Is temporal variation of seedling communities determined by environment or by seed arrival? A test in a neotropical forest

1 Both spatial and temporal processes are assumed to play an important role in driving seedling dynamics. We assessed the magnitude of these two processes in a neotropical forest in French Guiana. We first quantified temporal changes in seedling community structure, then evaluated the relative importance of environmental heterogeneity and temporal fluctuations in seed arrival in determining seedling dynamics. 2 We monitored the seedling dynamics of 6244 seedlings by censusing 370 1-m² seedling plots in 160 stations over 20 months. At each station, we quantified environmental variability (light availability, soil resources, litter depth, topography), and measured temporal fluctuations in seed input by sampling seed arrival into seed traps located in each station. 3 Temporal changes in seedling density and diversity between four consecutive censuses were compared with those predicted by a neutral model assuming random recruitment and mortality. Seedling density fluctuations were considerably more variable over time than expected under the neutral assumption. Diversity changes showed less consistent results. For the two first census intervals, seedling diversity was more constant than under the neutral expectations. For the last census interval, seedling diversity was more variable than expected under neutrality. 4 Seedling recruitment, mortality and diversity of recruits were modelled against environmental variables and seed arrival. Sites with higher light availability and soil fertility had more diverse recruits (P < 0.01) but lower seedling survival (P < 0.05). Both density and diversity of local seed arrival had a positive effect, respectively, on density and diversity of recruited seedlings (P < 0.05). 5 Our results show that temporal pulses in seedling community are mainly driven by large temporal fluctuations in seedling recruitment. Annual variation in seedfall and environmental filtering both contribute to explaining spatio-temporal variation in seedling dynamics to a large degree. Irrespective of species identity, tropical seedling communities are both seed- and establishment-limited. The temporal component of seed-limitation appears to be of critical relevance in the structuring of tropical seedling communities.