Germination characteristics and emergence time of annual Bromus species of differing weediness in Sweden L ANDERSSON*, P MILBERG  , W SCHU ¨ TZ à & O STEINMETZ à *Department of Ecology and Crop Production Science, Swedish University of Agricultural Sciences, Uppsala, Sweden,  Department of Biology- IFM, Linko ¨ping University, Linko ¨ping, Sweden, and àInstitut fu ¨r Wasserwirtschaft und Landschaftso ¨kologie, Christian-Albrechts-Universtita ¨t Kiel, Olshausenstrasse 40, Kiel, Germany Received 13 September 2000 Revised version accepted 14 September 2001 Summary The germination ecology of four annual Bromus species, which differ in weediness on arable land in southern Sweden, was investigated. The most problematic species is Bromus sterilis, while Bromus hordeaceus frequently occurs on arable land. In contrast, Bromus arvensis is a rare weed, and Bromus tectorum is found infrequently in fields despite being a widespread ruderal species. Five experiments were conducted to identify germination characteristics that could explain differences in habitat and abundance: (i) intraspecific variation in dormancy level; (ii) germination response to different light condi- tions; (iii) light and temperature interactions at germi- nation; (iv) timing of seedling emergence; and (v) seed persistence in soil. Bromus sterilis and B. tectorum behaved similarly in all tests. For both these species, there were large differences in dormancy level among populations and strong inhibition of germination by light. In addition, emergence from seeds sown on the soil surface was both delayed and reduced compared with buried seeds. In contrast, B. hordeaceus and B. arvensis showed generally weak dormancy, and germination was only slightly inhibited by light. It was concluded that germination characteristics alone do not explain the differences in weediness between these four species. Keywords: dormancy, grass, light, temperature, weed, winter annual. Introduction As cropping systems are changed, the abundance of some weed species might decrease while others increase. In western Europe, there has been a trend towards reduced soil cultivation and increased cultivation of autumn-sown cereal crops (Froud-Williams et al., 1981). Therefore, it is not surprising that a number of annual, autumn-germinating grass weeds are increasing and subsequently receiving attention. Among the most notorious in European agriculture is Bromus sterilis L. (Pollard, 1982; Eggers, 1990; Mortimer et al., 1993; Peters et al., 1993). A general question in weed research is which attrib- utes render some species more likely to become an increasing weed problem under particular circumstances (Milberg et al., 1999). This study focuses on four annual Bromus species that can be ranked according to how problematic they are in Sweden. These four species are B. sterilis L. (syn. Anisantha sterilis (L.) Nevski), B. hordeaceus L. (syn. B. mollis L.), B. tectorum L. (syn. Anisantha tectorum (L.) Nevski) and B. arvensis L. Bromus sterilis has become an important and increas- ing weed problem in the UK since the 1970s because of reduced soil cultivation and an increased area of winter cereals (Pollard, 1982; Mortimer et al., 1993; Peters et al., 1993). During the last two decades, B. sterilis has been recognized as a weed problem in the south of Sweden (Kraft, 1987; Hallgren, 1994). Bromus hordeac- eus has only recently been noticed as a serious, increasing grass weed on arable land in the UK (Mortimer et al., 1993). In 1998, this species was frequently seen in arable fields in southern Sweden (L Andersson, unpubl. obs.). Bromus tectorum is one of the most troublesome weed species in winter wheat in the central Great Plains in the USA (Wicks, 1997). However, in Sweden, B. tectorum is Correspondence: L Andersson, Department of Ecology and Crop Production Science, Swedish University of Agricultural Sciences, Box 7043, S-750 07 Uppsala, Sweden. Tel: (+46) 18 673 366; Fax: (+46) 18 672 909; E-mail: lars.andersson@evp.slu.se Ó European Weed Research Society Weed Research 2002 42, 135–147