Seedling Establishment in Vertical Gradients of Sunlight

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Abstract
The effect of vertical gradients of light on the morphogenesis of seedlings of 20 species of flowering plants has been examined by growing them within vertical black-walled cylinders. The facility with which a seedling escapes low shade is related to the amount of storage material available from the seed and the number and properties of extension sites in the shoot. Initial height growth in shade is well marked in seedlings of species occurring in dense grassland (Arrhenatherum elatius, Betonica officinalis, Plantago lanceolata), but it is negligible in herbaceous species which are restricted to low turf and bare soil (Arenaria serpyllifolia, Hieracium pilosella) and in trees which are pioneers of abandoned arable land (Betula populifolia, B. lenta, Rhus glabra). Survival of seedlings at low light intensities was greater in species with inherently slow rates of increase in dry weight. Shade fatalities were more frequent in "intolerant" trees, e.g. Ailanthus altissima and Rhus glabra which grow rapidly in full sunlight. Analysis of the relationship between growth rate and tolerance of shade is complicated by the vulnerability of intolerant species to fungi. The present investigation suggests that a late phase of forest succession involves the replacement of species which as seedlings are productive at both high and low light intensities, by species with slow-growing seedlings capable of enduring long periods in shade.

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