Effects of dispersal, shrubs, and density-dependent mortality on seed and seedling distributions in temperate forests

Abstract

Processes limiting recruitment of trees may have large impacts on forest dynamics. In this paper, we determined the effects of dispersal, shrubs (Rhododendron maximum), and density-dependent mortality on seed and seedling distributions of Southern Appalachian trees. We quantified the spatial distribution of seed rain, seed bank densities, first-year seedlings, and older than first-year seedlings in five vegetation plots. We fit models to these data assuming effects of limited dispersal, R. maximum (an understory shrub), and (or) density-dependent mortality (as well as a null model with none of these effects) and used best-fitting models to indicate which processes affected a particular species. We found that all factors examined limit species distributions, and thus, affect seedling dynamics. Seedling densities are higher near parent trees long after dispersal occurs. This pattern is less frequently observed for animal-dispersed species than for wind-dispersed species, presumably due to secondary dispersal of seeds by animals. Seedling densities of five species are decreased beneath R. maximum. Shade tolerance does not explain which species are affected, suggesting that factors other than low light are responsible for increased seedling mortality under this shrub. Our results suggest that density-dependent mortality affects four species, decreasing seedling densities close to parent trees. Dispersal, density-dependent mortality, and R. maximum all act in ways that may promote or limit diversity, illustrating that multiple factors are likely to control species diversity.Résumé : Les processus qui limitent le recrutement des arbres peuvent avoir des impacts importants sur la dynamique forestière. Dans cette étude, nous avons déterminé les effets de la dispersion, des arbustes (Rhododendron maximum) et de la mortalité liée à la densité sur la distribution des graines et des semis d'arbres du sud des Appalaches. Nous avons quantifié la distribution spatiale de la pluie de graines, de la densité des banques de semis, des semis dans leur première année et de ceux de plus d'un an dans cinq placettes échantillons. Nous avons estimé les paramètres de modèles à l'aide de ces données en assumant les effets d'une dispersion limitée, du R. maximum (un arbuste du sousétage) et (ou) de la mortalité liée à la densité (de même qu'un modèle de base ignorant tous ces effets) et nous avons utilisé les modèles avec le meilleur ajustement pour indiquer quels sont les processus affectant une espèce en particulier. Nous avons trouvé que tous les facteurs examinés limitent la distribution des espèces et par conséquent la composition de la communauté. La densité des semis est plus élevée à proximité des arbres parents longtemps après qu'il y ait eu dispersion. Ce patron est observé moins fréquemment pour les espèces zoochores que pour les espèces anémo-chores, probablement dû fait d'une dispersion secondaire des graines par les animaux. La densité des semis de cinq espèces diminue sous R. maximum. La tolérance à l'ombre ne perm...

Tree Seedling Establishment Under the Native Shrub, Asimina triloba

“…Shade tolerance was not related to seedling survival under R. maximum for A. rubrum, F. americana and Q. rubra (Lambers and Clark, 2003). In contrast, also under R. maximum, the relative order of survival of three species (Tsuga > Quercus > Prunus) was related to shade tolerance (Lei et al, 2002).…”

“…Shrub understories decrease seedling density, growth and survival (Beckage et al, 2000;Nilsen et al, 2001;Collier et al, 2002;Lei et al, 2002;Fagan and Peart, 2003;Gorchov and Trisel, 2003;Lambers and Clark, 2003). This effect has been found with invasive shrubs outside their native range, such as Lonicera maackii and Rhamnus frangula, as well as shrubs in their native range such as Rhododendron maximum, Buxus sempervirens and Juniperus communis.…”

“…Studies on forest understories have found reduced tree seedling growth and/or survival under Arecaceae (palms) (Denslow et al, 1991) and Gleicheniaceae (ferns) (Walker, 1994) in tropical environments, Poaceae (grasses) (Taylor et al, 1995) in forests in Japan and China, and Dennstaedtiaceae (ferns) (de la Cretaz and Kelty, 1999) in the Eastern United States. Forest understories composed of shrubs also impact seedling distributions (Beckage et al, 2000;Collier et al, 2002;Gorchov and Trisel, 2003;Lambers and Clark, 2003). The present study focuses specifically on the impact of a shrub understory in deciduous forests of the United States.…”

“…Whether shrubs are facilitative or competitive may depend on the life stage of the tree (Callaway and Walker, 1997). Shrubs have been shown to facilitate seed germination and emergence (Rousset and Lepart, 2000) but lower growth and survival at the seedling and sapling stages (Fagan and Peart, 2003;Lambers and Clark, 2003).…”

“…This effect has been found with invasive shrubs outside their native range, such as Lonicera maackii and Rhamnus frangula, as well as shrubs in their native range such as Rhododendron maximum, Buxus sempervirens and Juniperus communis. In the Southern Appalachians, where the effect of a R. maximum understory has been extensively studied, seedling densities for at least six canopy tree species were decreased under that species: Acer pennsylvanicum, Acer rubrum, Fraxinus americana, Quercus rubra, Tsuga canadensis, and Prunus serotina (Beckage et al, 2000, Nilsen et al, 2001Lei et al, 2002;Lambers and Clark, 2003). An understory of L. maackii decreased both the number of species and seedling density (Collier et al, 2002), while removing the shoots of L. maackii increased the survival of four native tree species that were transplanted into plots with the shrub (Gorchov and Trisel, 2003).…”