Optimal root growth has been found to occur only above a soil temperature of 15°C in many species (Alvarez‐Uria & Körner, 2007; Kuhns et al., 1985), though the exact optimal temperature varies between species – root growth rate will regardless progressively improve until the optimal temperature for the specific species is reached (Solfjeld, 2009). At below 5°C, root growth initiation and lengthening has found to have been inhibited drastically; certain species even have very low to no root growth, at slightly higher temperatures of 6°C. In fact, the critical temperature for significant root growth is ~6°C, which is close to the worldwide mean soil temperature at climatic tree-lines (Alvarez‐Uria & Körner, 2007; Solfjeld, 2009).
At soil temperatures of less than 7°C, root growth and suberization is therefore markedly reduced. Between 7-15°C, roots function at 33% of potential optimal capacity, and progressively increase in functionality at soil temperatures over 15°C. Once soils reach ~25°C however, root functionality declines due to the drying-out of the soil under such warmth – roots instead begin to suberize (Kuhns et al., 1985). At around 40°C, root growth is found to be almost, if not fully, inhibited (Kaspar & Bland, 1992).
Additional studies have shown that root elongation of major gymnosperms and angiosperms decreased from 1.5-2.0mm a day to a low of 0.5mm a day, as soil temperature (at a depth of 10cm) decreased from 12°C to 2°C (Tryon & Chapin, 1983).
In short, low soil temperature of under 8-10°C can be defined as an edaphic condition that can reduce root growth and, in seedlings in particular, subsequent establishment (Grossnickle, 2005; Solfjeld, 2009).
Alvarez‐Uria, P. & Körner, C. (2007) Low temperature limits of root growth in deciduous and evergreen temperate tree species. Functional Ecology. 21 (2). p211-218.
Grossnickle, S. (2005) Importance of root growth in overcoming planting stress. New Forests. 30 (2-3). p273-294.
Kaspar, T. & Bland, W. (1992) Soil temperature and root growth. Soil Science. 154 (4). p290-299.
Kuhns, M., Garrett, H., Teskey, R., & Hinckley, T. (1985) Root growth of black walnut trees related to soil temperature, soil water potential, and leaf water potential. Forest Science. 31 (3). p617-629.
Solfjeld, I. (2009) Root growth after transplanting: the role of transplant timing, root-zone temperature, and adequate soil volume. In Watson, G., Costello, L., Scharenbroch, B., & Gilman, E. (eds.) The Landscape Below Ground III: Proceedings of an International Workshop on Tree Root Development in Urban Soils. USA: International Society of Arboriculture.
Tryon P. & Chapin F. (1983) Temperature control over root growth and biomass in taiga forest trees. Canadian Journal of Forest Resarch. 13 (5). p827-833