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Light environment under Rhododendron maximum thickets and estimated carbon gain of regenerating forest tree seedlings


Canopy tree recruitment is inhibited by evergreen shrubs in many forests. In the southern Appalachian mountains of the USA, thickets of Rhododendron maximum L. restrict dominant canopy tree seedling survival and persistence. Using R. maximum as a model system, we examined available light under the thickets and the photosynthetic responses of seedlings of canopy tree species. We tested the hypothesis that the additional shading from under R. maximum drives carbon gain in seedlings below the threshold for growth and survival. A reduction in light under the thicket was found where canopy openness (derived from canopy photographs) under R. maximum was half the amount measured in forest without R. maximum. R.␣maximum also reduced direct radiation by 50% and diffuse radiation by 12–29% compared to forest without the shrub layer. Mean mid-day PPFD (photosynthetically active photon flux density between 1000 and 1400 h) under R. maximum (obtained from quantum sensors) was below 10 \(\upmu\)mol m−2 s−1 on both clear and overcast days and the amount of sunflecks greater than 10 \(\upmu\)mol m−2 s−1 PPFD was only 0–20 min per day. In contrast, forest without R. maximum received a mean PPFD of 18–25 \(\upmu\)mol m−2 s−1 on clear days and a cumulative sunfleck duration of 100–220 min per day in all sky conditions. Consistent with light availability between the sites, daily carbon gain in Quercus rubra L. seedlings was lower in forest with R. maximum compared to forest where the shrub was absent. The presence of the shrub layer also significantly suppressed average mid-day photosynthesis of both Q. rubra and Prunus serotina Ehrt. seedlings on 8 out of 11 measurement dates. However, parameters derived from light response curves between seedlings growing in forest sites with or without a thicket of R. maximum was significantly different only in A max (maximum photosynthetic rate), indicating a lack of further acclimation to the deeper shade under R. maximum. While the additional shade cast by R. maximum is sufficient to prevent the regeneration of tree seedlings under this shrub, there was sufficient heterogeneity in light under the thicket to imply that deep shade only partially explains the complete inhibition of regenerating canopy trees under R. maximum.

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This study was funded by USDA Forest/Range/Crop/Wetland Ecosystems Program, Grant # 95–37101–1902. Logistical assistance was provided by USDA Forest Service, Coweeta Hydrologic Laboratory. The use of a fisheye lens was kindly provided by Y. Maruyama. Canopy photo images were analyzed using FEW 4.0, a PC-based software developed by M. Ishizuka of Forestry and Forest Products Research Institute, Tsukuba, Japan.

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Correspondence to T. T. Lei.

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Lei, T.T., Nilsen, E.T. & Semones, S.W. Light environment under Rhododendron maximum thickets and estimated carbon gain of regenerating forest tree seedlings. Plant Ecol 184, 143–156 (2006).

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  • Appalachian
  • Light response
  • Photosynthesis
  • PPFD
  • Site factors
  • Sunflecks