Journal of Arboriculture 26(5): September 2000 255 GROWTH AND MORPHOLOGY DIFFER BETWEEN WIND-EXPOSED FAMILIES OF SORBUS AUCUPARIA (L.) by Arne Saebo and 0ystein Johnsen Abstract. Rowan (Sorbus aucuparia), a small and decorative tree with widespread distribution throughout Norway and Europe, is well suited for growth on street verges, parks, and urban woodlands. With a better knowledge of the ge- netic variation of its important traits, good genotypes could be selected and the appearance of the trees thus improved. The objective of this study was to describe the variation in growth, morphology, and phenology of rowan between and within seed sources and at different wind exposures. Seeds were collected from 10 to 15 trees within each of six popu- lations in a small area of 1,300 km 2 (500 mi 2 ) along the southwestern coast of Norway. Three populations were wind-exposed and three were sheltered. From each of the 78 mother trees included, 30 half-siblings were planted in the field at the Sserheim Research Centre (58.8°N, 5.5 C E). Height growth, stem diameter, number of stems and branches, branch angle, and growth start and termination were recorded annually in five successive years. Large dif- ferences in most traits (estimated heritabilities varied from 0.14 to 1.0) were found among families within the stands. However, no important differences could be observed be- tween progenies from wind-exposed trees and those from sheltered trees, and differences among stands were only mi- nor. The results are discussed in relation to the climatic adaptation of rowan and how this plant material can be used in urban settings. Key Words. Growth; morphology; phenology; seed source; selection; Sorbus. The visual and functional qualities of urban trees are largely dependent on the genetic quality of the plant material. While there is a need for both small and large trees, the larger trees often are too big for the available space, especially in gardens and paved areas. Rowan (Sorbus aucuparia, L, 2n = 34, mountain- ash) is one of 12 Sorbus spp. growing naturally in Scandinavia, and eight of the species are endemic to the area (Salvesen 1993). Many Sorbus species are apomictic, but not rowan, in which nonapomictic seeds are set even after self-pollination (Sperens 1997). Rowan usually reaches a maximum height of between 8 and 15 m (25 and 50 ft) in Norway, but farther south in Europe the tree can reach a height of 30 m (100 ft). Its compound leaves have between 11 and 15 lancelot leaflets, and the white flowers, ap- proximately 1 cm (0.4 in.) in diameter, have an inflo- rescence approximately 15 cm (6 in.) in diameter. Pollination is by insects and seed dispersal primarily by birds. The delicate leaf structure, the white and large inflorescence, the yellow to deep red fruits and often striking autumn colors are characteristics that make this moderately sized species valuable as an amenity tree in urban settings. Rowan is widely distributed, from the mountains of Asia Minor to Siberia. It tolerates a wide range of climatic conditions, both continental and oceanic, and grows well in the harsh climates of high eleva- tions (Hillebrand and Rosenberg 1996) and in sulfur-polluted environments (Nebe and Opfermann 1998). In Norway, it grows from the coast to eleva- tions more than 1,500 m (4,900 ft) above sea level, where it is a shrub. Rowan thrives in many soil types, even in soil with poor nutrient status, but it grows best on rich farmland. Its excellent hardiness and large plasticity also make the tree suitable for use in urban woodlands, in sheltering belts, along streets, or in parks. Some of the trees form many stems, but oth- ers have only one. The single-stemmed type is pre- ferred in paved areas, whereas in a sheltering belt, a tree with several stems might be more functional. Improvement of the plant material for urban areas is necessary if amenity values and establishment are to be improved and maintenance costs decreased. The primary selection criterion for plants in Norway is cli- mate adaptation, but other important criteria are growth potential, morphology, disease resistance, and decorative value. In urban woodlands, a plants ability to regenerate from seed is important. Habj0rg (1978) has shown the significance of pho- toperiod as a signal of the termination of growth in the autumn and the preparation for winter hardiness in im- portant tree species in Scandinavia. The rowan, how- ever, has been found to be less dependent on the photoperiod for growth cessation, although the results