THE TOLERANCE OF TREE ROOTS TO WATERLOGGING

SUMMARY Rooted cuttings of Picea sitchensis and Pinus contorta were grown with split roots in plastic troughs of soil, to produce 500 mm long woody roots on either side of each plant with the distal parts of the root system growing out into pots at the ends of the troughs. Some roots of primary structure were also present in the troughs. The soil in the trough on one side of the plant was waterlogged while all other parts of the root system remained in freely drained soil. The waterlogging treatment was applied out of doors from April to November. In both species all roots of primary structure in the waterlogged soil were killed, whereas most of the woody roots survived. In the pine, radial growth of the waterlogged part of the woody root was reduced relative to the freely drained control side, and the root tapered from the plant stem towards the centre of the waterlogged zone, then increased again towards the distal end. Growth of pine roots of primary structure in freely drained soil at the distal end of the waterlogged woody root was not reduced compared with the control side. Spruce was more sensitive, and growth of xylem and phloem of the woody root almost ceased at the centre of the waterlogged zone; there was also a carry‐over effect of the waterlogging treatment into other parts of the root system. Growth of the woody root proximal to the waterlogged zone and of the primary roots distal to it were reduced. Enlarged lenticels became abundant on the woody roots of both species at the ends of the waterlogged zone and their frequency decreased towards the centre. The results are discussed in relation to differences in oxygen transport known to occur in woody roots of the two species.