Closing the life cycle of forest trees: The difficult dynamics of seedling-to-sapling transitions in a subtropical rainforest

Experimental and observational studies on seedling dynamics posit mechanisms that can influence forest diversity, structure and function. However, high mortality and slow growth of seedlings make it difficult to evaluate the importance of this life-history filter to total tree life history. Quantifying the duration and transition of the seedling phase would help us understand this 'black box' in tree population biology. We used a 16-year dataset of comprehensive seedling-to-sapling demography from a subtropical rainforest to construct population models that capture temporal demographic fluctuations for eight major tree species. We used data-driven demographic models and simulations to estimate the transition ratios from newly recruited seedlings to saplings of 2-m height and the time taken to attain 2-m height for a newly recruited seedling conditional on its survival. Projections among species estimated that as few as 57 to more than 40,000 seedlings (with a median of 2,087) were required to make a single 2-m high sapling. Furthermore, it would take 22-200 years (with a median of 47) for a newly recruited seedling to become a 2-m high sapling. We found that temporal variation in demographic rates could greatly reduce the number of seedlings per established sapling, but not passage times (PTs). We also identified the importance of consistently fast growth rates for seedlings to escape the high mortality of early stages. Synthesis. Our findings demonstrate that high mortality in the very early seedling stage severely limits the probability that a newly recruited seedling will transition to the sapling stage. Although the PTs vary, we found this to be true across species with a range of life-history strategies. Only seedlings with consistently fast growth rates are expected to pass through this life-history filter. Findings from seedling studies should consider how short-term studies of seedling demography might capture the rare exceptional individuals and exceptional conditions that might define the dynamics of this seedling bottleneck.

Automated summary

Experimental and observational studies on seedling dynamics identify mechanisms that can affect forest diversity, structure, and function. However, high mortality and slow growth make it difficult to evaluate the impact of this life-history filter on total tree lifespan. Quantifying seedling phase duration and transitions can help understand this 'black box' in tree population biology.