Effects of microsite environment on tree regeneration following disturbance

This thesis explores the impact of disturbance on the availability of resources for plant growth. The central theme of this work is that disturbance events create heterogeneity in the resource base, thereby providing opportunities for regeneration of species with different resource requirements. The specific focus is on regeneration of canopy tree species on microsites created by hurricane blowdown. Effects of hurricane damage were simulated by pulling down selected canopy trees on two sites at the Harvard Forest in central Massachusetts. Environmental factors were measured, and seed dispersal, seedling recruitment, and growth and physiology of seedlings were studied.;Seedling mortality on some microsites was due primarily to extrinsic factors unrelated to growth rates. Mortality on other sites was caused by resource limitation, with the smallest seedlings being most susceptible. All species attained maximum growth rates on vertical portions of forest floor created by uprooting. The shade-intolerant colonizing species, Betula papyrifera, exhibited greatest growth rates and most flexible photosynthetic response to changing light levels, but the most shade-tolerant species, B. alleghaniensis, was best able to survive under residual vegetation on the blowdown site. Two different birch regeneration niches on tip-up mounds are described.;Vector models were developed to investigate seedling growth response to resource availability. Models for white birch response to light and nitrogen revealed single-resource limitation of growth at different combinations of these resources, increasing response to nitrogen at higher nitrogen levels, and partial compensation for low light levels by nitrogen. Results of all studies are synthesized in projection matrix models of tree seed and seedling demography on disturbed and undisturbed sites.;Patterns of light availability, CO...