Successful seedling establishment is generally limited by either low seed abundance or a limited number of microsites in which seeds can safely germinate, become established, and grow. Low seed abundance can result from either limited production or the limited dispersal of available seeds, while the factors defining safe-sites can be both biotic (e.g., competitors, seed predators) or abiotic (e.g., light levels, soil chemistry). There is an increasing awareness that the relative importance of seed- and safe-site limitation varies spatially and temporally. Nevertheless, studies simultaneously assessing the importance of these alternatives remain rare.
We use census data from 10 mapped plots distributed across a fragmented landscape in Amazonas, Brazil to (1) determine the relative magnitude of seed production, dispersal, and seedling establishment limitation for the understory forest herb Heliconia acuminata; and (2) assess variation in the importance of each of these processes between continuous forests and forest fragments embedded in a secondary forest matrix.
Our analyses demonstrated both seed and safe-site limitation in H. acuminata as well as striking spatial variation in the magnitude of the processes that determine seed production, dispersal, and establishment in the different study plots. Average estimated dispersal distance was less than 4 m and was corroborated with bird telemetry data. Results highlighted the importance of light and environmental suitability as the primary factors limiting recruitment of H. acuminata. We found a threshold light level beyond which seedling establishment fails as well as a linear positive relationship between light availability and seedling recruitment below the threshold.
There was a fivefold increase on the positive effect of light on seedling establishment between continuous forests and fragments. As a result, seedling establishment limitation was stronger in fragments than in continuous forests. Understory light levels were greater and more spatially heterogeneous in fragments than in continuous forests, possibly creating an environment in which higher photosynthetic activity is associated with greater evapotranspiration and lower net carbon assimilation. Minimizing water losses rather than maximizing carbon gain may be a more sensible strategy for shade tolerant species