Three-Dimensional Modeling of Hydrotropism Effects on Plant Root Architecture along a Hillslope

sawa, 1994). However, all of these models have been developed for planar surfaces; none are for hillslopes. A three-dimensional model of root system development and soil Several studies have shown that plants growing along water flow is described and applied to actual conditions along a hillslope. In the model, gravitropism, hydrotropism, and circumnutation slopes have asymmetric root systems (e.g., Scippa et al., were employed as the main factors controlling root elongation. Root 2001; Yamadera, 1990). To simulate root systems on systems of 2-yr-old pine trees (Pinus massoniana Lamb.) on natural slopes, it is necessary to understand the mechanisms of slopes in southern China were excavated and examined, and their root growth as well as to consider the effects of slope development was simulated through the use of continuously monion root growth. The influence of gravity on root elongatored temperature and rainfall data. In the simulated root systems, tion, or gravitropism (i.e., root elongation in the direcangles between first-order lateral root segments and the vertical direction of gravity), has been studied extensively (e.g., Ishition on the upslope portion of a tree were larger than those on the kawa et al., 1991; Kiss, 2000) and was used as a factor downslope portion of the tree; hence, root systems exhibited asymmetcontrolling root growth in all previous models (Fig. 1a). ric architectures. This asymmetry was more obvious for root systems The effect of soil mechanical resistance has also been developed on the downslope side of the hillslope. Because root systems simulated without the effect of hydrotropism did not develop asymincorporated in models (Clausnitzer and Hopmans, 1994; metric architectures, the direction of soil water flux and the effect of Diggle, 1988; Pages et al., 1989; Shibusawa, 1994), as has hydrotropism appear to be the main factors contributing to the obthe influence of soil water and nutrient flow (Clausnitserved architectural asymmetry, typical of root systems along hillzer and Hopmans, 1994; Dunbabin et al., 2002a, 2002b; slopes. Calculations with the proposed root system model were helpful Somma et al., 1998). Among these factors, tropism inin elucidating and understanding the predominant processes affecting duced by soil mechanical resistance (directional changes root system development on hillslopes. of roots that encounter compacted soil) is a possible cause of the asymmetric development of root systems in sloping soil profiles where soil density increases with T potential contribution of plant root systems depth (Fig. 1d). However, it is unlikely that roots can to slope stability is of interest in several areas of detect gradients of soil mechanical resistance at the root research, including erosion control and development of tip since differences in soil density may be too small. improved revegetation technology. Some studies sugTherefore, although a root might alter its elongation gest that plant root systems contribute to the stability rate according to changing soil mechanical resistance, of otherwise unstable hillslopes by increasing soil resisthe root doesn’t necessarily change its growth direction tance against shear stress and decreasing the soil water toward lower soil mechanical resistance. However, content via transpiration (Greenway, 1987; Tsukamoto, when the mechanical resistance changes suddenly at a 1987). These effects of root systems have been investiboundary, then it is plausible that a root will abruptly gated extensively by considering root strength, growth, change its direction (thigmotropism, as shown schematiand rate of decay (Abe, 1996; Kato and Syuin, 2001; cally in Fig. 1c; Darwin, 1880). Sidle, 1991; Syuin, 1998; Syuin et al., 1998; Tsukamoto, Moreover, a recent study indicated that the asymmet1987; Watson et al., 1999). As such modeling root system rical architecture of root systems on steep slopes indevelopment and soil water flow on hillslopes may encreases the plant’s stability by modifying the distribution hance our knowledge of the effects of root systems on of mechanical forces within the soil; considerable anaslope stability, including the mechanisms of development, tomical modifications in the shape and tissue organizamorphological architecture, and physiological functions tion of lateral roots on slopes support this hypothesis of root systems. Numerous models have been developed (Chiatante et al., 2002). However, the mechanism of that simulate root system development for various plant asymmetric root elongation along a slope has not been species and environmental conditions (Clausnitzer and adequately explained. Experimental studies (Takahashi, Hopmans, 1994; Diggle, 1988; Doussan et al., 1998; Dun1994; Takahashi and Scott, 1993; Takano et al., 1995) babin et al., 2002a, 2002b; Jourdan and Rey, 1997; Lynch showed that root growth is influenced by hydrotropism et al., 1997; Pages et al., 1989; Somma et al., 1998; Shibu(i.e., root elongation toward increasing water content), as shown schematically in Fig. 1b. Because soil water D. Tsutsumi, Div. of Fluvial and Marine Disaster, Disaster Prevention flows from upslope to downslope by gravity, root hyResearch Institute, Kyoto Univ., Gokasyo, Uji, Kyoto 611-0011, Japan; drotropism can be another factor that influences asymK. Kosugi and T. Mizuyama, Div. of Forest Science, Graduate School metric root system development along hillslopes. The of Agriculture, Kyoto Univ., Oiwakecyo, Kitashirakawa, Sakyoku, same question as encountered with mechanical resisKyoto 606-8502, Japan. Received 30 July 2003. Original Research Paper. *Corresponding author ([email protected]). tance then arises. That is, can a root detect a soil water potential or water content gradient? The answer may Published in Vadose Zone Journal 3:1017–1030 (2004). again be no. However, for hydrotropism, roots may de Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA tect the direction of water flow, even if the root tip is