Distance‐responsive natural enemies strongly influence seedling establishment patterns of multiple species in an Amazonian rain forest

Abstract

Summary1. In a faunally intact lowland Amazonian rain forest stand, we conducted a long-term multispecies experiment aimed at determining the primary mechanistic basis of seedling establishment patterns. We deployed a total of 1050 experimental seedlings, representing 11 common tree species in mixed compositions and at fixed, highly elevated densities in shaded understorey sites, representing extremes of distance from large conspecific trees. We used mesh exclosures to isolate the effects of distinct classes of natural enemies, and monitored survival for up to 45 months. 2. Final seedling survival of all species pooled represented a 40% increase at sites located far from ('F' sites) versus close to ('N' sites) large conspecific trees, and median seedling lifetime was 75% longer. These differences between N and F sites were significant for all species pooled, and for five out of 11 (survivorship) and four out of nine (lifetime) individual species examined. Survival analysis based on multiple censuses revealed that a 'distance effect' persisted and intensified over time, with the onset of significant distance-related differential mortality differing amongst species. 3. The use of mesh exclosures (<2 mm mesh size) and a factorial experimental design revealed that host-specific organisms <2 mm in size and ⁄ or below-ground soil-borne organisms are more strongly distance-responsive and depress seedling establishment primarily in the vicinity of large conspecific adults whereas above-ground organisms >2 mm in size appear to have a negative impact on seedling establishment at all distances. 4. No evidence was found for the effect of intra-cohort resource competition on seedling establishment even though initial density of experimental seedlings at all sites was elevated to c. 25 times the mean natural density of the mixed-species seedling layer in this forest. 5. Synthesis. Our study provides strong, multi-species support for the influence of host-specific distance-responsive natural enemies on seedling establishment, and suggests that negative densitydependent patterns of tree recruitment in tropical rain forests are at least partly produced at early life stages as an outcome of processes described by the classic Janzen-Connell model.

Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

“…This is further supported by the fact that the proportion of neighbors that were conspecific declined significantly with focal tree size at all sites, particularly for sapling-sized neighbors. Numerous studies have reported strong conspecific negative density-dependent mortality in both tropical and temperate forests (Augspurger 1984, Terborgh et al 2008, Uriarte et al 2004a, Stoll and Newbery 2005, Chen et al 2010, Swamy and Terborgh 2010, Bai et al 2012, Comita et al 2014), particularly at early life stages (e.g., seedling and sapling stages; Zhu et al 2015). Our results are consistent with these other studies and demonstrate that such density-dependent mortality patterns are strong enough to structure forest composition.…”

Interactions between repeated fire, nutrients, and insect herbivores affect the recovery of diversity in the southern Amazon

“…We hypothesized herbivores would increase diversity in accordance with Janzen's negative density dependence prediction (1970) and that this result would be strongest in burned plots with nutrient additions because these conditions might favor herbivore host-searching. Negative effects of herbivores and pathogens on recruitment are required to support the Janzen-Connell hypothesis, and such effects have been demonstrated multiple times (Coley 1983;Clark and Clark 1985;Norghauer et al 2006), although rarely at the community level (Swamy and Terborgh 2010;Alvarez-Loayza and Terborgh 2011). Our application of the JanzenConnell hypothesis is nontraditional in that we considered effects of all herbivores rather than specialists alone, but recent work has shown that even generalist herbivores can have important positive effects on diversity, resulting in Janzen-Connell patterns (Dyer et al 2010).…”

“…Based on these 7 studies, support of the Janzen-Connell hypothesis is equivocal. Recent studies show evidence of negative density dependence (Comita and Hubbell 2009;Chen et al 2010;Comita et al 2010), but more studies involving multiple species are needed (Carson et al 2008;Swamy and Terborgh 2010). Fire can be expected to influence the strength of these herbivore-plant interactions by decreasing plant diversity, effectively making regenerating seedlings more apparent to herbivores.…”

Early recruitment responses to interactions between frequent fires, nutrients, and herbivory in the southern Amazon

“…Changes in nutrients and fire both interact with herbivory, which has been shown to be important in determining tropical forest diversity Swamy and Terborgh 2010;Alvarez-Loayza and Terborgh 2011). The Janzen-Connell hypothesis postulates that herbivores create patterns of negative density dependence by selectively feeding on aggregated groups of conspecific seeds or seedlings (Janzen 1970;Connell 1971).…”