TREES AND SHRUBS SUITABLE FOR THE CONSTRUCTION OF AGROFORESTRY SYSTEMS IN A TEMPERATE CLIMATE AND THEIR USE IN ECONOMIC ACTIVITIES: A STUDY IN BULGARIA

). Trees and shrubs suitable for the construction of agroforestry systems in a temperate climate and their use in economic activities: a study in Bulgaria

Introduction.Agroforestry has become one of the sustainable approaches for achieving sustainable development in recent years.It refers to land use practices and land-used management systems where pasture or crops are combined with trees and shrubs [1].Furthermore, Burgess & Rosati [2] consider agroforestry as "the practice of deliberately integrating woody vegetation (trees or shrubs) with crop and/or animal systems to benefit from the resulting ecological and economic interactions".
The use of agroforestry as an economic activity emerged in the 1980s.However, there is evidence that agroforestry has been practiced since ancient times [3].By integrating different production actions, agroforestry systems (like forest-farming, alley, and forest-pasture agroforestry systems) allow the diversification of economic activities on the land, raising profitability per unit of area, and decreasing the risks of income losses caused by adverse market conditions and weather events [4].These characteristics demonstrate their importance in agricultural sustainability, social inclusion, and climate change issues.
Review of literature.Agroforestry research is diverse.Some papers focus on agroforestry's contribution to livelihoods and carbon sequestration [5] as well as to government policy for stimulating agroforestry in different counties [6].Others discussed the standardization and certification of agroforestry systems [7; 8], different partnerships and cooperation [9], various agroforestry systems like forest-farming [10], alley-cropping agroforestry systems or intercropping [11], silvopastoral systems/forest-pasture agroforestry systems [12; 13].There are differences between forest-farming, alley-cropping, and forest-pasture agroforestry systems.Forest farming is a type of agroforestry practice that cultivate high-value crops under the protection of a managed tree canopy.Alley-cropping or intercropping is another type of agroforestry practice that places trees within agricultural cropland systems in order to improve whole-farm yield.And forest-pasture agroforestry system or silvopastoral system is the practice of integrating trees, forage, and the grazing of animals in a mutually beneficial way, existences for millennia.
A few scientific researches focus on tree species suitable for agroforestry systems in particular counties (e.g., woody plant species in North-Western Himalaya, India [14] and trees and shrubs species in home gardens in Kampung Masjid Ijok Perak, Malaysia [15]).However, there is limited research on different trees and shrubs suitable for constructing agroforestry systems in a temperate climate, especially in Bulgaria.Moreover, the use of the products of these trees and shrubs in various economic sectors and activities, which can bring additional economic value for agroforestry farmers, is not discussed in one paper.
The current study aims to fulfill the above research gap and challenge by discovering the various trees and shrubs appropriate for building agroforestry systems in Bulgaria's temperate climate and presenting their possible use in various economic activities.Furthermore, the contributions of Bulgarian agroforestry systems to sustainable development have been analyzed.The case study method has been applied to gather the necessary information.The methodology and main findings are presented in the following sections.
Materials and methods.The study aims to (i) discover the various trees and shrubs appropriate for building agroforestry systems in Bulgaria's temperate climate and their possible use in various economic activities, (ii) identify the contribution of Bulgarians' agroforestry systems to the three dimensions of sustainable development.
Based on this research goal, the following two research questions are formulated: Q1.How do agroforestry systems contribute to sustainable development in Bulgaria?
Q2.What are the main types of tree species suitable for building agroforestry systems in Bulgaria, a country with a temperate climate?
The case study method was chosen to investigate various agroforestry systems in Bulgaria, the suitable tree species for their constructions, and their contributions to three aspects of sustainable development [16].The study was conducted in Bulgaria in 2019, a leading country in the European Union in agroforestry systems [17][18][19][20].The 17 top agroforestry experts and farmers from 14 leading organizations in the agroforestry field in Bulgaria were interviewed (see Appendix 1, Table A.1).They were selected based on three key criteria: (1) their extensive experience (above 7 years) in agroforestry systems and tree species; (2) their comprehensive and in-depth knowledge of agroforestry systems; and (3) the organization in which they work, i.e., the leading organizations in the field of agroforestry located in different regions of the country have been selected.
Semi-structured interviews were conducted with the respondents to allow them to freely and unrestrictedly answer ten identical, theoretically related questions based on their experience and knowledge in the agroforestry area.Four of these 10 questions were related to types of Bulgarian agroforestry systems and used tree species in them, as well as their contribution to sustainable development and society.The average duration of each interview was about 50 minutes.Face-to-face and telephone interviews were recorded with the respondents' consent and transcribed afterward.This tactic ensures that the reliability and validity of the study increase.A significant number of secondary documents (e.g., internal press releases, presentations, reports, newspapers, plans, and case studies of agroforestry systems: 40 documents, about 600 A4 pages) were collected, which supplemented the interview information and further increased the study's reliability and validity.Additionally, the respondent from the 12th organization provided (by email) above 100 pages of primary documents containing information about Bulgarian's most famous trees and shrubs.Moreover, it was conducted a field study in regions with agroforestry systems (including in the Rhodope Mountains) in July and August 2019, and June-August 2020.Thus, it was observed the various agroforestry systems, trees, and shrubs there (see Appendix 1, Table A The gathered information is managed as follows.The contributions of Bulgarian agroforestry systems to sustainable development have been grouped into the three dimensions of sustainable development: social, environmental, and economic.The information about tree species has been carefully analyzed.As a result, the tree species are classified into three main groups: trees with short rotation (between 2-4 years), trees with medium and long rotation (from 4 to 20 years), shrubs with medium and long rotation (above 2 years).

Results and discussion. The contribution of Bulgarian's agroforestry systems to
Journal of Innovations and Sustainability ISSN 2367-8151 2023, Vol. 7, No. 3 https://is-journal.comsustainable development.Agroforestry and agroforestry systems fully meet the concept of sustainability in its entirety [4].According to the respondents in the present study, agroforestry and agroforestry systems in Bulgaria contribute to the three dimensions of sustainable development as follows: Regarding the social dimension of sustainability, the respondents highlighted the social benefits of agroforestry.Agroforestry farming contributes to "raising the standard of living and the social status of the population" (Interviewee 12), "providing jobs and access to renewable energy" (Interviewee 8), "overcoming social inequality in individual regions" (Interviewee 3).Bulgarian mountains and agroforestry systems 1) "provide recreation opportunities for the population" (Interviewee 7.1), 2) "give social, educational, scientific, landscape and recreational benefits to society" (Interviewee 7.2), and 3) "generate firewood for society" (Interviewee 9).Furthermore, there are systems of protective forest belts in Bulgaria that protect against the strong winds in the northern part of the country (Interviewee 8; Interviewee 12, Interviewee 11).
The contribution of agroforestry in the environmental dimension is unquestionable.Agroforestry farming contributes to "environmental protection and the conservation of natural resources" (Interviewee 11), "improving environmental conditions and the sustainability of ecosystems" (Interviewee 6.1), "preventing erosion, improving the microclimate through conditions of a lasting tendency to warmth and dryness in Bulgaria, filtering air and water, providing shelters and protection to wild animals and birds" (Interviewee 8).
Finally, in the economic dimension, agroforestry agriculture, by not using pesticides or chemical fertilizers, reduces production costs.Agroforestry contributes to "increase the production of timber and non-timber forest products" (Interviewee 7.1).More revenue is generated per unit area via growing several plant species in one place as each plant species generates revenue streams (Interviewee 6.1; Interviewee 6.2; Interviewee 6.3; Interviewee 4; Interviewee 11; Interviewee 12).Therefore, agroforestry production in the mid-term period is no more expensive than conventional agriculture, and still has the advantage of creating life and not destroying it [21].
Main types of trees and crops suitable for the construction of agroforestry systems in Bulgaria.Tree species for agroforestry in Bulgaria.Tree species suitable for agroforestry in Bulgaria can be divided into three main groups: trees with short rotation, trees with medium and long rotation, shrubs with medium and long rotation.These groups are discussed in detail below.

Tree species with short rotation
The use of tree parts in particular economic activities (examples) Possible agroforestry system White willows (Salix alba L.) The wood/timber of the tree is used: i) for making small accessories; and ii) in the packaging.The leaves of the tree are utilized as fodder for domestic animals Coastal protection belts, plantations for biomass production, forest-farming agroforestry systems Basket willows (Salix viminalis L.) Cultivated for extraction of basketry rods, fiber for ropes, and others Forest-farming agroforestry systems

White poplars and aspens (Leuce Duby)
The wood/timber of the tree is used in the pulp and paper industry to manufacture various objects as well as in construction Coastal protection zones, plantations for biomass production, alley-cropping agroforestry systems Black poplars (Aigeiros Duby) The wood/timber of the tree is used in the pulp and paper industry to manufacture various objects as well as in construction and packaging Alley-cropping agroforestry systems, coastal protection zones, and the creation of plantations for biomass production

Balsamic poplars (Tacamahaca Spach)
The wood/timber of the tree is used in the pulp and paper industry to manufacture various objects as well as in construction Sources: created by the author based on the information from the interviews and primary sources, collected via the study.
The wood/timber of some of the above mentioned trees is used in i) furniture

Journal of Innovations and Sustainability
ISSN 2367-8151 2023, Vol. 7, No. 3 https://is-journal.comproduction (black (sticky) alder); ii) for making small objects/accessories (white willow); iii) the pulp and paper industry to manufacture various objects (e.g., poplars, aspens, black (sticky) alder); iv) in construction (e.g., poplars, aspens, black (sticky) alder); v) in the packaging (e.g., white willow, black poplar).The fruits of black (sticky) alder trees are used in medicine.The leaves of a piece of the above trees are utilized as fodder for domestic animals (white willow); dyeing, and in medicine (black (sticky) alder).
The trees, as mentioned above, can be applied in agroforestry to i) create protective belts (e.g., willows, poplars, white acacia, black (sticky) alder, the plane trees); ii) in forest-farming agroforestry systems (e.g., willows, white acacia, the plane trees, the hairy paulownia); iii) in forest-pasture agroforestry systems (the hairy paulownia); iv) in alley-cropping agroforestry systems (e.g., poplars, the plane trees, the hairy paulownia).A piece of the trees is suitable to create plantations for biomass production (e.g., willows, poplars, white acacia, black (sticky) alder, the plane trees, the hairy paulownia).
The wood/timber of some of the above-mentioned tall trees is used in i) furniture production (genus Oak (Quercus), walnut, maple (genus Acer), breccia, chestnut, birch, ordinary persimmon, coniferous species); ii) for making small objects/accessories (hornbeam) and parquet (walnut); iii) in construction (gledichiya, chestnut, coniferous species); iv) in carpentry (white alder, mahalebka, sumac); v) in turning (walnut) and carving (linden); vi) for making musical instruments (walnut, maple (genus Acer), breccia, chestnut, ordinary persimmon, coniferous species); shipbuilding (chestnut); vii) in the packaging (linden) and the extraction of charcoal for painting (linden).The fruits of trees are used as i) food (walnut, breccia, chestnut); ii) for confectionery, perfumery, painting (walnut); iii) in medicine and pharmaceuticals (breccia, chestnut, birch); iv) in the food industry (chestnut), chemical industry (chestnut, birch) and pulp and paper industry (birch).The leaves of a piece of the above trees are utilized as fodder for domestic animals (ash, gledichiya); dyeing, and in medicine (walnut, birch).A part of the tree's bark is obtained paint (ash) and extracted tannins/resin (birch).Some tree species' acorns are utilized to make fodder (orange (winter oak), blagun, tserat, red American oak, gledichiya).The flowers of a few trees are used for tea and in medicine (tree species of the genus linden (Tilia)), and as honey-bearing species (chestnut).The wood/timber is used in construction, furniture production, shipbuilding, manufacturing of musical instruments, and others.

Continuation of Table 2
The bark, leaves, domes of the fruits, and the wood contain tanning substances that are used in the chemical industry.The flowers are honey-like, and the fruits are tasty and nutritious Birch (Betula pendula Roth.) The wood/timber is used in furniture production for veneer and in the pulp and paper industry.Tannins, birch resin, and other substances are extracted from the bark.During active vegetation from the stem can be obtained juice, which is rich in sugars, and in some northern countries, it is used as a drink.The wood/timber is used in construction, in the production of packaging, pulp, and furniture.The bark is used to obtain tannins, resin, and dye substances.Essential oils can be extracted from the leaves and cones.The leaves are rich in vitamin C and have an ionizing and phytoncide effect on the air

Ornamental species in forestry
Sources: created by the author based on the information of the interviews and primary sources, collected via the study.
The wood/timber of some of the above medium and short trees is used in furniture production (manna (wild) pear, skorusha, rowan/mountain ash, white mulberry, janka), for making small objects ((wild) pear, janka), in underwater construction (white alder, elderberry) and carpentry (white alder, mahalebka, sumac), in turning (skorusha, white mulberry, mahalebka), carving (rowan/mountain ash), in cooperage (white mulberry), for making musical instruments (sumac), for heating and the extraction of charcoal (hairy oak).The fruits of trees are used as food (skorusha, rowan/mountain ash, mahalebka, pistachio), for beverages (skorusha), in medicine and pharmaceuticals (skorusha, rowan, white mulberry, almond, elderberry), in the food industry (white mulberry, almond, pistachio).The leaves of a part of the above species of trees are utilized for food by the larvae of the silkworm (white mulberry), as fodder for domestic animals (white alder, white mulberry), dyeing, and in folk medicine (manna ash, sumac).A piece of the trees' bark is obtained (black, yellow, brown) paint (pear, white alder, white mulberry, sumac).
For the first time, it has been given a new insight into the trees and shrubs suitable for various agroforestry systems in Bulgaria, a country with a temperate climate.The above-mentioned knowledge species with short, medium, and long rotations, and their possible use in various agroforestry systems and economic activities will support agroforestry farmers in creating efficient and sustainable agroforestry farms suitable for their region and economic industries operating there.Furthermore, the study contributes to agroforestry literature by providing knowledge on the rich palette of critical trees (46 trees) and shrubs (15 shrubs) appropriate for agroforestry systems in temperate climates.What is more, if these tree species are promoted by decisionmakers and professional/farmer associations, and adopted by the farmers can support and reinforce agroforestry in both Bulgaria and regions with temperate climates.