Betula : arter och sorter

Sammanfattning: As birch is one of the most common trees in Sweden, and probably one of the easiest for someone who lives here to identify, it is interesting to note that most people do not know how variable the genus is and how widely it is distributed across the northern hemisphere. One can find birch growing at both high and low elevations, demanding different soil conditions and preferring varying amounts of sunlight. Many people may also be surprised to hear that the colour of the bark can vary from bight white to almost black, and different shades of pink, orange and red can also be displayed. This, together with the astonishing beauty of the genus, the important role as a pioneer in many ecological systems and the difficulty in sorting out the taxonomy of the genus, constitutes the background of this thesis. The thesis is written at the Department of Landscape Development in Alnarp, Sweden with Kenneth Lorentzon, researcher at the Swedish University of Agriculture as a supervisor. The aim of this thesis is to highlight which species and cultivars within the genus Betula could be appropriate for horticultural use in Sweden. It intends first and foremost to illuminate the species that possess a great aesthetic value, and is not a taxonomical inquiry of the genus. The method used in this thesis was a literary study and interviews with both researchers at SLU Alnarp as well as other highly qualified people such as Kenneth Ashburner. Visits to several botanical gardens and arboretums in Sweden, Denmark and Great Britain were also made. Great Britain was chosen not only because of appropriate geographical location but also for the access to a great assortment of Betula species and cultivars if you are situated in Sweden. Betula is a part of the family Betulaceae. It contains between 30 and 150 species depending on what literature one chooses to rely on. This thesis has come to the conclusion that a number of somewhere around 60 species would seem plausible. It seems that some of the species are more appropriate than others for horticultural usage in Sweden, and that often it is within these species that you find most of the birch cultivars that are available in trade. The Asiatic species Betula albosinensis, B. ermanii, and B. utilis are a few of the important ones, together with the North American Betula alleghaniensis and the domestic B. pendula. Whilst the latter is very common in the Swedish landscape, it is easy to forget the value of this species, but with its strongly hanging habit and white stems, it is one of the most beautiful birches. This is not to say that there aren't many other very beautiful and valuable species that also possesses great qualities and interesting usage. The breadth of the genus when it comes to aesthetics and soil demands makes it possible to find a suitable birch for a variety of situations. There are also small birches available for the "small garden" that has become quite common in Sweden today. A lot of the plant material that has been researched in this thesis is quite new, and it is not yet commonly cultivated in Sweden. But as the assortment and availability of birch species and cultivars increase, an opportunity arises for increased usage and this enables the experience of the genus to grow and develop.