Original article Analysis of the relationship between photosynthetic photon flux density and natural Taxus baccata seedlings occurrence Grzegorz Iszkulo *, Adam Boratyn ´ski Polish Academy of Sciences, Institute of Dendrology, ul. Parkowa 5, 62-035 Kórnik, Poland Received 14 May 2004; accepted 4 August 2005 Available online 16 September 2005 Abstract The aim of the present work was to analyse the relationship of seedlings and saplings of Taxus baccata to the photosynthetic photon flux density (PPFD) reaching the forest floor under natural conditions. Two permanent plots, subdivided into 1 × 1 m square plots, were estab- lished in a naturally regenerating population of T. baccata formed during last decades in the Kórnik Arboretum, Poland. All seedlings in every 1 × 1 m plots were counted. Relative PPFD was measured for every plot at the canopy height of the yew seedlings. The dependence of seedling density upon PPFD was examined. We found, that the frequency of the smallest seedlings (to 6.0 cm tall) was highest in the most shaded plots and decreased in plots with increasing PPFD. Thus, the youngest yew seedlings can germinate and grow in very shady conditions. However, the older seedlings (6.1–25.0 and 25.1–100.0 classes) were observed most frequently in 2–7% PPFD. The small numbers of older, taller seedlings in deep shade likely indicate a higher mortality rate of seedlings less than 6 cm in height without promotion to the next height class. Probably the low value of PPFD under the canopy of the stand significantly reduces the competition of other plants with the youngest yew seedlings. At higher light levels they may not be able to compete with more light-demanding plants, such as herbs and seedlings of broad- leaved trees. The seedlings of the second (6.1–25.0 cm) and third (25.1–100.0 cm) height classes were observed most frequently in the plots with 2–7% PPFD (Fig. 1b and c). © 2005 Elsevier SAS. All rights reserved. Keywords: PAR; PPFD; European yew; Light demands; Light optimum 1. Introduction Yew seedlings occur quite frequently in natural and sec- ondary stands of the species, under a variety of light and soil conditions and in various plant communities, but saplings are rarely observed, if at all (Dolukhanov, 1959; Kos ´cielny and Król, 1970; Król, 1975; Boratyn ´ ski et al., 1997; Garcia et al., 2000; Boratyn ´ski et al., 2001; Iszkulo, 2001). Mortality of Taxus baccata seedlings under the canopy of mature trees is commonly observed and has been the subject of several inves- tigations in natural habitats (Svenning and Magård, 1999; Thomas and Polwart, 2003; Iszkulo and Boratyn ´ski, 2004; 2005). Unfavourable site conditions and/or the damages caused by fungi, insect pests and mammalian herbivores, have been considered as the main reasons for the lack of yew tree regeneration and recruitment under the canopy of mature trees of the species (Paczoski, 1928; Gieruszyn ´ski, 1961; Man ´ka et al., 1968; Stypin ´ ski et al., 1984; Findo and Stefancik, 1988; Mysterud and Østbye, 1995; Hulme, 1996; Boratyn ´ ski et al., 1997; Svenning and Magård, 1999; Thomas and Polwart, 2003; Mysterud and Østbye, 2004). The October and Novem- ber absolute minimal temperatures were also found as a pos- sible reason for strong reduction, and even elimination of yew seedlings (Iszkulo and Boratyn ´ ski, 2005). One of the most important factors that can reduce the num- ber of seedlings is an insufficient amount of light under the canopy trees. The yew is considered to be one of the most shade-tolerant tree species in Europe (Ellenberg et al., 1991; Brzeziecki and Kienast, 1994; Zarzycki et al., 2002; Thomas and Polwart, 2003), but its response to various photon flux densities has not been object of detailed study until now. Only Köpp and Chung (1997) demonstrated experimentally that T. baccata seedlings can survive in light levels of 2% PPFD of full sun. * Corresponding author. Fax: +48 061 817 0166. E-mail address: iszkulo@man.poznan.pl (G. Iszkulo). Acta Oecologica 29 (2006) 78–84 www.elsevier.com/locate/actoec 1146-609X/$ - see front matter © 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.actao.2005.08.001