GENET DYNAMICS OF THE CLONAL PLANT RUBUS-SAXATILIS

1 Populations of Rubus saxatilis were investigated between 1988 and 19 91 in a study area (c. 2.5 km2) in central Sweden. Different phases in the life cycle were studied: flowering, pollination, fruit-set, fruit -removal and seedling recruitment. Identification of genets was made b y use of RAPD (random amplified polymorphic DNA). Information concerni ng these life-cycle phases were then used to infer processes in the po pulation dynamics of R. saxatilis. 2 A path analysis suggested a stron g effect of fruit-set on total patch fruit production. Fruit-set varie d between 6.6% and 8.3% (yearly averages) and was influenced by distan ce to nearest flowering conspecific patch. Isolated patches tended to have a low fruit-set. Size of patches was of minor importance for frui t-set and fruit-production, whereas intrapatch ramet density influence d fruit production positively via an effect on the proportion of ramet s that flowered. 3 A pollination experiment confirmed that R. saxatili s is self-incompatible. Within-patch pollinations indicated that isola ted patches comprise single genets (or incompatibility types). Between -patch pollination indicated that fruit-set may be pollen limited, at least some years. Deficiency of compatible pollen (or 'partners') is t he most likely mechanism behind the distance-effect on fruit productio n found in the path analysis. 4 A bagging experiment suggested that c. 50% of the fruits (drupelets) were removed by animals. No relationshi p was found between fruit removal rate and either fruit production of patches or distance to neighbour patch. 5 Seedlings of R. saxatilis we re carefully searched for during four years in three different localit ies 'coniferous forest', 'deciduous forest' (DF) and 'mixed semi-open woodland' (MSOF). Seedlings were found only during one year and in one locality (MSOF). Ramets from carefully mapped 20-m x 30-m plots in lo calities DF and MSOF (one plot per site) were sampled for genet identi fication with RAPD analysis. In DF, 20 ramets were found to belong to 10 different genets. In MSOF, 24 ramets belonged to eight different ge nets. In addition, a sample of three ramets from an isolated road-side patch indicated that it consisted of one genet. 6 The dynamics of the genet population of R. saxatilis in the study area was interpreted by a source-sink concept. Source populations represent local clusters of genets with high fruit production which enhances further local recrui tment in source populations and provide the surrounding sink areas wit h propagules. Sink populations represent low-density populations compr ising isolated genets suffering from partner-limited fruit production. These isolated genets may be loci for future colonization. The source -sink structure implies an apparent positive genet density effect on r ecruitment, but to what extent this effect is due to variation in the local 'quality' of sites or is a real density effect, remains unresolv ed.