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Citation. Walters, M. B.; Reich, P. B. 1996. Are shade tolerance, survival, and growth linked? Low light and nitrogen effects on hardwood seedlings. Ecology 77:841-853.   [1704  CC]

Abstract. Variation in shade tolerance is a primary mechanism driving succession in northern deciduous forests. However, little is known about interspecific diffrerences in the traits responsible for shad tolerance. Is shade tolerance due to the ability to grow or survive in deep shade, or both? How do plan morphology and photosynthesis relate to growth in shade? Is low light the sole critical stress determining differences in "shade tolerance: or do belowground resources interact with low light to affect growth and survival? In this study we address these questions for seedling of Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum (Marsh., and Quercus rubra L. grown for 2 yr in outdoor shade houses in a complete factorial of low light (2 and 8% open sky) and nitrogen (forest soil and forest soil plus 200 kg N ha-1 yr-1). For these seedlings, we examined effects of light and nitrogen on the interrelationships among survival, growth, and shade tolerance and explored the physiological bases of shade tolerance by examining the relationship of plant morphology and photosynthesis to growth. Nitrogen amendments did not have a significant effect on any plant trait at either light level. In 8% light, growth and survival were highest for shade-intolerant Betula papyrifera and mid-tolerant Betual alleghaniensis, lower for shade-tolerant Ostrya and Acer, and lowest for disturbance adapted Quercus. In 2% light, species ranking reversed as Ostrya and Acer had higher growth and survival than the other species. Second-year survival was strongly related to 1st-yr growth (P<0.001), whereas relationships with 1st-yr plant mass and 1st-ys absolute growth rates were weak. Therefore, survival of shade-tolerant species at 2% light was related to their maintenance of positive growth, whereas intolerant species has growth near zero and high rates of mortality. In both 2 and 8% light photosynthetic rates on mass (but not area) bases and the proportion of the plant in leaves (leaf area ration and leaf mass ratio) were positively related to growth. Greater rates of growth and survival for shade-tolerant species in very low light, and for intolerant species in higher light, suggest that there is a species-based trade-off between maxinmizing growth in high light and minimizing the light compensation point for growth. This trade-off may be an important mechanism driving forest community dynamixs in northern hardwood forests.

Keywords. Acer saccharum, allocation, Betula alleghaniensis, Betula papyrifera, forest succession, growth rate, nitrogen, Ostrya virginiana, photosynthesis, Quercus rubra, shade tolerance, survival

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