; 34 belts (26%) from 20 to 60 years old; and 78 belts (60%) over 60 years old.

For the coppice FPFBs, the distribution of the studied sites by age was as follows: 7 belts (12%) up to 10 years old; 31 belts (51%) from 11 to 30 years old; and 22 belts (37%) over 30 years old.

According to the belts’ exposition (function), the studied sites were distributed as follows: 123 studied sites in main forest belts (exposed perpendicularly to the prevailing north-easterly winds) and 67 sites in auxiliary belts (placed perpendicularly to the main ones).

For each of the 190 studied sites, the five evaluation indicators (Table 1) were determined in the field, and an integrated assessment of their condition was made according to the classification scheme proposed in Section 2.3. This study performed a preliminary test of the proposed criteria and parameters for assessment of the FPFBs’ condition with the appropriate level of detail and accuracy.

3. Results

3.1. Results per Tree Species

The results of the integrated assessment of the condition of the studied FPFBs are presented in Figure 6. The majority of the belts of Q. cerris and Q. robur were in good condition (88% and 75%, respectively), and the rest were in moderate condition. No belts were found in poor condition. Of the honey locust (G. triacanthos) belts, 74% were in good condition. A small percentage (5%) of the honey locust belts were in poor condition, which were of coppice origin. Two-thirds of the studied northern red oak belts (Q. rubra) (67%) were in good condition, and the remaining 33% were in poor condition. Between 42% and 47% of the black locust and ash belts were in poor condition, and 39%–42% were in moderate condition, as biotic damage was reported in them. Only about 15% were in good condition, and these were mainly young stands, which had higher vitality. All surveyed field elm belts were in poor condition.

3.2. Contribution of Individual Indicators

The contribution of individual indicators to establishing the deterioration causes of the condition of the FPFBs in the formation of the integrated assessment was analyzed (Figure 7).

The participation of Q. cerris in the FPFBs was well preserved (98%). The health condition of the trees was assessed as good (95% of the belts had no or a low degree of defoliation and dieback, and the rest, amounting to 5%, were of a medium degree). Frost cracks were found on most of their stems, but they were callused and did not threaten the trees’ vitality. The main problem in the Q. cerris belts was their high density due to the large amount of intermediate tree species and shrubs. A share of 43% was impermeable, and thinning activities are needed to form a permeable structure (Figure 7a).

Quercus robur trees were well preserved in 75% of the studied belts and partially preserved in the remaining 25%. They were assessed as resistant, and unlike Q. cerris trees, they had no damage from frost. All studied belts showed a low damage degree on the stems and dieback, but 25% of the trees were assessed as moderately damaged. The percentage of impermeable belts was very high (75%). This was due to the large and highly branched crowns that the species had formed, as well as the many other trees and shrubs that were afforested together with them (Figure 7b).

Gleditsia triacanthos was resistant to biotic damage, as 95% of the belts were assessed to be in good health (Figure 7c). However, in a quarter of the belts, its preservation was partially or severely impaired. This was mainly observed in the honey locust belts and was the result of a regulatory weakness that allows their clear-cutting at the age of 50, with subsequent shoot regeneration. The G. triacanthos belts were mostly impermeable (75%). Thinning activities were not avoided in them, probably due to the large and dangerous thorns on the stems.

Quercus rubra was very well preserved in the FPFBs, but one-third of the studied belts were in poor health (Figure 7d). The cause for their deterioration was ‘charcoal’ disease, caused by the fungal pathogen B. mediterranea. The percentage of their permeability was higher compared to other species. They usually were not afforested with shrubs, and regular thinning was carried out in them.

The health condition of the R. pseudoacacia belts was assessed as poor (Figure 7e). This was due to both the impaired preservation of the species in them and problems with their health status. These belts were of coppice origin and were already at 2–3 coppice rotations. The tree stands were obviously in poor condition; their resistance and vitality were greatly reduced. They were of a very low height, which practically makes them unfit to provide their protective functions. In addition, they suffered greatly from dry rot and a process of decay. The majority of them (59%) were impermeable and had a shrubby appearance.

The ash (Fraxinus spp.) belts were in a severely deteriorated health condition. Since 2020, they had begun to dry out, with defoliation reaching 80%–100%. The causative agents were singing cicadas (Hemiptera: Cicadidae) and other pathogens, which are considered opportunistic and usually damage trees under stress conditions. The ash trees in the belts died at both a young and mature age. Due to sanitary thinning, the permeability of F. excelsior belts was good—only 28% were impermeable (Figure 7f). In the case of the F. americana belts, the permeability was much worse (67% were impermeable) (Figure 7g) since they were mainly of coppice origin and no thinning was carried out in them.

The belts of Ulmus minor were in an extremely poor condition—both in health condition and preservation of the dominant tree species (Figure 7h). They were practically completely degraded. The elm trees died because of Dutch elm disease (Ohiostoma novo-ulmi). The reduction in the canopy allowed the understory species to be impermeable. However, the understory was of low height and did not have the capacity to perform field-protective functions.

3.3. Territorial Analysis of the Results

The proposed methodology allows for the preparation of maps showing the integrated assessment of the FPFBs. In the belts of the four most widely distributed tree species (Q. cerris, G. triacanthos, R. pseudoacacia, and F. excelsior), the integrated assessment did not depend on their spatial location (Figure 8). The belts of Quercus cerris (Figure 8A), G. triacanthos (Figure 8B), and R. pseudoacacia (Figure 8C) were predominantly in good and moderate condition. The belts of Fraxinus excelsior were mainly in poor condition, and they were relatively evenly distributed over the studied territory in SHE Balchik, SFE Dobrich, and SFE General Toshevo (Figure 8D). Since the tree composition of the belts was the same in the three forest enterprises, the problems in them were universal for the Southern Dobrudzha region, and common solutions must be sought.

Nevertheless, the possibility of preparing maps showing the condition of the belts at different territorial levels (land of a settlement, forestry enterprise, region, etc.) significantly facilitates forest management decision-making and the planning of necessary forestry works in the belts.

3.4. Planning of Forestry Operations

The proposed methodology provides information on the type of forestry activities that should be carried out to improve the condition of the FPFBs. In Subgroup 1.2., including the belts in good condition with an impermeable structure, and Subgroup 2.2., including the belts in moderate condition with an impermeable structure, thinning cuts should be carried out to form a permeable structure. They are mainly carried out by removing shrubs and part of the intermediate tree species. At the same time, sufficient growth space for the dominant tree species and conditions for self-pruning of the trees, by at least one-third of the height of the stem, should be ensured. In Subgroup 2.2., in addition to increasing permeability, thinning cuts should also have a sanitary function by removing diseased and damaged trees to improve the health of the tree stand.

Thinning measures are necessary in all the belt types (the dotted areas in the columns in Figure 6). The failure to carry out thinning greatly reduces the capacity of the belts to perform their protective functions. In addition, it leads to a deterioration of the growth conditions for the main tree species, and hence their health. Thinning is needed to prevent further deterioration of the belts in moderate condition (Group 2) and their transition to Group 3, or at least to slow down this process as much as possible.

The belts in poor condition (Group 3) have too little participation of the dominant species, and they are in a poor health condition. These are shown in the red areas in the columns in Figure 6. In practice, without dominant tree species, the belts are degraded and do not provide their protective functions because they do not have enough height. So, reforestation is the only way to improve these belts. Reforestation is necessary in the large areas of the ash, elm, black locust, and northern red oak belts. Moreover, in most cases, the dominant tree species needs to be changed, as ash, elm, and northern red oak have suffered from diseases and pests, and black locust is unsuitable as a dominant tree species in the FPFBs in Dobrudzha.

In addition to the type of forestry activities required, the methodology could also provide information on their volume. If the methodology is applied to assess the condition of all belts in the country, it will provide complete data on the type and volume of the activities required in absolute values (hectares).

4. Discussion

The results provide strong support for the current assessment approach, showing that there are no differences among the integrated assessment of the condition of the studied tree species in the field protective forest belts in the territory of the three State Forest/Hunting Enterprises (Figure 8). This finding validates our work and the decision not to include indicators that reflect the natural and climatic conditions of the studied area.

The field protective forest belts in Bulgaria were created and developed under comparable natural and climatic conditions. There is no significant variation in altitude, relief, exposure, soil conditions, or habitat types, which would be a prerequisite for differences in their condition or management [10]. For this reason, the methodology does not include indicators reflecting natural and climatic conditions.

The five indicators were formulated to be familiar and understandable to forest workers so that they can be quickly and easily applied in practice.

Initially, we defined more indicators for assessment, which we measured during field research. However, in the final version of the methodology, some of these indicators were not included, as it turned out that they were not decisive for the condition of the belts or that their influence was significantly accounted for by the five indicators included. Below, we describe some of the indicators that were dropped in the process of this work and the reasons for this:

Exposition (function) of the belt—There are no differences whatsoever in the condition of the main and auxiliary belts. For this reason, this indicator was dropped from the methodology. At the same time, when planning forestry activities to improve the condition of the belts (thinning and reforestation), priority should be given to the main belts since they protect against the prevailing winds.

Preservation of the belt pattern—In the field, we determined to what extent the integrity of all the rows in the belts (rows of the dominant, intermediate, and shrub species) is preserved. In practice, however, mostly the dominant tree species provide the protective functions of the belt. The understory tree species serve to improve their growth at a young age, and the shrubs are afforested as outer rows to protect the young belts. For this reason, instead of the preservation of all the rows in the belts, we included only the preservation of the rows of the dominant tree species in the methodology (Indicator 1). The reduced participation of the intermediate and shrub species does not worsen the condition of the belts, but often, even on the contrary, improves it, since it increases their wind permeability.

Average height of the belts—The average height of a belt is a very important indicator since it depends on how far the belt’s influence on wind speed extends (Figure 2). Figure 2 shows that this influence reaches a distance of about 30 times the height of the tree stand. Given that the average distance between the main belts is 400–500 m, the height of the tree stand must be at least 14 m to have an effect on the entire agricultural field. After 40 years of age, all of the dominant tree species used reach such a height that makes the inclusion of this indicator in the methodology meaningless.

Origin of the dominant tree species (seed or coppice)—Initially, all the belts were of seed origin. After the mid-1980s, sanitary felling began in part of the ash and elm belts to address the deterioration in their health, and they were mainly regenerated by coppice stands [10]. The belts of R. pseudoacacia and G. triacanthos are coppice-managed, with a rotation of 15 years and 50 years, respectively. We consider that this was a serious regulatory error, as it did not meet the main purpose of the field protection belts. However, the origin of the tree stands in itself is not decisive for the condition of the belts. If the dominant species has been successfully regenerated by coppice and is in good health, then it performs its field protection functions just like a tree stand of seed origin. The main problem with coppice belts is the extent to which the participation of the dominant species has been preserved, and this is reflected in Indicator 1. Also, coppice stands are undoubtedly more unstable and short-lived than seed stands. But the deterioration in their health is reflected in Indicators 3, 4, and 5. That is, the main negative effects of coppice origin are reflected in the indicators set out in the methodology.

Number of coppice rotations—Most coppice belts are on one coppice rotation. The exception is the honey locust belts, which are coppice-managed with a rotation of 15 years; as a result, three coppice rotations have already been carried out in most of them. In the third rotation, a stand is obviously in very poor condition. It has reduced coppice productivity, reduced vitality, and a very low height, which practically makes it unfit to fulfill its main purpose. Reduced shoot productivity and vitality are successfully accounted for by Indicator 1. This, as well as the fact that the number of coppice rotations is different only for the honey locust belts, suggests that this indicator does not need to be assessed separately in the methodology.

Age of the dominant tree species—The age of the stands is an important indicator since, in the dry growing conditions of Dobrudzha, most of the tree species used are unlikely to be long-lived. At present, the maximum age of the field protective forest belts in Bulgaria is 70 years for seed belts and about 40 years for coppice belts. A mass process of natural deterioration has not yet been observed, from which we can judge the maximum age to which they survive. The mass drying of the ash, elm, and red oak belts is due to diseases and pests that affect all age groups equally, i.e., it does not depend on age [18]. At the same time, the belts of Q. cerris, Q. robur, and G. triacanthos are in good condition (Figure 4 and Figure 5) and have the capacity to show relative longevity. For these reasons, no reliable age criteria can currently be established to determine the condition of the FPFBs in Bulgaria.

Frost cracks—Severe damage to the stems of Q. cerris was reported, but the stems were callused, and the damage did not threaten the health of the belts.

Insect pests and fungal pathogens—The presence of aggressive insect pests and fungal pathogens that can cause mass damage to trees is an indicator that the condition of the belts will seriously deteriorate in a short time. However, the impact of destructive insect pests and fungal pathogens is indirectly expressed in the values of Indicators 3, 4, and 5.

The FPFBs are linear forest stands surrounded by extensive agricultural areas. For this reason, the populations of forest pests and pathogens are concentrated in the relatively small area of the belts and, accordingly, the extent of the damage they cause could be very high. In addition, because of the dry growing conditions and the deteriorated physiological condition of the tree stands in the belts, forest pests that are usually not considered too dangerous can cause serious damage and mass drying of the belts. This has led to the most serious problem at the moment—the mass drying of the ash forest belts in South Dobrudzha, caused by singing cicadas (Hemiptera: Cicadidae) [18]. Although the pathogens are different, the causes of the massive health deterioration in the FPFBs are common; these include the extremely dry growing conditions (reduction in the amount of precipitation and its uneven distribution throughout the year, the presence of prolonged periods of drought and heat waves), which lead to the physiological weakening of the tree stands.

The FPFBs in Southern Dobrudzha were artificially afforested in the mid-20th century on agricultural lands. Before that, there were no forests in these areas. The growing conditions are almost steppe, and natural forest tree vegetation is observed only in some small valleys. The natural regeneration of the afforested tree and shrub species is almost unnoticeable, so the stand dynamics are determined only by the afforested individuals.

A lack of scientific and practical experience in the afforestation and management processes of the FPFBs in the first decades of their creation was the reason for making serious mistakes. The oversaturation of the belts with intermediate species and shrubs, some of which, with their rapid growth at a young age, drowned out the dominant tree species. Timely and systematic thinning fellings were not conducted in a larger part of the FPFBs in order to give the dominant tree species growth space. The large participation of intermediate and shrub species, and the failure to carry out thinning, led to the formation of dense belts with an impermeable structure. As a result, the dominant tree species were suppressed in some places, and their participation in the composition was reduced. The function of intermediate tree species and shrubs is to improve the height growth of the dominant species and their self-pruning at an early age. However, most of the stands are now about 70 years old, and the intermediate species form a dense understory, which leads to an impenetrable structure of the belts.

After the death of the dominant tree species, the intermediate species cannot replace their field-protective functions because they are of small height. For these reasons, thinning should be primarily aimed at removing intermediate tree species and shrubs in order to provide more favorable conditions for the dominant species and reduce the density (increase the permeability) of the belts.

In addition, according to Bulgarian forest regulations, pruning the edge rows of the FPFBs is permissible. Earthen or stabilized roads are usually located parallel to the belts. Large-sized agricultural machinery moves along these roads to process the adjacent agricultural fields, as well as the heavy machinery (including cranes) that serves the many wind power generators that have been built. Pruning the edge rows makes it possible to move this machinery and reduces conflicts with the owners and tenants of the agricultural land and wind generators.

5. Conclusions

The developed methodology is an approach for the integrated assessment of the condition of the FPFBs in Bulgaria; it serves as a practical tool for foresters and decision-makers in management, territorial planning, and development. The methodology was successfully tested in Southern Dobrudzha, and the results obtained are reliable and logical.

The methodology has a universal nature, applying to all the FPFBs in Bulgaria, regardless of their tree composition, age, and origin. It is easy and practical enough to be used in forestry practice, and it is very suitable for monitoring the condition of all the forest belts in the country, which the Executive Forestry Agency carries out annually. Based on the results obtained, multifactorial analyses and decisions can be made for the management of the FPFBs at different levels (forestry state enterprise, region, country). The methodology can also be applied in other countries where similar FPFBs have been established. The proposed approach for the integrated assessment of structural–functional characteristics and health status of forest belts can be applied in other countries after adaptation to their specific characteristics and natural conditions.

The methodology is a cost-effective and informative methodological approach to support decision-makers in the country’s forestry sector. It has a proactive nature—it allows for early identification of the main problems in field protective forest belts and their intensity, as well as planning the type and volume of the necessary forestry activities (cultivation and reconstruction) in them. The results obtained can justify the choice of the most suitable species for the reconstruction of degraded belts, as well as for the possible expansion of the system of FPFBs in the country.

The results of the application of the methodology provide fundamental information about the capacity of the belts to fully perform their protective functions. In this regard, it is a tool that can be implemented to protect agricultural land and soil and prevent desertification in conditions of climate change. It can be used to attract the attention of governing bodies for sustainable territorial development and guarantee the food security of a country. Its results can be used to raise awareness among the owners and tenants of agricultural lands, various stakeholders, and the general public about the functions of FPFBs, thus enriching and strengthening society–forest interrelationships.