Ecology of Alnus Glutinosa (L.) Gaertn: III. Seedling Establishment

Experiments on the relations of seedlings to soil moisture are described, Betula pubescens being used for comparison: (1) Seedling establishment in waterlogged soil. (2) Effect of waterlogging at various developmental stages. (3) Establishment at various heights above a constant water-table. (4) Effect of periodic drying of the soil. It is concluded that in regions of low rainfall (50-65 cm.) alder seedlings will only establish where the surface soil falls within the capillary fringe of the water-table so that it remains constantly moist for 20-30 days in spring (March-May). Heavier rainfall and a cover of vegetation modify this picture. The alder seedling is better adapted to establishment in very wet and waterlogged soils than is the birch, and pure stands of alder can be obtained from mixed sowings by appropriate treatment. It is also more resistant to complete inundation at all stages of development. A water-table at a depth of 10-30 cm. is optimal for early alder establishment, but subsequent growth is best on drier sites. The birch seedling is better adapted to establishment in habitats subject to periodic drying out, and pure stands of birch can thus be obtained from mixed sowings, though not so easily as pure alder stands can be obtained by flooding. The different buoyancies of the seed, the different susceptibilities of the radicles to drying out and the different ratios of shoot to root development are all concerned in this. The growth of seedlings on different soils is described, and establishment found to be satisfactory on all except acid peats and highly calcareous soils. Pure fen peat appears to lack some factor, present in mineral soil, necessary for the optimum development of the seedlings. An experiment on the addition of various fertilizers to the peat indicated that the phosphate radicle is largely concerned. Development of nodules and mycorrhiza is found to have little effect on establishment except perhaps on the most infertile sands. Field and pot experiments on light intensity and root competition are described. It is concluded that low light intensity, supplemented by insect attack, flooding, etc. is a frequent cause of regeneration failure in woods and thick herbaceous vegetation. Root competition appears to be of greatest importance in the drier grass turfs. The significance of these factors in natural population structures and succession is briefly discussed.