Seedling emergence, survival and size in relation to light and water availability in two bottomland hardwood species

1 Windstorm disturbances create a wide range of microsites which can have complex effects on forest regeneration patterns. We investigated the combined effects of light and microtopography on emergence, mortality and size of seedlings of two bottomland hardwood canopy tree species, Quercus michauxii and Liquidambar styraciflua, over a 2-year period. A split-plot design in experimental tanks represented the range of light levels and the pits and mounds found in a disturbed floodplain forest. 2 Emergence was always higher on mounds than in pits, except for L. styraciflua in full sunlight. For both species, mortality was consistently lower, and seedlings of both species grew better in both years on mounds. Light levels did not affect the two later stages. 3 There were species-specific interactions between the effects of two factors on seedling emergence, Lower emergence of L. styraciflua on mounds in full sunlight suggested that full sunlight at this stage can eliminate the advantage to later stages of being on a mound. The combined stresses of low light and a high water table significantly reduced emergence of Q. michauxii in pits at low light. 4 Microsites optimal, for one regeneration component of a species were not always optimal for others as seen for L. styraciflua. The relative significance of environmental factors also varied with regeneration stages, such that neither light nor a light-water interaction influenced regeneration after emergence. 5 Environmental factors may have independent or interacting effects on regeneration, and the nature or presence of these effects can vary among demographic stages. Within each environmental combination, effects may be consistently positive or negative across stages; alternatively, demographic conflicts may develop.