Lianas are an important component of species richness and structural diversity in lowland tropical forests accounting for up to 20% of woody plant diversity and 40% of stem density (Putz 1984; Putz & Mooney 1991; Schnitzer & Bongers 2002, 2011). Nonetheless, assessments of liana community composition and structure remain rare, perhaps because of a lack of resources for field identification, and because standardized protocols for liana surveys have only recently been developed (e.g. Parren et al. 2005; Gerwing et al. 2006; Schnitzer, DeWalt & Chave 2006; Schnitzer, Rutishauser & Aguilar 2008). In the absence of these data, it remains unclear whether lianas behave like trees with regard to the distribution of coexisting taxa, and in their responses to variation in resource availability.
For trees, recent analyses conducted in large (>10 ha) census plots have revealed the importance of habitat variability in shaping the distribution patterns and trait composition of communities (Harms et al. 2001; Potts et al. 2004; Valencia et al. 2004; Engelbrecht et al. 2007; John et al. 2007). These analyses have shown that tree species frequently have distributional biases with respect to topography (e.g. Valencia et al. 2004; Gunatilleke et al. 2006), which suggest sensitivity to underlying variation in soil moisture or nutrient availability. For example, 52 of the 171 most common species of shrubs and trees that occur in the Barro Colorado Island (BCI) 50-ha forest dynamics plot in Panama show distributional biases with respect to the plot’s swamp habitat and 44 species with respect to slopes (Harms et al. 2001). Similarly, 104 of 258 tree species in the BCI plot show non-random associations with either high or low concentrations of one or more soil chemical variables (John et al. 2007).
As yet, only limited evidence is available to assess whether lianas are equally sensitive to topographic variation and soil nutrient availability. At the landscape scale, liana stem density has been shown to increase, albeit modestly, with soil fertility (Proctor et al. 1983; Putz & Chai 1987; Gentry 1991; Laurance et al. 2001), and more strikingly at the regional scale with decreasing precipitation, and increasing seasonality of rainfall (Schnitzer 2005; DeWalt et al. 2010). As liana species richness scales with stem density (Schnitzer & Carson 2001, 2010), diversity might be expected to show similar patterns. In small scale plot surveys, liana species richness has been shown to vary between floodplain and upland habitat types in the western Amazon, with higher richness on the upland sites (Burnham 2002, 2004), and across a gradient of soil fertility and nutrient availability in northern Borneo, with higher richness on more fertile, less drought-prone sites (DeWalt et al. 2006). Individual liana species distributions have also been shown to be related to soil nutrient and light availability (Chettri et al. 2010; Malizia, Grau & Lichstein 2010).
Alternatively, lianas may be relatively insensitive to soil and topography, at least at local scales, if their distributions are primarily determined by local disturbance history. To varying degrees, lianas have often been classified as early successional species (e.g. Putz 1984; Putz & Chai 1987; Hegarty 1991; Campbell & Newbery 1993), and are often noticeably more abundant in young secondary forest and at forest margins (e.g. DeWalt, Schnitzer & Denslow 2000; Laurance et al. 2001; Arroyo-Rodríguez & Toledo-Aceves 2009) and in treefall gaps (Putz 1984; Schnitzer, Dalling & Carson 2000; Schnitzer & Carson 2001, 2010). However, no study has tested whether resource availability or local disturbance provides a better explanation for liana distribution within a forest.
In this study, we use a complete census of the lianas (≥1 cm diameter) in the BCI 50-ha plot (S.A. Schnitzer, unpublished data) to compare liana densities across topographic habitat types, and to compare the frequency of liana vs. tree species associations with habitat type, and with soil chemical variables surveyed across the plot. In addition, we use a high-resolution map of forest canopy height determined using light detection and ranging (LiDAR) to test for liana species associations with low canopy heights related to gap disturbance.
Understanding the drivers of liana community assembly has become a priority given recent observations of increasing liana abundance in neotropical forests (e.g. Phillips et al. 2002; Foster, Townsend & Zganjar 2008). In turn, high liana loads in forest canopies have been associated with reduced tree growth rates (Schnitzer & Carson 2010), elevated tree mortality rates (Ingwell et al. 2010) and reduced above-ground biomass (van der Heijden & Phillips 2009). Based on previous work at BCI showing that liana species richness is significantly higher in gaps than in the forest understorey (Schnitzer & Carson 2001), we predict that liana species will be associated primarily with areas of low canopy height, rather than with habitats defined by topography or soil nutrients, previously shown to be important for tree species. If so, regional increases in liana abundance may reflect increasing disturbance rates in tropical forests, potentially resulting either from faster forest turnover rates or from elevated tree mortality rates (Condit, Hubbell & Foster 1995; Phillips et al. 2009). Alternatively, if observed regional variation in liana abundance reflects a competitive advantage for lianas in more seasonal environments (Schnitzer 2005), then we would predict that lianas would be most frequently associated with plateaus, the most seasonally moisture-limited topographic habitat types.