RESEARCH ARTICLE Short-distance gene flow in Populus nigra L. accounts for small-scale spatial genetic structures: implications for in situ conservation measures Georg Rathmacher Æ Marc Niggemann Æ Melanie Ko ¨hnen Æ Birgit Ziegenhagen Æ Ronald Bialozyt Received: 2 December 2008 / Accepted: 10 July 2009 Ó Springer Science+Business Media B.V. 2009 Abstract The European black poplar (Populus nigra L.) is a major species of riparian softwood forests. Due to human influences, it is one of the most threatened tree species in Europe. For restoration purposes, remaining stands may act as source populations. We analysed a nat- ural population of P. nigra for genetic diversity and spatial genetic structure using seven microsatellite markers. For the first time, paternity analysis of seedlings as well as juveniles from a restricted area of natural regeneration was used to quantify pollen- and seed-mediated gene flow, respectively. In both cases, cultivated P. x canadensis trees in vicinity could act as potential parents. Spatial genetic patterns of the adult tree population suggest small-scale isolation by distance due to short-distance gene flow, the major part of which (i.e. 70%) takes place within distances of less than 1 km. This helps to explain the reduced diversity in the juveniles. It has implications for the spatial management of natural regeneration areas within in situ conservation measures. Keywords Parentage analysis Á SSR Á Microsatellite Á Natural regeneration Á Pollen dispersal Á Seed dispersal Introduction The European black poplar (Populus nigra L.) is dioecious and wind-pollinated. It is a keystone species for softwood floodplain forest ecosystems. As a pioneer species, it regenerates through colonisation of bare soil along the riverbank created by heavy flooding events. The wind- and water-dispersed seeds have a short viability period. They need highly specific water/soil conditions for germination (Braatne et al. 1996; Guilloy-Froget et al. 2002). Suc- cessful regeneration therefore only occurs when the mois- ture of sediment remains high enough for seedling roots to establish (Legionnet et al. 1999). Due to inappropriate conditions in many years, successful regeneration is absent. Accordingly, the history of flooding is reflected in a strong age structure that frequently exists in naturally occurring stands (Heinze 1998). The natural regeneration of P. nigra, which is already patchy and sporadic, is additionally restricted by human activities. The drainage of rivers or management of riverbanks prevents natural flooding dynamics, which causes a lack of suitable areas for seed- ling establishment. Furthermore, today, many native pop- ulations of P. nigra have been replaced or fragmented by the widespread cultivation of commercially exploited hybrid poplars. From an economic point of view the most important hybrid combination is P. x canadensis (Dode) Guinier produced by crossings of P. nigra and the North American species P. deltoides Bartr. (FAO 1979). Culti- vated P. x canadensis is a potential mating partner for P. nigra. Several studies have already reported on the pro- duction of viable progeny (e.g. Vanden Broeck et al. 2004; Smulders et al 2008a; Ziegenhagen et al. 2008). Forest fragmentation leads to a breakup of pollen- and seed-mediated gene flow (Jump and Penuelas 2006). Limi- tations of pollen and seed dispersal result in spatial aggre- gation of related individuals that is called ‘‘isolation by distance’’ (Hardy and Vekemans 1999; Born et al. 2008). However, genetic diversity is important to allow a population to survive and reproduce under changing environmental conditions (Primack 2000). G. Rathmacher (&) Á M. Niggemann Á M. Ko ¨hnen Á B. Ziegenhagen Á R. Bialozyt Faculty of Biology, Conservation Biology, Philipps-University of Marburg, Karl-von-Frisch-Straße 8, 35032 Marburg, Germany e-mail: georg.rathmacher@gmx.de 123 Conserv Genet DOI 10.1007/s10592-009-9961-6