The value of arboreta in South Africa

Since antiquity, collections of living trees, or arboreta, have been established for diverse religious and aesthetic purposes.1 Naturalists became increasingly aware of their scientific value as early as the 16th century2, in particular as a living resource with which to study taxonomic questions that arose with the influx of species into Europe from botanical exploration3. The economic and scientific importance of introduced tree species was realised by many of the major landowners in the United Kingdom and from 1750 and 1850 many arboreta were established on private estates and in botanic gardens.4 During the 19th century, arboreta were increasingly used as acclimatisation trials for introduced tree species in Europe1,5,6, America7, Australia8 and South Africa9. The first arboreta to be established in South Africa were Arderne Gardens in 184510 followed by the Durban Botanic Gardens in 185111. Acclimatisation studies developed into a new research field during the 19th century and the term ‘acclimatisation garden’ became widely used by professional and amateur botanists.12

• Arboreta are a valuable resource for the forestry industry as gene banks.
• Arboreta, together with botanic gardens, form a useful network of sentinel sites for plant pathogens and invasive species.
• A survey of the species composition and status of arboreta in South Africa would be an important contribution to our botanical knowledge.
Since antiquity, collections of living trees, or arboreta, have been established for diverse religious and aesthetic purposes. 1 Naturalists became increasingly aware of their scientific value as early as the 16th century 2 , in particular as a living resource with which to study taxonomic questions that arose with the influx of species into Europe from botanical exploration 3 . The economic and scientific importance of introduced tree species was realised by many of the major landowners in the United Kingdom and from 1750 and 1850 many arboreta were established on private estates and in botanic gardens. 4 During the 19th century, arboreta were increasingly used as acclimatisation trials for introduced tree species in Europe 1,5,6 , America 7 , Australia 8 and South Africa 9 . The first arboreta to be established in South Africa were Arderne Gardens in 1845 10 followed by the Durban Botanic Gardens in 1851 11 . Acclimatisation studies developed into a new research field during the 19th century and the term 'acclimatisation garden' became widely used by professional and amateur botanists. 12

Defining an arboretum
Definitions of an arboretum vary from vague, such as a botanical tree garden 13 , to specific, for example, a comprehensive collection of arboreal species, varieties and forms, of both alien and indigenous species, that can be cultivated in one locality 14 . In some cases, such as the Morton Arboretum 15 , University of Wisconsin Arboretum 16 , the Holden Arboretum 17 , the Dawes Arboretum 18 and Connecticut Arboretum 19 , the term is used as a synonym for a commons -a relatively large legally protected area (in the aforementioned cases 485, 1255, 728 and 182 hectares, respectively) that includes prairies, natural woodland, wetlands and landscaped areas, with some sections receiving little, if any, management. The optimum size of an arboretum is largely determined by its purpose, whether it is to address particular research questions or is intended for education and display. 1 Baas-Becking 1 differentiated between a forestry and dendrological arboretum, recommending a minimum of 50 hectares for the latter which would have wider research applications than strictly silviculture. This is a recommendation that will undoubtedly vary with the amount of land available. Hartley 20 observed that the renowned 19th-century Scottish botanist J.C. Loudon considered the ideal size of an arboretum that which allows sufficient space for each species to attain its full size and character.
Although he never defined an arboretum in his Arboretum et Fruticetum Britannicum, J.C. Loudon's concept of an arboretum is similar to that of a botanical garden except that it is specifically aimed at ligneous plants, displayed following a natural system. 20 In South Africa, the term arboretum is associated with the systematic cultivation of trees and shrubs for display and research purposes. 21 While there is some overlap between the definition of an arboretum and a botanical garden (Supplementary table 1), we support a narrow arboretum definition as a documented or labelled collection of living, ligneous taxa, non-native or indigenous, cultivated for scientific research, educational and display purposes.

Arboreta in forestry
One of the most important stages in forestry plantation establishment is selecting appropriate species for a particular region or site conditions 22 and arboretum experiments are the most practical way to facilitate this 23 . The fact that different species or provenances require particular climatic and edaphic conditions to perform favourably, is known as the Genotype x Environment interaction. 24 Thus it is important that species are tested in the setting for which they are intended. 25 In this way, new genotypes could be selected for particular applications and new sites, either for commercial timber industry 26 or for reforestation of degraded land 27 . Arboreta have been instrumental in selection studies for commercial timber species in South Africa and Tokai arboretum was the first to be established for this purpose (Supplementary table 2). Research into the performance and commercial suitability of kauri pines (Agathis species) was also done at Port Durnford arboretum (established in 1923), in KwaZulu-Natal, South Africa. 28 Whereas species selection trials are often a mixture of species, plant breeding programmes and provenance trials are often a series of ongoing trials 25 of a single species or cultivar where the goal is to select superior genotypes 29 .

Arboreta in horticulture
Urban forests are valuable as amenity plantings and for improving the aesthetic value of municipal areas. 30 Urban plantings are increasingly used as a resource in detecting new plant pests and pathogens and investigating the potential effects of climate change. 31 However, the selection of street trees is often based on what will grow at a site, the preferences of local people, and the availability of nursery stock. 32 Arboreta are labelled, catalogued collections with information on their establishment date and often the origin of the species they contain, giving them greater value over and above the substantial plantings of trees in urban settings because tree selection is more often driven by research questions. Arboretum trials are useful in assessing species for various urban and industrial settings. 33 Selecting trees with the appropriate size and shape decreases the maintenance costs of both the trees as well as the surrounding infrastructure. 34 To streamline the process of introducing new cultivars to horticulture in the USA, a National Arboretum introduction programme was started in 1971 to evaluate the performance of new cultivars. 35 The National Arboretum co-ordinates this evaluation programme but the growth and monitoring of plants is done at botanic gardens, arboreta, universities and nurseries that have signed a memorandum of understanding to be a co-operator in the programme, thereby converting established botanic gardens and arboreta into a network of test sites to determine the hardiness range of new cultivars. 35 Starting with Tokai arboretum, the Department of Forestry established a number of arboreta across South Africa which provided valuable information on the performance of tree species in the various climates. This information enabled the Department of Forestry to recommend tree species to farmers, municipalities and the public for amenity planting or particular applications such as windbreaks or soil stabilisation. 36

Arboreta as sentinel sites
The International Plant Sentinel Network functions as an early warning system through sharing information about new outbreaks of pests on alien collections in botanic gardens and arboreta. 37 Surveying introduced tree species in such collections has proven useful in detecting new insect introductions, as well as undescribed insect species that use the trees as refugia. 38 The  40 The polyphagous shot hole borer beetle, and it is associated fungal symbiont Fusarium euwallaceae, was detected from such a survey at the KwaZulu-Natal Botanical Garden in 2017. 41 The continued introduction of pests is likely to continue with continued trade and this is a significant threat to biodiversity across Africa. 42 Because arboreta contain introduced species, they can also act as point sources for future plant invasions. With a change in climate, new horticultural species will be introduced, providing potentially new invasive species. 43 The sharp increase in the numbers of botanic gardens and arboreta since 1950 suggests that they still have the potential to represent a pathway of plant invasions. 44 In addition, the rate of new plant introductions has increased steadily since the 19th century and shows no sign of decreasing. 45 The evaluation and development of tools for detecting and monitoring new plant invasions was highlighted by Van Kleunen et al. 46 among the important points in managing future horticultural invasions. Arboreta within South Africa represent a valuable resource for this purpose.

Arboreta in conservation
The role of arboreta and botanic gardens in botanical exploration continues; however, there is an increasing emphasis on their involvement in botanical research and the conservation of genetic resources. 47 Ex-situ conservation is the safeguarding of particular genotypes outside of their indigenous range and is useful when a threatened species has a very restricted indigenous range and arboreta and botanic gardens are currently the greatest contributors to ex-situ tree conservation. 48,49 An important question to be considered is the conservation value of the arboreta as gene banks, in particular, those containing unique and redlisted species. Accurate species inventories are an important foundation. It is nevertheless necessary to determine the genetic diversity represented in a collection itself. 50 Adaptation to a new environment occurs along with sexual reproduction and this could cause a gradual change in the genotype being conserved ex situ from the genetic composition of the populations in the indigenous range. 51,52 Ideally, in-situ and ex-situ conservation should be combined for the conservation of a species to be most effective. 52  The ex-situ conservation and silvicultural research on threatened species is the goal of the non-profit Central American and Mexican Coniferous Resources Co-operative (Camcore) focusing on four genera: Pinus, Eucalyptus, Gmelina and Tectona. 55 This initiative was started in 1980 and currently includes 29 forestry companies from 11 countries as active members. 55 Conservation is an obligation of membership of Camcore and each company is responsible for the well-being and maintenance of those species under its care. 56 South Africa has been part of the Camcore initiative since 1983, with six conservation parks containing 12 Pinus and 1 Eucalyptus species. 57 The research questions and projects of which arboreta are becoming a part, seem to be of an increasingly national and international nature over time (Supplementary  table 2). Where arboreta were mainly used in the early 20th century to select suitable commercial or amenity planting species, in the late 20th and early 21st centuries, they have been effectively used in biosecurity research globally as well as in climate change research, such as the REINFFORCE network 58 across Europe.

Conclusion
Longstanding living collections of alien species have considerable research value for improving our understanding of pure and applied research problems. 59 The first assessment of South Africa's forestry arboreta was done by Poynton in 1957 60 followed by an unpublished survey by Poynton and Rycroft in 1986 21 . Arboreta represent an important resource for education, research, conservation and recreation and a survey of their current status would be an important step in using them for these purposes. Replacement cost theory is one way of placing a value on living plant collections. A complicating factor in this, is that living plant collections (as with herbarium collections) cannot be replaced, and the replacement value is more a measure of the investment made in establishing such collections. 61 Alien tree species are of prime importance to forestry industries in the tropics 62 , and in the past there has been little in the way of procedures governing the movement of tropical tree germplasm between countries 63 . The Nagoya Protocol on Access and Benefit Sharing that was launched in 2014 64 creates barriers to the exchange of biological material; those countries with existing collections of alien plant material will be at an increasing advantage over time 65 .