Journal of Vegetation Science 18: 163-174, 2007 © IAVS; Opulus Press Uppsala. - Patterns of woody plant species abundance and diversity - 163 Patterns of woody plant species abundance and diversity in the seedling layer of a tropical forest Comita, Liza S.1*; Aguilar, Salomón2; Pérez, Rolando2; Lao, Suzanne2 & Hubbell, Stephen P.1,2 1Department of Plant Biology, University of Georgia, Athens, Georgia 30602, USA; 2Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948, USA; E-mails: [email protected] (SA); [email protected] (RP); [email protected] (SL); [email protected] (SPH); *Corresponding author; E-mail [email protected] Abstract Introduction Questions: 1. How does the composition and diversity of established seedlings compare to that of larger size classes in Recent studies have highlighted the importance of seed tropical forests? 2. How do species abundances in the seedling dispersal and seedling establishment in the regeneration layer vary with adult abundance and life history strategies? of tree species and the maintenance of diversity in both Location: Barro Colorado Island (BCI), Panama. Methods: We inventoried woody seedlings ≥ 20 cm tall and < temperate and tropical forests (e.g. Dalling et al. 1998a; 1 cm DBH in ca. 20 000 1-m2 quadrats within the BCI 50-ha Lambers et al. 2002; Nathan & Muller-Landau 2000; Forest Dynamics Plot, where all trees and shrubs ≥ 1 cm DBH Wang & Smith 2002). However, once established, tree have been identified. We compared diversity and composition seedlings can spend decades in the forest understorey of seedlings to that of larger size classes and tested whether waiting for openings in the canopy to provide adequate adult abundance, growth form, and shade tolerance contributed light for growth into the canopy layer (Antos et al. 2005; to variation in seedling abundance among species. Delissio et al. 2002). Therefore, a substantial portion of a Results: We encountered 60056 seedlings of 332 tree, shrub, treeʼs life can be spent as an established seedling or small and liana species. Diversity of tree seedlings was lower than sapling in the shaded understorey (Hubbell 1998). This life that of trees ≥ 1 cm DBH. Species abundances in the seedling layer increased non-linearly with reproductive adult abundance, stage may be particularly crucial to the success of shade- such that per capita seedling abundance declined with adult tolerant species and species lacking viable seed banks, abundance. Per capita seedling abundance was highest for which comprise the majority of species in most tropical canopy tree species and lowest for understorey trees. For canopy tree communities (Vazquezyanes & Orozcosegovia 1993; trees, shade-tolerant species had significantly higher per capita Whitmore 1989). seedling abundance than more light-demanding species. When an opening does occur in the forest canopy, Conclusions: The woody seedling layer on BCI is composed these gaps are often filled in by preexisting seedlings of a subset of the species present in larger size classes. Most and saplings that established in the shade prior to gap species were present in less than 1% of seedling plots, sug- formation (Brokaw & Scheiner 1989; Connell 1989; Uhl gesting strong recruitment limitation. Tree species abundance in the seedling layer is largely a function of reproductive adult et al. 1988). The seedling bank therefore serves as the abundance, but is also influenced by life history strategies and template upon which gap phase dynamics act to produce compensatory processes. patterns of adult tree species diversity and relative abun- dances (Brokaw & Busing 2000; Brokaw & Scheiner 1989). Thus, identifying community-wide patterns of Keywords: Advanced regeneration; Barro Colorado Island; established seedling abundance and diversity in the for- Forest dynamics plot; Life history strategy; Recruitment limita- est understorey is critical to our overall understanding tion; Seedling bank; Shade tolerance. of tropical forest dynamics. A speciesʼ abundance in the seedling bank will be a function of three demographic rates: recruitment into Abbreviations: BCI = Barro Colorado Island; DBH = Diam- eter at 1.3 m. the seedling bank, survival of established seedlings, and growth out of the seedling layer into larger size classes. The upper limit of recruitment into the seedling layer is Nomenclature: Correa et al. (2004). governed by seed production of reproductive adults in the community. Species abundance in the seedling layer is therefore likely to increase with reproductive adult abundance. If the number of seedlings produced per adult 164 Comita, L.S. et al. is constant, then we would expect a linear relationship largest census of woody seedlings at a single study site between seedling and adult species abundances. How- to date. In addition, these data can be directly linked to ever, studies from both temperate and tropical forests an existing 25-year dataset on the dynamics of trees and have shown that seedling establishment and survival shrubs ≥ 1 cm DBH in the BCI 50-ha plot, providing an are negatively influenced by conspecific density (Harms unprecedented opportunity to explore community-wide et al. 2000; Lambers et al. 2002; Wright et al. 2005a). patterns of species abundance and diversity across life Negative density dependence can generate a rare species stages. advantage, or community compensatory trend (Con- Here we present results from the initial BCI 50-ha nell et al. 1984), whereby species that are abundant in plot seedling census and examine established seedling the community have lower rates of seed and seedling density, diversity and abundance for 396 tropical tree, survival compared to rarer species. Thus, while the shrub, and liana species. We model the relationship be- absolute number of established seedlings is expected to tween established seedling and adult tree abundances in increase with increasing reproductive adult abundance, the BCI tree community. We then test whether variation the number of seedlings per adult may decline as a result in per capita seedling abundance among species can be of compensatory processes during establishment. explained by differences in life history strategies, namely Variation in the number of seedlings per reproduc- shade tolerance and growth form. tive adult (hereafter referred to as per capita seedling abundance) may also result from differences in life his- tory strategies among species. For example, per capita Methods seedling abundance may vary with growth form. Trees in the canopy will have more resources to devote to Study site reproduction than understorey trees, which typically reproduce in the shade and have smaller canopies and The study was conducted in the 50-ha Forest Dynam- root systems with which to capture resources. Thus, we ics Plot (500 m × 1000 m) on Barro Colorado Island would expect canopy tree species to have more seedlings (BCI), Panama (9°9' N, 79°51' W) (Hubbell & Foster per reproductive adult than smaller-stature species. 1983; Hubbell et al. 1999). BCI is classified as tropical Variation in per capita seedling abundance may also re- moist forest and receives a mean of 2600 mm of rain per sult from differences in speciesʼ shade tolerance. Tropical year, most of which falls during the 8-month wet season tree species are known to vary greatly in the amount of from May to December (Windsor 1990). More detailed light required for regeneration (Whitmore 1998). descriptions of the climate, geology, and flora of BCI can Shade-tolerant, climax species are capable of estab- be found in Croat (1978), Leigh et al. (1982), and Leigh lishing and persisting in the shaded understorey. At the (1999). The BCI 50-ha plot was established in 1980, other end of the spectrum, light-demanding pioneer spe- and all trees and shrubs ≥ 1 cm DBH in the plot were cies require canopy gaps for establishment and survival, mapped, identified to species, and measured between and grow quickly in high light conditions (Hubbell & 1982 and 1983 (Condit 1998; Hubbell & Foster 1983). Foster 1992). Pioneer species are therefore expected The BCI plot has been recensused at 5-year intervals to have fewer individuals per reproductive adult in the since 1985. seedling bank compared to more shade-tolerant species, since pioneers cannot persist in the shade, and they grow Field methodology quickly into larger size classes when in gaps. Testing community-level hypotheses concerning the In 2001, we established a permanently marked 1 seedling layer in tropical forests is challenging because m × 1 m seedling plot in the center of each 5 m × 5 m most tropical tree species are rare and clumped in dis- subquadrat of the 50-ha plot for a total of 20 000 plots persion (Condit et al. 2000). Numerous samples spread (total area sampled = 2 ha). Between January and Octo- over a large area are therefore needed to assess fully ber 2001, all woody plants greater than 20 cm in height community-wide patterns of seedling abundance and and less than 1 cm DBH (hereafter referred to simply as diversity. With this objective in mind, we began a long- seedlings) were tagged, mapped and identified to species term, community-wide survey of established seedlings within each 1-m2 plot. Seedlings were not censused in and small saplings in the 50-ha Forest Dynamics Plot on 366 seedling plots to avoid pre-existing research plots Barro Colorado Island (BCI), Panama (Hubbell & Foster within those areas. Lianas were not measured in the 1983). We established ca. 20 000 1-m2 seedling plots at census of stems ≥ 1 cm DBH, however liana seedlings 5-m intervals across the BCI plot and censused all woody were included in the seedling census if they were not seedlings and small saplings ≥ 20 cm in height and < 1 yet entwined in or climbing on another plant. Palm cm DBH within each plot. To our knowledge, this is the seedlings (family Arecaceae) were excluded from the - Patterns of woody plant species abundance and diversity - 165 seedling census because they lack measurable woody ent seedling production per reproductive adult, and b stems.