Many forest trees exhibit reduced growth or are killed when the soil is low in oxygen. Anaerobic soil conditions are associated with flooding or compaction of soil but also occur commonly in soils of heavy texture. Reductions in height growth, leaf growth, cambial growth and reproductive growth of trees growing on poorly aerated soils are well documented. The amount of growth reduction varies widely among species and duration of anaerobic soil conditions during the growing season. Inhibition of growth is preceded by changes in physiological processes, including food, water, hormone, and mineral relations. Some species can adapt to soil anaerobiosis by (1) producing hypertrophied lenticels which assist in aeration of the stem and release of toxic compounds, and (2) growing new roots to replace loss of original roots under anaerobic conditions. The replacement roots assist in absorption of water and mineral nutrients and in oxidizing the rhizosphere and detoxifying soil toxins. Ethylene, together with other compounds, appears to play an important causal role in morphological adaptations to soil anaerobiosis. Some forest trees can also adapt metabolically to poor soil aeration.