Plant Ecol (2008) 195:131-141 DOI 10.1007/s 11258-007-9310-0 The vascular epiphytes of a lowland forest in Panama- species composition and spatial structure Gerhard Zotz • Steffen Schultz Received: 4 October 2006 / Accepted: 15 May 2007 / Published online: 12 June 2007 © Springer Science+Business Media B.V. 2007 Abstract We conducted a comprehensive census of data-set for the direct observation of the long-term the vascular epiphytes in a lowland forest in Panama dynamics in a diverse epiphyte community. by means of a canopy crane. In 0.4 ha of ca. 40-m tall forest, 103 species of vascular epiphytes with 13,099 Keywords Biodiversity • Bromeliaceae • individuals were found. The orchids were the most Orchidaceae • Tropical forests • San Lorenzo crane important family both in species and individual site • Patchiness numbers, accounting alone for >40% of all species and >50% of all individuals. There was a clear vertical segregation of species with a concentration at inter- Introduction mediate heights: more than 50% of all individuals were found between 15 m and 25 m above ground. The control of alpha diversity, i.e. the number of Tree species identity, tree size and the position of a species coexisting at a small spatial scale, remains a tree in the forest ("space") all influenced species central question of community ecology, particularly composition. However, none of the two environmen- for hyper-diverse tropical rain forests. While enor- tal variables nor space alone explained more than 10% mous progress has been made in understanding the of the total variation in epiphyte assemblages in mechanisms underlying the diversity of trees in several canonical correspondence analyses. By far the tropical forests (Harms et al. 2000; Hubbell et al. largest proportion of the observed variation remained 1999; Volkov et al. 2005) this is much less true for unexplained and is arguably due to mere chance. In other life forms. Considering that "non-trees" gen- the future, our results will be used as a baseline erally form by far the larger portion of tropical phytodiversity (Galeano et al. 1999; Gentry and Dodson 1987) there is a clear need for descriptive G. Zotz (El) and experimental data on community composition Institut fur Biologic und Umweltwissenschaften, and community dynamics of the co-occurring groups Universitat Oldenburg, AG Funktionelle Okologie, of epiphytes, hemiepiphytes, lianas and vines, shrubs Postfach 2503, 26111 Oldenburg, Germany e-mail: [email protected] and forbs. Vascular epiphytes frequently represent the most species-rich life form in tropical forests G. Zotz (Benzing 1990; Gentry and Dodson 1987). Although Smithsonian Tropical Research Institute, Balboa, Panama their contribution to local species richness is generally S. Schultz greater in montane forests (Kelly et al. 2004), they Greening Australia, 525, Heidelberg, Vic 3084, Australia still account for at least 10-20% of all vascular 4y Springer 132 Plant Ecol (2008) 195:131-141 species in most lowland forests (Baislev et al. 1998; (depending on species). The delimitation of individ- Benavides et al. 2005; Bordenave et al. 1998; Croat ual plants is often difficult in vascular epiphytes, and 1978; Whitmore et al. 1985). we followed Sanford's (1968) definition of an Here we present a complete inventory of the "individual": a group of rhizomes and leaves vascular epiphytes found in 0.4 ha of lowland forest belonging to one species, which forms a clearly on the Caribbean slope of Panama and provide delimited stand. With the exception of small seed- detailed data on their spatial distribution. A tower lings, which could frequently not be identified to crane allowed repeated access to the forest canopy and species, all plant sizes were included in the census. facilitated a comprehensive sampling of the epiphytes Species names of flowering plants follow the Flora growing on almost 1,400 trees. The paper describes of Panama Checklist and Index (D'Arcy 1987), the current composition of the epiphyte community authorities for ferns are according to Lellinger comprising > 13,000 individuals, analyses vertical and (1989) and Croat (1978), for filmy ferns compare horizontal distribution patterns, and studies the effect also Zotz and Biiche (2000). Encyclia aemula (= of tree size and tree species on the composition of Prosthechea aemula) was treated as a separate species epiphyte assemblages. Future descriptive census work following Dressier (1993). Voucher specimens are will determine whether the current patterns in this deposited in the herbarium of the Smithsonian Trop- species-rich epiphyte community are rather stable in ical Research Institute, Panama (Tupper Center). time and space, while on-going experiments explore the underlying mechanisms of these distributions. Data analysis The spatial structure and the influence of host tree Materials and methods characteristics (tree identity and tree size (dbh = diameter at breast height)) on the composition of Field site and census work epiphyte assemblages (= epiphytes on a single host tree) were explored with ordination techniques using This study was conducted at the San Lorenzo Canopy CANOCO 4.5 (Leps and Smilauer 2003). With Crane site, which is located within the former Fort canonical correspondence analysis (CCA) one can Sherman area near the Atlantic coast of the Republic partition the variation in community composition of Panama (Wright et al. 2003). The average annual among different environmental variables, allowing rainfall is estimated to be around 3500 mm. Canopy also for the inclusion of a spatial component (Borcard height of this primary rain forest is quite variable and et al. 1992). We could, however, not include all three reaches a maximum of ca. 40 m. The use of a small expanatory variables (i.e. tree species, tree size, and gondola allowed access to all strata of the forest. spatial component) in a single analysis for a number Each tree in a roughly square area of ca. 0.4 ha was of reasons (see below), but rather performed one inspected for the occurrence of vascular epiphytes. series of analyses focusing on the spatial component While the census included woody hemi-epiphytes of the and another one focusing on the role of tree species genera Ficus, Coussapoa, Clusia and Havetiopsis and identity. To reduce heteroscedasticity, species abun- abundant climbing aroids such as Philodendron schot- dance data were log-transformed before analysis. tianum, the analysis of this paper is limited to holo- The distances between trees could only be epiphytes. There are a number of species in the assessed reliably for the subset of epiphytes growing Aracaeae, which may grow either as an epiphyte or as on trunks and the branches close to the trunk. secondary hemiepiphyte (Croat 1978). In this study, Moreover, considering the usual vertical stratification both life forms were observed in individuals of Philo- in epiphyte distributions (Nieder et al. 2000; Pittend- dendron radiatum and P. sagittifolium. Again, only truly righ 1948; Zotz 2007) a meaningful analysis of a epiphytic individuals are included in the analysis. spatial component can only be accomplished by The following data were collected for each restricting the vertical extension within the forest: individual: height of attachment (minimum and we confined our analysis to epiphytes growing in the maximum in creeping species), leaf/frond number, lowermost 10 m above the ground and only to those leaf length, stem length and/or number of shoots trees with, respectively, at least two or three species 4y Springer Plant Ecol (2008) 195:131-141 133 in this region. The spatial coordinates and the dbh the number of colonized trees and the total abundance of data for this analysis were obtained from unpublished each species as an index (PI) in an analysis of covariance. plot data (Rick Condit, STRI, unpubl.). A first CCA Only taxonomic groups (families with the exception of included 2181 epiphytes on 201 trees belonging to 71 "ferns") with at least 3 species and >10 individuals species of epiphytes, another analysis encompassed each were included. Data were again log-transformed 129 assemblages with 1934 epiphytes of 70 species. before analysis to reduce heteroscedasticity. Most of the, respectively, 68 and 52 tree species of the two data sets as environmental variables were rare (e.g. 33 of the 68 species were singletons), so tree Results species identity was not included in the analyses and variance was only partitioned between dbh and a In the forest plot of 0.4 ha, we inspected a total of spatial component. The matrix of geographical coor- 1373 trees with a diameter at breast height dinates (x and y) was completed by adding all terms (dbh) > 1 cm. Fewer than 30%, or 389 trees, hosted of a cubic trend surface regression using the software at least one vascular epiphyte, while 934 trees lacked package SpaceMaker2 (Borcard and Legendre 2004). epiphytes. Trees with epiphytes had a significantly To avoid an artificial increase of the explained greater dbh than those without (f-test t = 12.5, variation by chance, the nine terms of the resulting P < 0.001). There were, however, a few larger trees equation were reduced to seven by applying the without epiphytes in the study plot, e.g. a Vochysia CANOCO procedure of "manual forward selection ferruginea with 37 cm dbh and a Marila laxiflora of explanatory variables". with 26 cm dbh. Among epiphyte-bearing trees we A second series of CCAs focused on the potential found the expected correlations between tree size influence of tree species identity on epiphyte assem- (dbh) and epiphyte occurrence (dbh and abundance: blages. Since precise spatial positions of epiphytes r = 0.59, P < 0.05; dbh and species number: r = 0.72, were only available for those growing on trunks, this P < 0.01), respectively. In the plot there were 126 analysis ignored a spatial component. The following species of trees. Epiphytes were found on 90 (=71%) requirements had to be met by a tree species to be of these tree species.