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<I>Rhododendron maximum</I> L. (Ericaceae) is an evergreen shrub that grows in dense thickets and currently covers large areas of the understory in the deciduous forests of the southeastern United States. Thickets of R. maximum are inhibitory to recruitment and regeneration of many understory plants including canopy tree seedlings. By effectively lowering the survivorship of woody species trying to establish within thickets, <I>R. maximum</I> could influence stand level regeneration patterns and ultimately the community structure of these deciduous forests. This dissertation outlines research conducted to determine if: 1) below and above ground resources are lower within thickets of <I>R. maximum</I> when compared to forest sites where <I>R. maximum</I> is absent; 2) <I>Quercus rubra</I> and <I>Prunus serotina</I> seedlings growing in thickets have lower mid-day photosynthetic rates; 3) <I>Quercus rubra</I> and <I>Prunus serotina</I> seedlings growing within thickets are low light acclimated when compared to seedlings growing in forest without <I>R. maximum</I>; 4) the presence of <I>R. maximum</I> constrains CO₂ assimilation of <I>Quercus rubra</I> seedlings exposed to light flecks of different durations and intensities; 5) the presence of <I>R. maximum</I> constrains the light fleck responses of <I>Quercus rubra</I> seedlings exposed to eight light flecks in rapid succession; and 6) canopy openness regulates the capacity of <I>Quercus rubra</I> seedlings to assimilate carbon when exposed to eight consecutive light flecks. <I>Rhododendron maximum</I> thickets altered resource availability for seedlings when compared to areas of forest without <I>R. maximum</I>. Diffused photosynthetically active radiation (PAR) averaged less than 5 μmol m⁻² s⁻¹ throughout the growing season in sites with <I>R. maximum</I> in comparison to 10-30 μmol m⁻² s⁻¹ in sites without <I>R. maximum</I>. Soil moisture content, measured using Time Domain Reflectometry was approximately 6% lower in forest sites with <I>R. maximum</I> compared to sites without <I>R. maximum</I> throughout the growing season. Most nutrient concentrations (e.g.,, C, N and most cations) and nitrogen mineralization rates were significantly lower in sites with <I>R. maximum</I>. Temperature and atmospheric relative humidity are slightly lower under thickets of <I>R. maximum</I>. In general, sites with <I>R. maximum</I> are associated with lower resource availability above and below ground in comparison with sites without <I>R. maximum</I>. Attenuation of below canopy PAR by thickets of <I>R. maximum</I> negatively influences the photosynthetic capacity of <I>Quercus rubra</I> and <I>Prunus serotina</I> seedlings as indicated by measurements of mid-day photosynthesis. In 1996, the seasonal mean mid-day photosynthetic rate of first year <I>Q. rubra</I> seedlings growing in <I>R. maximum</I> thickets (1.3 μmol m⁻² s⁻¹) was 62% lower than the seasonal mean mid-day photosynthetic rate (2.1 μmol m⁻² s⁻¹) of seedlings growing in forest sites without <I>R. maximum</I>. For second year seedlings in 1997, seasonal mean mid-day photosynthesis was 183% higher for plants growing outside of thickets (1.7 μmol m⁻² s⁻¹) compared to the mean rate (0.6 μmol m⁻² s⁻¹) for plants located within thicket sites. The mean mid-day PAR available to seedlings located in forest sites without <I>R. maximum</I> during measurements of photosynthesis was 354% higher in 1996 and 257% higher in 1997. First year <I>Prunus serotina</I> seedlings growing in forest without <I>R. maximum</I> also had greater seasonal mean mid-day photosynthesis (0.7 μmol m⁻² s⁻¹) when compared to the mean rate (-0.1 μmol m⁻² s⁻¹) for plants growing within thickets. <I>Prunus serotina</I> seedlings located in the presence of <I>R. maximum</I> received on average 67% less PAR. Photosynthetic acclimation to low light was assessed for <I>Q. rubra</I> and <I>P. serotina</I> seedlings growing under both forest conditions by measuring photosynthetic responses to light <I>in situ</I> using even aged one-year old seedlings. <I>Quercus rubra</I> seedlings growing in forest sites without <I>R. maximum</I> had significantly higher light saturated rates of photosynthesis. For both species, photosynthetic responses to light were otherwise similar irrespective of the presence or absence of <I>R. maximum</I>. The impact of the <I>R. maximum</I> subcanopy on understory PAR and subsequent influence on canopy tree seedling photosynthetic capacity implies that sunflecks are critical for seedling net carbon gain in these forest understory environments. To determine the effect of <I>R. maximum</I> on the photosynthetic response to sunflecks of oak seedlings, light flecks were simulated on 288 randomly chosen, even aged, two-year old seedlings <I>in situ</I>. Half of the seedlings were located within <I>R. maximum</I> thickets. Seedlings were randomly assigned one of four light fleck durations (30, 60, 120, and 300s) and one of three intensities (100, 500, 1000 μmol m⁻² s⁻¹). Half of all seedlings were dark pre-acclimated prior to light fleck simulations by covering with aluminum foil for at least 12 hours, while the remaining seedlings were pre-acclimated under ambient conditions. Analysis of covariance showed that a significant, positive, linear relationship exists between the length of a light fleck and total carbon gain during a light fleck for seedlings in forest sites with and without <I>R. maximum</I> regardless of pre-acclimation status, or light fleck intensity. Furthermore, there was a significant effect of <I>R. maximum</I> on the slope of the relationship such that following ambient pre-acclimation, seedlings located within thickets assimilated significantly less carbon with increasing light fleck length than seedlings located in forest sites without <I>R. maximum</I>. When seedlings were dark pre-acclimated there was no difference in carbon gain with increasing fleck length between seedlings in forest with and without <I>R. maximum</I> except for flecks of 1000 μmol m⁻² s⁻¹. The data lead to the conclusion that under natural conditions the presence of <I>R. maximum</I> likely prohibits <I>Q. rubra</I> seedlings from utilizing sunflecks as effectively as seedlings growing in forest sites where <I>R. maximum</I> is absent. Because sunflecks often occur clustered together during a short period of time during the day, another field study was conducted to further characterize the effect of <I>R. maximum</I> on the photosynthetic response of oak seedlings to eight consecutive light flecks. Within 10 paired sites, (i.e., with and without <I>R. maximum</I>) 3 even aged three-year old <I>Q. rubra</I> seedlings were selected. Over each seedling, a hemispherical canopy photograph was taken and analyzed for percent canopy openness. Each seedling was dark pre-acclimated for 12 hours and then exposed to eight light flecks in rapid succession during which time photosynthesis was logged every two seconds. Each light fleck was 500 μmol m⁻² s⁻¹ in intensity and lasted for 120s. Following each light fleck, leaves were exposed to 10 μmol m⁻² s⁻¹ PAR for 60s before the next light fleck. Mean carbon gain and maximum photosynthesis achieved during each light fleck was significantly lower for seedlings located in the presence of <I>R. maximum</I> for all flecks in an eight-fleck simulation. In addition, seedlings located within thickets generally had significantly lower pre-illumination photosynthesis following the first of eight light flecks. The mean photosynthetic light use efficiency of seedlings located in forest with <I>R. maximum</I> was significantly lower for the first six of eight light flecks in succession. Using regression analysis and analysis of covariance, percent canopy openness was used to explain the variation in carbon gained from all eight light flecks in succession for seedlings under both forest conditions. However, significant relationships failed to exist between under either forest condition and precluded using analysis of covariance. The results from these studies lead to the conclusion that light limitation is a major mechanism responsible for the extirpation of canopy tree seedlings from within thickets of <I>R. maximum</I>. Tree seedlings growing in forest sites with <I>R. maximum</I> receive less solar irradiance, have lower mid-day photosynthesis, fail to acclimate to the lower light conditions within thickets, and utilize sunflecks less effectively as well as less efficiently when compared to plants growing in forest sites without <I>R. maximum</I>. / Ph. D.

In this Ph.D. thesis I am dealing with the effect of dynamic irradiance on (i) photosynthesis, growth and bundle sheath leakiness (for CO2) of C4 plants and (ii) bryophyte photosynthesis. Part of this thesis is literature review on broader aspects of the dynamic light effects on photosynthetic and growth processes in plants.