Ontogenetic niche shifts in an understory shrub (Itea virginica) from southeastern Cypress-Tupelo swamps

Nearly all understory plant species in cypress-tupelo swamps grow on elevated microsites and not on the flooded forest floor; the swamp specialist Itea virginica (Saxifragaceae) is no exception.  Interestingly, the strict microhabitat association of Itea appears to relax through ontogeny.  In a field survey, we found that less than 0.2% of Itea seedlings (n=6912) occurred on the forest floor and the remaining 99.8% seedlings grew on fallen logs or cypress knees; in contrast, 5% of adults (n=119) were rooted directly in the forest floor.  Flooding may inhibit seedling establishment, but not adult growth, thus potentially explaining the restricted distribution pattern of this species. We conducted a greenhouse experiment on seedlings and cuttings (i.e. adults) of a range of sizes to determine whether flooding is a more severe stress for seedlings than older plants. Indeed, seedling performance (survivorship and growth rate) decreased with flood severity, whereas adult performance did not vary with the amount of flooding.  We also conducted experiments to investigate the effects of different substrates and flooding durations on germination success.  Seeds had similar germination rates on soil collected from the forest floor, wood collected from cypress knees, and filter paper.  Germination rate also did not differ between seeds submerged in water for 7 weeks, 4 weeks, and 0 weeks.  These results suggest that seeds germinate readily on the forest floor or elevated microsites once the flood stress is removed, even if they had previously been flooded for substantial periods of time.  This study is noteworthy because it demonstrates that juvenile microhabitat requirements can restrict adult distribution patterns.