Soil Aeration, Flooding, and Tree Growth
Abstract: Roots require a supply of soil oxygen, water, and mineral nutrients. Oxygen is necessary to maintain aerobic root respiration so as to supply energy needed for mineral uptake, synthesis of protoplasm, and maintenance of cell membranes. In poorly aerated soils the anaerobic respiration of roots does not release enough energy to maintain root functions. Furthermore, many phytotoxic compounds accumulate in poorly aerated soils. Inadequate soil aeration occurs commonly as a result of soil compaction, filling in with soil over roots, impermeable layers (e.g. pavements) around roots, and flooding of soil. Root growth in compacted soils is reduced not only because of unfavorable aeration and moisture conditions, but also because of high mechanical impedance of soil to root growth. Flooding results in elimination of soil oxygen, accumulation of CO2, transformation of nitrogen, and production of toxic compounds. Physiological responses of trees to flooding include closing of stomata, reductions in the rate of photosynthesis and uptake of essential mineral nutrients, as well as alterations in plant growth hormones. These physiological changes lead to inhibition of tree growth. Because root growth is reduced more than shoot growth by flooding, drought tolerance often is reduced after the flood waters drain away. Flood tolerance varies widely among species. Important morphological adaptations for flood tolerance include (1) initiation of adventitious roots which assist in uptake of water and mineral nutrients and (2) production of hypertrophied lenticels which assist in aerating the stem and roots and serve as openings through which toxic compounds are released.