Inhibition of Canopy Tree Seedlings by Thickets of Rhododendron maximum L. (Ericaceae) in an Eastern Deciduous Forest
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Rhododendron maximum thickets altered resource availability for seedlings when compared to areas of forest without R. maximum. Diffused photosynthetically active radiation (PAR) averaged less than 5 mmol m2 s-1 throughout the growing season in sites with R. maximum in comparison to 10-30 mmol m2 s-1 in sites without R. maximum. Soil moisture content, measured using Time Domain Reflectometry was approximately 6% lower in forest sites with R. maximum compared to sites without R. maximum throughout the growing season. Most nutrient concentrations (e.g.,, C, N and most cations) and nitrogen mineralization rates were significantly lower in sites with R. maximum. Temperature and atmospheric relative humidity are slightly lower under thickets of R. maximum. In general, sites with R. maximum are associated with lower resource availability above and below ground in comparison with sites without R. maximum.
Attenuation of below canopy PAR by thickets of R. maximum negatively influences the photosynthetic capacity of Quercus rubra and Prunus serotina seedlings as indicated by measurements of mid-day photosynthesis. In 1996, the seasonal mean mid-day photosynthetic rate of first year Q. rubra seedlings growing in R. maximum thickets (1.3 mmol m-2 s-1) was 62% lower than the seasonal mean mid-day photosynthetic rate (2.1 mmol m-2 s-1) of seedlings growing in forest sites without R. maximum. For second year seedlings in 1997, seasonal mean mid-day photosynthesis was 183% higher for plants growing outside of thickets (1.7 mmol m-2 s-1) compared to the mean rate (0.6 mmol m-2 s-1) for plants located within thicket sites. The mean mid-day PAR available to seedlings located in forest sites without R. maximum during measurements of photosynthesis was 354% higher in 1996 and 257% higher in 1997. First year Prunus serotina seedlings growing in forest without R. maximum also had greater seasonal mean mid-day photosynthesis (0.7 mmol m-2 s-1) when compared to the mean rate (-0.1 mmol m-2s-1) for plants growing within thickets. Prunus serotina seedlings located in the presence of R. maximum received on average 67% less PAR.
Photosynthetic acclimation to low light was assessed for Q. rubra and P. serotina seedlings growing under both forest conditions by measuring photosynthetic responses to light in situ using even aged one-year old seedlings. Quercus rubra seedlings growing in forest sites without R. maximum 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 R. maximum.
The impact of the R. maximum 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 R. maximum on the photosynthetic response to sunflecks of oak seedlings, light flecks were simulated on 288 randomly chosen, even aged, two-year old seedlings in situ. Half of the seedlings were located within R. maximum thickets. Seedlings were randomly assigned one of four light fleck durations (30, 60, 120, and 300s) and one of three intensities (100, 500, 1000 mmol 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 R. maximum regardless of pre-acclimation status, or light fleck intensity. Furthermore, there was a significant effect of R. maximum 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 R. maximum. When seedlings were dark pre-acclimated there was no difference in carbon gain with increasing fleck length between seedlings in forest with and without R. maximum except for flecks of 1000 mmol m-2 s-1. The data lead to the conclusion that under natural conditions the presence of R. maximum likely prohibits Q. rubra seedlings from utilizing sunflecks as effectively as seedlings growing in forest sites where R. maximum 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 R. maximum on the photosynthetic response of oak seedlings to eight consecutive light flecks. Within 10 paired sites, (i.e., with and without R. maximum) 3 even aged three-year old Q. rubra 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 mmol m-2 s-1 in intensity and lasted for 120s. Following each light fleck, leaves were exposed to 10 mmol 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 R. maximum 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 R. maximum 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 R. maximum. Tree seedlings growing in forest sites with R. maximum 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 R. maximum.
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