REGENERATION OF 3 SYMPATRIC BIRCH SPECIES ON EXPERIMENTAL HURRICANE BLOWDOWN MICROSITES

Tip-up mounds, pits, and other microsites created by hurricanes may pr omote diversity in many forests by providing opportunities for differe nt species to regenerate. To see if we could detect differences in mic rosite preference among closely related species, we studied the regene ration of three sympatric Betula species on five types of microsites o n experimental mound-pit complexes. Microsites were created by pulling down canopy trees to simulate damage from past hurricanes in southern New England. Seeds were collected in litter traps and experimentally released over mounds and pits to determine effects of microtopography on fine-scale dispersal patterns. The fate of naturally germinating se edlings was monitored on the disturbed site, and seedlings were also t ransplanted onto microsites to examine growth patterns, causes of mort ality, and leaf-level physiology. Seed rain onto the disturbed site wa s abundant and spatially heterogeneous because of scattered residual c anopy trees and surviving uprooted trees. Seeds tended to disperse awa y from vertically oriented surfaces of mounds and to accumulate in pit s. Most seedlings germinated on scarified level areas rather than on m ounds or in pits, but mounds became more favorable for germination the second year following disturbance. Two fundamentally different types of mortality were observed in transplanted seedlings. Extrinsic factor s such as frost heaving, burial by soil and litter, and browsing were dominant on some microsites. Mortality due to these factors occurred p rimarily during the winter and was unrelated to seedling size. On othe r microsites, resource limitation (low light levels and lack of water or nutrients) was the major cause of death. Small seedlings were most susceptible to mortality on these microsites, and most deaths occurred during the growing season. White birch (Betula papyrifera Marsh.) exh ibited the fastest growth and most flexible photosynthetic response to changing light levels but suffered greatest mortality on shaded micro sites. Black birch (B. lenta L.) showed increased leaf area ratio in s haded conditions. Yellow birch (B. alleghaniensis Britt.) was least fl exible and grew more slowly than the other species but was best able t o survive on shaded microsites. All species attained maximum maximum g rowth on tip-up mounds. After three growing seasons, the tallest seedl ings reached nearly 3 m above the forest floor, enabling us to predict which individuals would ultimately reach the canopy to complete the r egeneration process.