Introduction
Understanding the ecological processes that maintain high species diversity in natural environments, such as tropical forests, is fundamental in community ecology (He et al. , 1996). The main processes maintaining species coexistence in tropical forests are deterministic or stochastics (Chase and Myers, 2011). The deterministic processes of interspecific competition and habitat filtering are based on the species’ niche. Species that explore the same limiting resources or that have similar tolerance limits to the same environmental conditions compete intensely, especially in the case of sessile organisms such as trees. In a two-species model with asymmetric competition, the stronger species exclude the weaker species (Schluter, 2000) or constrains the spatial distribution of the weaker species to less favourable environments (Baraloto et al. , 2007). Thus, if these deterministic processes are important forces structuring a community, species coexistence is possible when each species has a different niche or when environmental heterogeneity is high enough so that species with similar niches are restricted to somewhat different environments (Silvertown, 2004). On the other hand, according to neutral theory (Hubbell, 2001), species diversity is the balance between stochastic emergence and disappearance of species at the regional scale (Hubbell, 2005). Locally, seed arrival in vacant space is unpredictable, considering the spatio-temporal variation in availability (stochasticity). However, dispersal and recruitment limitation, common in tropical forests (Hubbell et al. , 1999), prevent the most abundant species from occupying all available recruitment sites and thus dominating the community over time (competition exclusion), i.e. “winning-by-forfeit” (Hurtt and Pacala, 1995; Hubbell, 2001).
The arrangement of plants in space is the result of processes acting on each individual throughout its life, with each ecological process generating a characteristic spatial structure at a given spatial scale (Hubbell et al. , 2001). Habitat filtering, for example, is expected to result both in spatial segregation of species with different environmental requirements at spatial scales larger than the patch scale where the environment is approximately similar (Itoh et al. , 2003; Getzin et al. , 2006) and in spatial association of species with similar environmental requirements at spatial scales smaller than the patch scale (Burns and Strauss, 2011). When these species additionally compete within patches, reduced resource availability should result in a decrease in growth rate (Kenkel, 1988), thereby causing nearby neighbours to be smaller than distant trees (Getzinet al. , 2008). Also, in extreme cases, the stronger competitor can eventually cause mortality of the weaker competitor (Kenkel, 1988), resulting in spatial segregation of competing species at the small neighbourhood scale (say < 5 m; Velázquez et al. , 2015).
In contrast, if species stochasticity governs community structuring as assumed in neutral theory, conspecific interactions should be stronger than heterospecific interactions, producing an approximate independent spatial relationship between species pairs (Volkov et al. , 2007). Additionally, species distribution should not be related to environmental characteristics, as all species are expected to respond to the environment in a similar way (Hubbell, 2005). Also, dispersal limitation is expected to result in aggregation of seeds and small trees, and in spatial associations of seeds and smaller trees to large trees (Murphy et al. , 2017), because mortality is usually not as strong, even at high-density patches, as to spatially uncouple seeds from their parent trees (Hubbell, 1980; Hubbell, 2005; but see Getzinet al. , 2014).
Even though habitat filtering and dispersal limitation both result in aggregation and spatial association of individuals, the former leads to species distribution related to local environmental variables, whereas the latter causes high density of small trees centred on conspecific large trees (Wang et al. , 2015). Additionally, the spatial pattern of individuals of different sizes indicates which ecological process has acted more strongly on the population (Comita et al. , 2007; Shen et al. , 2009). Species-habitat association might change through plant development, possibly resulting in spatial dissociation between size classes. Moreover, species-habitat association is expected to be stronger for large than for small trees (Comitaet al. , 2007; but see Baldeck et al. , 2013). On the other hand, aggregation of individuals of different sizes and spatial association between size classes are expected under strong dispersal limitation (Wiegand et al. , 2007).
Investigating ecological processes with spatial point process models may be especially important in harsh environments, such as areas of rocky soils (Pollock et al. , 2012) and areas subject to periodic flooding (Baraloto et al. , 2007; Colmer and Voesenek, 2009), as these environments can act as strong drivers of species selection (Chapin et al. , 1993; Reich et al. , 2003). Selective pressures on plants, such as gradients in soil moisture, can change functional traits and the niche relationship of co-occurring species (Werner and Platt, 1976). However, few studies have evaluated the spatial structure of trees in environments with seasonal flooding (e.g. Baraloto et al. , 2007; Oliveira et al. , 2014), even though habitat filtering is commonly associated to topographic and edaphic variation in tropical forests (Bagchi et al. , 2011; Baldecket al. , 2012). Thus, species with different flooding tolerances should show different species-habitat associations according to the local environmental heterogeneity (Baraloto et al. , 2007).
Congeneric species are a good model to investigate the relative importance of ecological processes that maintain high species diversity because they tend to have similar niches due to their close phylogenetic relationship (Losos, 2008). Therefore, strong interspecific competition within the same tolerable environment should be easily detected, resulting in spatial association between species at large scales (< 30 m) due to habitat filtering and dissociation at small scales (< 5 m) due to interspecific competition (Velázquezet al. , 2015). However, congeneric species may show niche differentiation due to selective pressures acting on each one of them at the evolutionary scale, including competition within the same tolerable environment (Wiegand et al. , 2007). This “ghost of competition past” results, at present day, in each species specialized in a different environment (Stubbs and Wilson, 2004; Yamada et al. , 2005) or in different use of resources within the same tolerable environment (Schluter, 2000). Alternatively, species’ niche may not be important, and so the spatial structure of populations should reflect dispersal limitation and stochasticity (May et al. , 2014).
The present study aims to find evidence for the action and relative importance of different ecological processes hypothesized to maintain species coexistence in a tropical forest subject to seasonal flooding, using the spatial structure of populations of three congeneric species. Specifically, we ask: (1) which environmental variables are associated with the spatial distribution of the different size classes of each species? (2) What is the spatial relationship between congeners in the same size class? (3) Is the size of an individual influenced by the proximity to congeners in the same size class? (4) What is the spatial relationship between small and large conspecific trees? The combination of different spatial patterns will indicate which ecological processes are more important to the maintenance of species coexistence (see Table 1).