Are all seeds equal? Spatially explicit comparisons of seed fall and sapling recruitment in a tropical forest

Abstract

Understanding demographic transitions may provide the key to explain the high diversity of tropical tree communities. In a faunally intact Amazonian forest, we compared the spatial distribution of saplings of 15 common tree species with patterns of conspecific seed fall, and examined the seed-to-sapling transition in relation to locations of conspecific trees. In all species, the spatial pattern of sapling recruitment bore no resemblance to predicted distributions based on the density of seed fall. Seed efficiency (the probability of a seed producing a sapling) is strongly correlated with distance from large conspecific trees, with a >30-fold multiplicative increase between recruitment zones that are most distant vs. proximal to conspecific adults. The striking decoupling of sapling recruitment and conspecific seed density patterns indicates near-complete recruitment failure in areas of high seed density located around reproductive adults. Our results provide strong support for the spatially explicit predictions of the Janzen-Connell hypothesis.

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

“…Thus, the presence of an intact community of animal dispersers appears critical for seeds to escape the effects of NDD [3]. If loss of animal-mediated seed dispersal results in seed deposition near or beneath the parent crown, as seems likely for Miliusa, very few seeds would be likely to escape NDD, because the scale of NDD can be tens of metres greater than the crown radius of the parent tree [26]. Theoretical models for spatial plant population dynamics show that lower population size is a predictable consequence of decreasing the scale of seed dispersal relative to the scale of NDD [52]; our empirical study shows that this theoretical prediction is relevant to tropical forest conservation.…”

Reproductive phenology, seed removal and early regeneration in relation to distance from parental plants of a native palm in small Atlantic forest fragments

“…), seedling densities increase with distance from parental plants, indicating that distanceand density-dependent processes may affect recruitment in this early stage like expected from the Janzen-Connell model. This supports the importance of seed dispersal for recruitment at this early stage (Hyatt et al 2003;Swamy et al 2011). However, the density of later stages is not affected by distance from parental plants up to 20 m, suggesting that mortality related to distance from parents affect especially early stages at this spatial scale.…”

“…Thus dispersal enhances the per capita likelihood of recruitment (Harms et al 2000;Hyatt et al 2003;Swamy et al 2011;Terborgh 2013). Distanceand density-dependent mechanisms, as those predicted by Janzen-Connell hypothesis, seem to be pervasive in tropical tree communities (Harms et al 2000;Swamy et al 2011). However, the role of those processes that influences plant recruitment in small forest fragments is not clear.…”

“…Many plant pests are host-specific and congregate near dense aggregations of hosts. Most seeds produced usually fall beneath the parental plants or are dispersed at short distances from it (e.g., Swamy et al 2011). The high density of seeds near parental plants enhances the risk of spread of pathogens and the attraction of specialized seed predators, such as weevil beetles (Wright & Duber 2001).…”

Dispersão e Caracterização de Frutos de Myrceugenia euosma em Floresta Ombrófila Mista no Sul do Brasil

“…A frequência de coletores contendo algum propágulo de M. euosma diminuiu com o aumento da distância da planta matriz, corroborando a hipótese da densidade dependente da distância de Janzen-Connell (Janzen, 1970;Howe & Smallwood, 1982;Francisco & Galetti, 2001;Swamy et al, 2011). Porém, na comparação com diferentes tipos de propágulos, observaram-se comportamentos distintos entre frutos imaturos, frutos maduros e sementes.…”