Most roots will grow within the first 50-75cm of soil as, below such levels, soil aeration progressively falls. As roots need to breathe in order for respiration to occur, such depths beyond 1m are typically less desirable; unless soils are very fine and sandy, or cracks within rock faces allow for greater penetration depth of oxygen (Davis, 2015; Shigo, 1986). For most species, when the oxygen levels falls below 10–15 % in the soil, root growth is inhibited – growth stops completely at 3–5%. Such conditions occur when airspaces in the soil are replaced by more soil (compaction), water (flooding – particularly detrimental during the growing season and where flood waters are static and warm) or gases such as carbon dioxide, hydrogen sulphide or methane (gas leaks are perhaps the principal driver) (Crow, 2005; Davis, 2015).
Poor aeration is largely a consequence of increased bulk density and means that tree root growth – particularly fine root growth – is limited (Imada et al., 2008; Shigo, 1986; Watson & Kelsey, 2006). With the loss of macro-pore space brought about at higher bulk densities, water infiltration and gas diffusion is reduced, soil oxygen concentration is decreased, and carbon dioxide concentration increases (Watson & Kelsey, 2006) – this concoction of soil conditions is certainly less than desirable for potential root growth.
Reduced soil aeration may also be triggered by water-logging. Water-logging has a drastic effect on soil aeration and consequently on tree root development (Newsome et al., 1982). Waterlogged soils are characterised by a lack of oxygen, increased levels of carbon dioxide and ethylene, and together with many other chemical changes (Coutts, 1989). The tips of growing tap-roots and sinkers are killed when the water table rises, and regeneration takes place when it falls during drier periods. Roots may even develop adventitiously upon the stem if the water-logging conditions extend above the soil (Coutts, 1989; Newsome et al., 1982). Some flood-tolerant species have however developed tolerances to help cope with reduced soil aeration due to flooding – Salix spp., Populus spp., and Alnus spp. can for example survive flooding for up to 40-45% of the growing season (Crow, 2005). Taken to the extreme, species of the Taxodioideae family (Taxodium spp., Metasequoia glyptostroboides, and Glyptostrobus pensilis) have developed ‘knees’ – aerial roots – that function principally as a means of negating or off-setting the incredibly low oxygen levels within the soil beneath the swamps that they naturally inhabit (Lamborn, 1890; Kramer et al., 1952; Martin & Francke, 2015; Moorberg et al., 2015).
References
Coutts, M. (1989) Factors affecting the direction of growth of tree roots. Annales des Sciences Forestières. 46 (Supplement). p277-287.
Crow, P. (2005) The influence of soils and species on tree root depth. Edinburgh: Forestry Commission.
Davis, M. (2015) A Dendrologist’s Handbook. UK: The Dendrologist.
Imada, S., Yamanaka, N., & Tamai, S. (2008) Water table depth affects Populus alba fine root growth and whole plant biomass. Functional Ecology. 22 (6). p1018-1026.
Kramer, P., Riley, W., & Bannister, T. (1952) Gas exchange of cypress knees. Ecology. 33 (1). p117-121.
Lamborn, R. (1890) The knees of the bald cypress: a new theory of their function. Science. 15 (365). p65-67.
Martin, C., & Francke, S. (2015). Root Aeration Function of Baldcypress Knees (Taxodium distichum). International Journal of Plant Sciences. 176 (2). p170-173.
Moorberg, C., Vepraskas, M., & Niewoehner, C. (2015) Phosphorus dissolution in the rhizosphere of bald cypress trees in restored wetland soils. Soil Science Society of America Journal. 79 (1). p343-355.
Newsome, R., Kozlowski, T., & Tang, Z. (1982) Responses of Ulmus americana seedlings to flooding of soil. Canadian Journal of Botany. 60 (9). p1688-1695.
Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.
Watson, G. & Kelsey, P. (2006) The impact of soil compaction on soil aeration and fine root density of Quercus palustris. Urban Forestry & Urban Greening. 4 (2). p69-74.
Arboriculture 