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Journal of Vegetation Science 22 (2011) 868–877 Liana co-occurrence patterns in a temperate rainforest R.A.J. Blick & K.C. Burns Keywords Compartments; C-score; Liana; Negative co-occurrence; Network; Null model. Nomenclature Allan (1961), Brownsey & Smith-Dodsworth (2000), Moore & Edgar (1970) Received 31 March 2010 Accepted 28 February 2011 Co-ordinating Editor: Tom Philippi Blick, R.A.J. (corresponding author, [email protected]): School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand, Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052, Sydney, Australia Burns, K.C. ([email protected]): School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Abstract Questions: Are liana–host interactions structured at the community level? Do liana–host interactions differ between species, growth form guilds or habitats? Location: Otari-Wilton’s Bush, on the southern tip of North Island, New Zealand. The forest contains 75 ha of mature and regenerating conifer–broadleaf forest. Methods: Nine liana species were quantified among 217 trees to test for negative co-occurrence patterns. We also conducted additional analyses within and among compartments embedded in the community-level matrix. Liana and host abundance distributions were assessed across two contrasting habitats. Results: Community-level analyses revealed negative co-occurrence patterns. Positive, neutral and negative co-occurrence patterns were found among compartments within the community-level matrix. Host species compartments were consistent with randomized expectations, while positive co-occurrence patterns were found within the host species matrix. Negative co-occurrence patterns were found inconsistently among lianas that share the same region of host space, and those that do not. Conclusions: Overall, results indicate the liana community is structured nonrandomly. Liana–host interactions appear to follow an opportunistic growth strategy and interactions are due mostly to habitat partitioning. Introduction Species interaction matrices provide a useful method for assessing deterministic, neutral or random associations of species assemblages (Gotelli & Graves 1995; Gotelli 2000; Bascompte et al. 2003; Vázquez 2005; Almeida-Neto et al. 2007; Guimarães et al. 2007; Hausdorf & Hennig 2007; Ings et al. 2009). If communities are assembled deterministically, similar species should replace one another among communities (Diamond 1975). Furthermore, co-evolved traits or geographically isolated individuals may generate compartments (e.g. modules, blocks or species subgroups) within ecological networks (Prado & Lewinsohn 2004; Lewinsohn & Prado 2006; Olesen et al. 2007). Typically, large pollination networks consist of compartments with highly ‘nested’ organization and are linked by generalist species (Olesen et al. 2007). However, there remains little consensus on the processes that generate compartments among species assemblages. Few studies have used an individual-based approach (i.e. matrices of interactions between individuals rather than 868 species) to identify the processes structuring communitylevel interactions. Here we investigate liana–host interactions in a temperate New Zealand forest. Patterns in the distribution of lianas have been studied extensively in tropical (Talley et al. 1996b; Clark & Clark 2000; Chittibabu & Parthasarathy 2001; Carsten et al. 2002) and temperate systems (Talley et al. 1996a; Allen et al. 1997, 2005; Baars et al. 1998; Malizia et al. 2010) and their role in ecosystem functioning has recently been emphasized (Schnitzer & Bongers 2002; Schnitzer 2005). Lianas represent a significant proportion of vascular plants in tropical regions. Their distribution in cooler climates may be limited due to an increased rate of embolism, therefore temperate regions consist of few species capable of withstanding harsh environmental conditions (Schnitzer 2005). The influence of environmental conditions on liana species composition have been identified (DeWalt et al. 2000, 2006; Burnham 2002; Ibarra-Manriquez & Martinez-Ramos 2002; Burnham 2004; Malizia et al. 2010); however, the Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. mechanisms responsible for community organization are less clear when structural support requirements are considered (Putz 1984; Balfour & Bond 1993; Campbell & Newbery 1993; Carsten et al. 2002). Two mechanisms may structure liana–host interactions. First, lianas may undergo ‘vertical’ partitioning of host tree space. The primary stems of lianas are typically small in diameter and incapable of supporting their own weight. Therefore lianas require neighbouring trees to provide structural support. Host tree structural characteristics, such as branch height or bark type, may alter liana–host interactions by influencing colonization rates (Nabe-Nielsen 2001; Campanello et al. 2007), altering liana distribution (DeWalt & Chave 2004; Senbeta et al. 2005) and maintaining liana diversity (Schnitzer & Carson 2001; also see Putz 1984; Balfour & Bond 1993; Campbell & Newbery 1993; Wright et al. 1997; Malizia & Grao 2008; Watanabe & Suzuki 2008). Alternatively, ‘horizontal’ partitioning among spatially distinct habitats may also influence liana–host interactions. Liana distribution can change in response to abiotic conditions (Baars & Kelly 1996; Burrows 1996; Hättenshwiler 2002; Schnitzer & Bongers 2002; Schnitzer 2005; Swaine & Grace 2007; Foster et al. 2008; but see DeWalt & Chave 2004; van der Heijden & Phillips 2008), which can simultaneously affect the distributions of host trees (DeWalt et al. 2006). Relationships between liana structural requirements and host characteristics (i.e. ‘host preferences’ Burns & Dawson 2005) could lead to community-level patterns, such as negative co-occurrence patterns and host specialization. We investigated these hypotheses by documenting the lianas growing among host tees in a temperate New Zealand forest. In an effort to pinpoint the mechanisms responsible for negative co-occurrence patterns, we followed up community-level analyses with additional analyses of compartments within the community-level matrix. Our aims are to (1) characterize general patterns of lianas and their hosts, (2) identify whether these underlying patterns of distribution promote negative co-occurrence patterns, and (3) investigate compartmentalization in a temperate plant–plant interaction network. Overall, this study highlights the importance of abundance and distribution in structuring interaction networks. Methods Sampling Liana distributions were quantified in Otari-Wilton’s Bush, which is located on the southern tip of the North Island, New Zealand (41114 0 S, 174145 0 E). The area experiences a mild climate with approximately 1249 mm of annual rainfall and an average monthly temperature of approximately 12.8 1C. The forest contains 75 ha of mature and regenerating conifer–broadleaf forest. There are five dominant canopy tree species, Elaeocarpus dentatus, Prumnopitys ferruginea, Beilschmiedia tawa, Dysoxylem spectabile and Melicytus ramiflorus, and two canopy emergent trees, Dacrydium cupressinum and Knightia excelsa. Easily accessible regions of old-growth forest, and the presence of a canopy walkway made this site an ideal location for assessing epiphyte and liana communities. Burns & Dawson (2005) provide a more detailed description of the site and its epiphyte and liana assemblages. Ten randomly located 10 m 10-m plots were established in old-growth forest. Five plots were situated on a slope with northern exposure (north) and five plots were situated on an opposite valley slope with southern exposure (south), thus ensuring an even distribution between the two major forest habitats within the reserve. All woody plants within each plot were inspected for lianas, and species identities of liana–host interactions were noted. Host size has been found to influence liana– host interactions (Chittibabu & Parthasarathy 2001). We included all host species in our analysis, as Burns & Dawson (2005) showed that host size had little effect on liana distribution at this site. Most liana species were attached to the forest floor by multiple stems, which were typically intertwined and difficult to distinguish individually. Therefore, accurate counts of interactions between individual plants were not possible. Instead, the frequency of liana occurrence was quantified as the number of host trees occupied by each liana species following Burns (2007). Furthermore, all liana–host interactions were recorded individually, regardless of multiple host interactions, as primary and secondary host trees could not be distinguished in such cases in which ‘canopy bridges’ had formed. Liana guilds and habitat associations Liana species were allocated to guilds a priori according to their vertical distribution of host trees. Three guilds were established. (1) The ‘canopy’ guild included all species that grew to the top of host trees. (2) The ‘trunk’ guild encompassed species that produce the majority of their foliage in close association with the main stem of host trees, directly below the canopy. All species within this guild were in the genus Meterosideros and climbed hosts using specialized roots. (3) The ‘base’ guild encompassed species that typically grew along the forest floor and climb only the base of host trees. All members of this guild were ferns that climb hosts with specialized roots, and seldom grow above 3 m. To test the accuracy of guild designations, we measured the maximum heights of all lianas encountered during sampling. One-way fixed factor Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science 869 Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. ANOVA assessed differences in maximum height obtained from each plant guild. To characterize the general patterns of lianas and their hosts we used a series of individual analyses. We tested whether liana and host distributions differed among habitats using individual one-tailed exact non-parametric Mann-Whitney U-tests for each species. Separate chisquared tests were conducted to test whether lianas from the same guild were partitioning habitats. For all lianas, we assessed whether the number of host interactions differed between habitats using simple logistic regression. Tests for interspecific differences in habitat preferences differ from tests for guild-based negative co-occurrence patterns (see co-occurrence analyses below) in that they investigate interspecific differences in horizontal distributions, rather than vertical differences in distribution. However, in both instances, differences in distribution would promote negative co-occurrence patterns. All data were transformed to meet normality assumptions when required and all analyses were conducted using the R environment. Co-occurrence analyses First, a community-level matrix was constructed, which assessed broad-scale liana–host interactions. This matrix was then subdivided to assess structural patterns displayed by compartments within the matrix. All compartments were generated by subdividing the rows and columns of the community-level matrix by known biological information (Fig. 1). As such, we investigated both quantitative Fig. 1. An example of matrices defined at five levels (i.e. community-level matrix, host species compartments, host species matrix, liana guild compartments and liana guild matrix). Host tree species are differentiated by symbols (square, triangle or circle: e.g. the first three columns represent three individuals of the same species) and liana species are differentiated by letters (rows A to I). Liana species are labelled on each host tree to indicate their position of occurrence. Dashed lines with attached arrows indicate the approximate region of host colonization and guild classification. In all matrices, interactions are represented by 1 in the corresponding matrix cell. Absences are represented by empty cells for clarity. 870 Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. and binary matrices to resolve the patterns of interactions found among lianas and host trees. We first subdivided the columns (individual host trees) of the community-level matrix to create compartments for each host species. Here, we defined these as ‘host species compartments’ and they represent binary interactions of individual trees with each liana species. The number of host species compartments was dependent on the number of host species observed. Therefore matrix size differed between compartments, and the patterns that they might display indicate the role of host preferences in communitylevel interactions. In the final step of analysing columns of the community-level matrix, we amalgamated all ‘binary’ columns of each host species compartment to create the ‘host species matrix’. The host species matrix represents a typical species interaction matrix without interaction frequency between species pairs. Second, we subdivided rows (liana species) of the community-level matrix to create compartments of liana species that are typically found on the same regions of host space. Here, we defined these as ‘liana guild compartments’ that contained three groups corresponding with the liana guilds defined above. Null model results indicate whether lianas within each guild co-occur nonrandomly. In the final step of analysing rows of the community-level matrix, we amalgamated all rows of each liana species compartment to create the ‘liana guild matrix’. The liana guild matrix now consisted of three rows representing presence or absence of each guild. Null model results indicate whether liana species that utilize disparate regions of host space co-occur together. In all matrices, when an interaction between liana species and a host tree was observed the cell was given a value of ‘1’; otherwise it was given a value of ‘0’. Null model simulations re-arrange these 1 and 0 values according to null model constraints. Binary matrices were assessed using fixed-equiprobable null model constraints (i.e. fixed rows and relaxed column constraints) and randomized using the sequential swap method of matrix fill. Quantitative matrices followed a similar procedure while accounting for liana and host abundance distributions. The total number of lianas for each species, in each plot, was fixed. Hosts were randomly sampled without replacement from a list of pooled host frequencies found in each habitat. Each list included host interaction frequencies, rather than the number of individuals, to account for multiple interactions with each host tree. Matrices generated for north and south habitats were merged to create the community matrix. Null model simulations of liana guild compartments and the liana guild matrix were also derived from the community matrix. This is a quantitative approach that accounts for the underlying distribution of abundances and provides a set of constraints that accounts for habitat partitioning. We used the c-score to calculate negative co-occurrence patterns among liana species at each level. The c-score calculates the average checkerboard distribution of all species interactions within each liana–host interaction matrix. The c-score index was obtained by calculating the number of checkerboard units ‘cu’ for each species pair: cu = (Oi S)(Oj S), where Oi is the total number of host species occupied by liana species i, Oj is the total number of host species occupied by liana species j, and S is the number of host species occupied by both species (Stone & Roberts 1990). The observed c-score was compared to 5000 simulation replicates, and all observed values greater than null model expectations indicated support for negative co-occurrence patterns. All c-score simulations were carried out using ECOSIM version 7 (Gotelli & Entsminger 2001) for binary matrices or the R environment for quantitative matrices. All results were assessed using a Bonferoni correction (a/5 = 0.01) to adjust for exploratory analyses of different compartments. Null model simulations are susceptible to matrix size and fill (Gotelli 2000). Therefore, to account for small matrix size in compartments, all species recording less than five interactions were omitted from analysis. Therefore, two dicotyledonous lianas (Clematis paniculata and Rubus cissoides), one canopy emergent (Podocarpus totara), one canopy tree (Pittosporum eugenioides), six shrub species (Alectryon excelsus, Brachyglottis repanda, Coprosma lucida, Myrsine australis, Pseudopanax arborius, Olearia rani), two tree ferns (Cyathea medulla, Cyathea dealbata), two liana species represented as hosts (Metrosideros fulgens and Ripogonum scandens) and two unknown host species were omitted. To ensure all matrices were comparable, the standardized effect size (SES) was assessed in each null model simulation. Matrix size (number of cells) was transformed to meet normality assumptions and correlated with SES values using linear regression. If a relationship was not found, these matrices were considered comparable. Results A total of 217 trees occurring in all plots met the criteria for analysis. Approximately 60% of trees hosted (n = 125) lianas and the remainder (n = 92) lacked lianas. Two dicotyledonous lianas, Parsonsia heterophylla (A. Cunn.), Passiflora tetrandra, and one monocotyledonous liana, Ripogonum scandens (G. Forst.) were assigned to the ‘canopy’ guild. Three dicotyledonous lianas, Metrosideros fulgens (G. Forst.), M. perforata (A. Rich.) and M. diffusa (G. Forst.) were assigned to the ‘trunk’ guild. Three climbing ferns, Blechnum filiforme (A. Cunn.), Microsorum pustulatum Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science 871 Liana co-occurrence patterns 872 125 3 3 5 7 9 4 8 6 8 14 11 36 11 68 14 8 45 21 11 36 24 10 237 1 0 0 2 0 0 0 2 0 5 2 1 0 0 0 0 1 0 2 6 3 1 0 2 0 0 1 0 0 7 7 0 2 0 0 0 0 0 0 9 6 0 4 1 0 0 1 0 0 12 2 0 0 4 3 0 0 3 0 12 0 0 0 7 2 0 1 3 0 13 1 1 0 4 4 2 2 1 0 15 4 1 0 7 2 0 3 2 1 20 7 3 1 2 2 3 0 3 0 21 7 5 0 4 3 1 3 1 3 27 5 1 1 7 4 1 2 7 0 28 Canopy 23 Canopy 1 Canopy 0 Trunk 5 Trunk 1 Trunk 4 Base 22 Base 2 Base 4 62 Ripogonum scandens Passiflora tetandra Parsonsia herterophylla Metrosideros fulgens Metrosideros perforata Metrosideros diffusa Blechnum filiforme Microsorum pustulatum Microsorum scandens Frequency of host interactions Number of host trees Frequency of liana occurrence Cyathodes fasciculata Corynocarpus laevigatus Hedycarea arborea Geniostoma rupestrae Dacridium Macropiper cupressium excelsum Dicksonia squarosa Prumnopitys ferruginea Melicytus Knightia ramiflorus excelsa Beilschmidia tawa Elaeocarpus dentatus Dysoxylum spectabile Guild Tree density differed among habitats (north = 77, south = 140); although the distribution of individuals did not differ (Mann Whitney U-test: z = 0.437, P = 0.662). At the species level, four out of 13 host tree species showed significant differences between habitats (one tailed exact Mann-Whitney U: E. dentatus, z = 2.300, P = 0.032; G. rupestre, z = 2.635, P = 0.008; M. ramiflorus, z = 2.652, P = 0.008; and D. spectabile, z = 2.795, P = 0.008). All remaining host tree species showed no significant differences (n = 8; P 4 0.134). This result indicates that there is an overlap in tree species distribution; however, certain species characterize the north- and south-facing habitats in this reserve. Liana density was similar among habitats (north = 106, south = 131; Mann-Whitney U: z = 0.408, P = 0.683), however, when we considered each species individually, two out of nine lianas showed greater abundance in the south-facing habitat (one-tailed exact Mann-Whitney U: R. scandens, z = 2.220, P = 0.032; and B. filiforme, z = 2.471, P = 0.016) and one liana species showed greater abundance in the north-facing habitat (M. pustulatum, z = 2.471, P = 0.016). All remaining liana species showed no significant differences between habitats (n = 6; z 4 1.928, P 4 0.095). Height measurements generally support quantitative guild designations. There was a significant difference in maximum height obtained by each climbing guild (oneway ANOVA; f2, 211 = 19.97, P o 0.001). The canopy guild reached an average maximum height of 7.52 m; the trunk guild reached an average maximum height of 5.07 m; and the base guild reached an average maximum height of 2.2 m. M. diffusa and P. heterophylla recorded different overall heights to other members in their respective guilds. In this study, these species were observed as Host tree species Liana guild and habitat associations Liana species (G. Forst.) and M. scandens (G. Forst.) were assigned to the ‘base’ guild. Lianas occurred on two canopy emergent trees (Dacrydium cupressinum and Knightia excelsa), six canopy trees (Melicytus ramiflorus, Elaeocarpus dentatus, Beilschmiedia tawa, Dysoxylem spectabile, Corynocarpus laevigatus, Prumnopitys ferruginea), one sub-canopy tree species (Hedycarya arborea), three shrub species (Macropiper excelsum, Geniostoma rupestre, Cyathodes fasciculata) and one tree fern (Dicksonia squarrosa) (Table 1). D. spectabile, G. rupestre and M. excelsum were almost exclusively found in the southfacing habitat and collectively represented the highest number of individual trees without lianas (n = 62). M. excelsum and G. rupestre occurred in the north-facing habitat, however they typically remained free of lianas (n = 6 and n = 4, respectively). Table 1. Host tree and liana species with 5 or more interactions are ordered by frequency of occurrence in descending order. Rows are further divided into guild categories. All host trees and liana species registering less than 5 interactions were omitted from analysis and not included in this table. Blick, R.A.J. & Burns, K.C. Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. 60 South North 31 30 22 19 20 10 Trunk Canopy Base Trunk 6 Par het 2 2 Met ful 0 2 Met per Guild Pas tet Par het Rip sca Guild Met ful Met per Met dif Guild Ble fil Mic sca Mic pus Base Mic pus 2 0 77 65 Pas tet 10 2 18 14 Met dif 4 34 Ble fil 6 40 Mic sca Observed abundance Mean maximum plant height 8 0 50 50 10 Rip sca 12 Canopy Fig. 2. Mean maximum height (m) obtained by three liana guilds and their host trees. Bars with open fill represent liana species. Bars with closed fill represent liana guilds. Error bars indicate standard errors. Species names abbreviated to first three letters of genus and species. Fig. 3. Observed abundance of each liana in the north-facing habitat (closed fill) and south-facing habitat (open fill). Species names abbreviated to first three letters of genus and species. juveniles and were encountered less frequently than all other species. For all remaining species, our method for classifying lianas into guilds accurately accounted for host tree partitioning (Fig. 2). When we considered guild associations, we found that liana species in the base guild and the trunk guild differed significantly between habitats (x2 = 53.479, df = 2, P o 0.001; x2 = 7.512, df = 2, P = 0.023, respectively). This result was not consistent with the canopy guild (x2 = 3.165, df = 2, P = 0.205) (Fig. 3). These results indicate that lianas typically grow in preferred habitats and that lianas with similar growth strategies grow in divergent locations. However, lianas that use twinning or tendrils to reach the canopy do not partition the habitat clearly. Not all trees were occupied by lianas. Trees that lacked lianas (n = 92) were generally small (mean height = 2.9 m, mean DBH = 23.6 mm), although some trees were from larger size classes (max height = 15.1 m, max DBH = 199 mm). The number of occupied hosts was similar between habitats (n = 53, n = 72, respectively; one-tailed exact Mann-Whitney U: z = 0.335, P = 0.738). However, five out of nine liana species occupied host trees more frequently in their preferred habitat (simple logistic regression: M. perforata, score = 30.713, df = 1, P o 0.001; M. fulgens, score = 27.674, df = 1, P o 0.001; M. pustulatum, score = 37.207, df = 1, P o 0.001; M. scandens, score = 5.776, df = 1, P = 0.016; B. filiforme, score = 16.887, df = 1, P o 0.001. R. scandens showed a weak significant difference in host use between habitats (score = 3.514, df = 1, P = 0.061). All remaining liana species did not occupy host trees preferentially in either habitat (n = 3, score o 0.721, df = 1, P 4 0.396). Co-occurrence analyses Co-occurrence patterns differed strongly between the community-level matrix and generated compartments. Negative co-occurrence patterns were found in the community-level matrix as the observed c-score was significantly higher than randomized expectations (CS = 4.472, ZS = 6.543, P o 0.001) (Fig. 4). However, the individual matrices generated for north and south habitats revealed contrasting results to the community-level matrix (CS = 0.286, ZS = 4.702, P o 0.001; CS = 1.583, ZS = 0.302, P = 0.381, respectively). Host species compartments revealed results consistent with null model expectations for all species (P 4 0.041). The host species matrix revealed positive co-occurrence patterns, as the observed c-score was significantly lower than expected by chance (CS = 4.47, P = 0.004). Liana guild compartments and the liana guild matrix gave a range of results. The observed c-score for the base guild (CS = 5.000, ZS = 1.974, P = 0.024) showed some evidence for negative co-occurrence, while the trunk and canopy guilds were not significantly different to randomized expectations (CS = 0.000, ZS = 4.795, P o 0.001; CS = 3.330, ZS = 0.498, P = 0.309, respectively). The liana Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science 873 Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. Fig. 4. Null model simulations for the community and associated compartments following the layout of Fig. 1. Matrix evaluations failed for the liana guild matrix and one host species compartment (Geniostoma rupestrae) and are omitted here. If the observed c-score (indicated by arrows on the x-axis) is significantly greater than 5000 simulations, it would indicate negative co-occurrence patterns. Host species names are abbreviated to the first letter of genus and species. guild matrix gave a null result (CS = 1.000), as matrix evaluations gave a single index of 0.816. This may have occurred due to an inflated number of links (i.e. 1 values) in the amalgamated matrix. No significant correlation was found between matrix size and SES (r2 = 0.102, P = 0.668). The results were interpreted as matrix size was not responsible for differences in null model simulations. Discussion Negative co-occurrence patterns were observed at the community level, and analyses of compartments within the community-level matrix provided some resolution of 874 the mechanisms underpinning community-level analyses. Null model simulations of each habitat and each host species compartment revealed habitat-level divergence in species interactions. Analysis of liana and host abundance distributions support the hypothesis that negative co-occurrence in the community matrix is derived from lianas that interact with host species in preferred habitats. When we generated liana guild compartments, we found negative co-occurrence patterns in the base guild. This was due to differences in horizontal partitioning of habitat space and interactions with host trees that grow in the same location. The community-level matrix revealed significant negative co-occurrence patterns indicating the presence of Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. host-specific interactions. This type of community structure is observed when many species interact in reciprocal pairs, with the absence of generalist species interactions (Bascompte & Jordano 2007). In plant–plant interactions, two contrasting community-level patterns are recognized (Blick & Burns 2009). Mistletoe–host interactions often reveal a high degree of host specificity, in which establishment success appears to be host limited (Norton & De Lange 1999; Mathiasen et al. 2008; Watson 2009). Alternatively, epiphyte–host interactions reveal nested patterns (Burns & Dawson 2005; Burns 2007) similar to those seen in plant–pollinator and plant dispersal networks; in which the community is structured with a core set of generalist species and relatively few specialists (Olesen et al. 2007). Here, the community-level matrix indicates similar structural patterns to those seen in mistletoe–host interactions, indicating host preferences in the community. In contrast to the community matrix, each habitat alone and all host species compartments revealed results consistent with random expectations. This suggests that lianas colonize host trees randomly within each habitat, and negative co-occurrence patterns found in the community-level matrix are derived from divergent species assemblages. Liana and host abundance distributions support the notion that geographically separated species constrain the number of possible liana–host interactions. Similarly, ‘functional biogeographical’ units have been found embedded in the community matrix of nectarivorous birds (Carstensen & Olesen 2009) and in marine food webs (Rezende et al. 2009). These functional units consist of spatially organized species assemblages; however, the processes underlying these patterns remain largely unresolved. Here, we suggest that liana–host interactions are structured through indirect processes relating to habitat; however, further testing is required to validate this hypothesis. Liana guild compartments showed varying results. The base guild revealed negative co-occurrence patterns, while the trunk and canopy guilds revealed neutral or positive results. Alone, negative co-occurrence patterns indicate that the base guild is partitioning host space vertically through unshared allocation of resources (competitive exclusion principle) (Hardin 1960). However, if we consider abundance distributions across the northand south-facing habitats we find an alternative explanation. The base guild partitioned habitat almost exclusively, while the trunk and canopy guilds showed less marked differences. Therefore, co-occurrence patterns reflect abundance distributions in each compartment and the community-level matrix. More broadly, this suggests that one guild may be influencing the outcome at the community level. We do not preclude structural characteristics as a mechanism altering liana–host interaction; however, our results indicate that spatial organization of individuals among habitats is important. Further testing is required to identify the specific processes that lead to these habitat associations. The host species matrix revealed positive co-occurrence patterns – a result opposite to that of the community matrix. This result suggests that lianas are co-occurring on host species (e.g. facilitation). Facilitative interactions have been recorded previously (Putz 1984; Pérez-Salicrup & de Meijere 2005), and community-level positive co-occurrence patterns have been found in other ecosystems (see Sfair et al. 2010). However, this result does not support what was found using an individual-based approach to analyse the community-level matrix or host preferences previously found at this site (Burns & Dawson 2005). This illustrates the necessity for investigating species interactions using individual-based null models that account for abundance or intensity of interactions among individuals (Krishna et al. 2008; Blick & Burns 2009). Liana–host interactions are an important part of tropical and temperate ecosystems (Schnitzer 2005). Lianas compete for nutrients and exploit other trees to obtain available light in the forest canopy. Accessing available light may influence co-occurrence of liana species (Putz 1984; Pérez-Salicrup & de Meijere 2005). However, our results indicate that habitat preference is an important component underlying community-level patterns of liana–host interactions in Otari-Wilton’s Bush Reserve. Further investigation into liana–host interactions should account for habitat preferences, which accurately characterize host availability and host use. In summary, we found habitat preferences among liana species that would have otherwise been interpreted as partitioning of host space (liana guild compartments). Overall, liana–host interactions found in this study system follow an opportunistic growth strategy, and interactions are due mostly to habitat partitioning. While we have used a relatively species-poor plant–plant interaction network, we have illustrated how the underlying patterns of distribution can affect negative co-occurrence patterns. Acknowledgements We thank Tom Philippi, Bruce Allen and two anonymous reviewers for their helpful comments. We also thank Victoria University of Wellington for financial assistance and the Otari-Wilton’s Bush Reserve staff for permission to conduct this study. References Allan, H.H. (1961) Flora of New Zealand, Vol. 1. Government Printer, Wellington, NZ Journal of Vegetation Science Doi: 10.1111/j.1654-1103.2011.01287.x r 2011 International Association for Vegetation Science 875 Liana co-occurrence patterns Blick, R.A.J. & Burns, K.C. Allen, B.P., Pauley, E.F. & Sharitz, R.R. 1997. Hurricane impacts on liana populations in an old-growth southeastern bottomland forest. Journal of the Torrey Botanical Society 124: 34–42. Allen, B.P., Sharitz, R.R. & Goebel, P.C. 2005. 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