Perspectives in Plant Ecology, Evolution and Systematics 13 (2011) 163–172 Contents lists available at ScienceDirect Perspectives in Plant Ecology, Evolution and Systematics j o ur nal homepage: www.elsevier.de/ppees Research article Lianas escape self-thinning: Experimental evidence of positive density dependence in temperate lianas Celastrus orbiculatus and C. scandens Stacey A. Leicht-Young a,∗ , Andrew M. Latimer b , John A. Silander Jr. c a U.S. Geological Survey, Lake Michigan Ecological Research Station, 1100N. Mineral Springs Rd., Porter, IN 46304, USA b University of California, Davis, Department of Plant Sciences, Davis, CA 95616, USA c University of Connecticut, Department of Ecology and Evolutionary Biology, 75N. Eagleville Rd., Storrs, CT 06269-3043, USA a r t i c l e i n f o Article history: Received 28 June 2010 Received in revised form 21 March 2011 Accepted 8 April 2011 Keywords: Competition Facilitation Lianas Invasive species Temperate forests Vines a b s t r a c t The neighborhood density of plants strongly affects their growth, reproduction, and survival. In most cases, high density increases competition and negatively affects a focal plant in predictable ways, leading to the self-thinning law. There are, however, situations in which high densities of plants facilitate focal plant performance, resulting in positive density dependence. Despite their importance in forest gap dynamics and distinctive growth form, there have been very few studies of the effect of density on lianas or vines. We grew an invasive (Celastrus orbiculatus) and a native (Celastrus scandens) liana species together in three different density treatments, while also manipulating the light and support availability. We found that across treatment conditions, C. orbiculatus always out-performed C. scandens, showing greater relative growth rate in height and diameter, greater biomass and higher survival. Both species responded similarly to the density treatments: with plants in high density not showing a decrease in relative height growth rate compared to medium density. Aboveground biomass for C. scandens was not affected by density, while for C. orbiculatus, the most massive plants were growing in medium density without support. More surprisingly, survival analysis indicated that the two species both had significantly lower mortality rates in the highest density treatment; this trend held true across the other treatments of light and supports. More generally, this study demonstrates that these lianas can escape the consequences of high density and thus the self-thinning law that affects self-supporting plants. This suggests a broader hypothesis about lianas in general: their greater flexibility in allocating growth resources allows them to grow taller and thinner without collapsing and thereby potentially escape shading and mortality even at high densities. Published by Elsevier GmbH. Introduction Despite their unique growth form and structural importance on the landscape, the ecology and biology of lianas (woody vines) remains poorly understood. They are probably best known from the tropics (Schnitzer and Bongers, 2002), but can be quite prevalent in some temperate areas (i.e., East Asia) to uncom- mon or rare in others (i.e., eastern North America). Exotic liana species can easily invade these liana-depauperate areas, form- ing dramatic tangles on the landscape (Fike and Niering, 1999; Forseth and Innis, 2004; Schierenbeck, 2004). Lianas use other plants (i.e., trees) and neighboring lianas (Putz, 1984, 1995b) as supports, which often results in large liana tangles (Schnitzer et al., 2000; Gerwing, 2001). Dense tangles can perturb succes- sion (Schnitzer et al., 2000, 2005; Schnitzer and Carson, 2010) ∗ Corresponding author. Tel.: +1 219 926 8336x426; fax: +1 219 929 5792. E-mail address: slyoung@usgs.gov (S.A. Leicht-Young). and have impacts on tree growth (Dillenburg et al., 1993a,b; Schnitzer et al., 2005). Even in the absence of trees or other host species, lianas can climb on each other (Putz, 1995a) and effectively increase their height, avoiding being overtopped by other species (Letcher and Chazdon, 2009). The resulting tan- gles are prominent in tropical systems where tree fall gaps form (Putz, 1984; Schnitzer et al., 2000, 2004), and in temperate sys- tems in gaps and at forest edges (Robertson et al., 1994; Kodani, 2006). The invasive liana, Celastrus orbiculatus Thunb. (oriental bitter- sweet), is expanding its range across the eastern North American landscape and is commonly observed in large tangles overtopping the surrounding vegetation (Fike and Niering, 1999). Interestingly, its native congener, Celastrus scandens L. (American bittersweet), appears to have a more conservative life form (Steward et al., 2003), and while it can grow successfully into trees, is not often observed forming dense tangles or actually overtopping the host tree it is growing on (Dreyer et al., 1987; Steward et al., 2003). In many of the locations where C. scandens once occurred, it 1433-8319/$ – see front matter. Published by Elsevier GmbH. doi:10.1016/j.ppees.2011.04.002