Deep root systems

Mycorrhizas are critical to plant survival and production in arid soils. Hyphae transport water in both directions. Water from patches of moist soils to the plant is provided by fungal hyphae and rhizomorphs that explore large volumes of soil ranging outward from the canopy edge. Mycorrhizal hyphae even extend from deep roots into the bedrock to access sources of water that cannot be reached or transported in the time scales necessary to be important to plant water balance. Mycorrhizal hyphae also benefit from hydraulically lifted water from plants that have deep root systems that reach groundwater or perched water tables. 

Upper soil levels. They also show that secondary forests may re-establish deep root systems (6 m deep) within l6 years. They further show that abundant fine roots infected with mycorrhizal fungi characterize the deep roots of secondary forest. These circumstantial data suggest that secondary forests are mining nutrient reserves in deeper soils, but conclusive data are still unavailable. 

Under normal conditions of water availability, values for LAI in pastures of B. brizantha have been measured above 4.0. However with the establishment of a water deficit in the soil, these values decrease to below two or even lower in pastures of P. maximum (Roberts et al. 1996). A similar situation is found in abandoned pastures in eastern Amazonia, where a reduction of approximately 68% of green tissue has been observed in the dry season, while in an adjacent area of primary forest this reduction was only 16% (Nepstad et al. 1994). Primary forests, which have deep root systems and little seasonal variation in LAI, maintain stable subcanopy microclimatic conditions and transpirational flux, even during the dry season. Because of an evergreen forest canopy, the return of the rainy season has less impact on the microclimate near the soil in the forest than in the pastures, and the deep soil water stores are also more efficiently recharged in the forest. 

The recovery of deep soil water uptake in the Paragominas secondary forest was possible because of the re-establishment of deep root systems following pasture abandonment (Fig. 9.2). The root systems of secondary forest trees, vines, and palms rapidly penetrate to at least 8 m depth during the first 15 years of regrowth. We identified one third as many morphos-pecies of roots to 8 m depth in the secondary forest as in the neighboring mature forest, with a prevalence of vine and palm roots in the secondary forest. The vine Davilla kunthii, for example, penetrates to at least 8 m depth by the time its stem has attained 1 m height (Restom 1998). Vines in... 

Another very important benefit realized from the roots of green manure crops is the big effect on the aggregation of the finer-textured soils. Crops with very finely divided and extensive, but not necessarily deep, root systems are most effective. Small grain crops meet this requirement and are most used for this purpose (Benoit et al., 1962). A single heavy cover crop of rye, planted in the early fall, has been known to produce a very marked improvement of the soil. The chief factors and mechanisms involved in aggregate formation and stabilization, and the role that root systems play in the process, are discussed in Chapter 16. 

The texture and structure of a soil also affect plant response to nitrogen additions through effects on root development and depth of penetration. A restricted root system limits the ability of the plant to assimilate nutrients and also limits its uptake of water. Since the movement of water through soils by capillarity is slow and limited, the plant roots must literally go after it. A deep root system, therefore, assures that a much greater proportion of the supply in the soil is available to the plant. In addition, soil compaction limits the absorption of the rainfall. Such unfavorable conditions do of course lower the efficiency of applied nitrogen. 

Vitis rupestris Scheele is also called the sand grape. It occurs in the mid-South states of Arkansas, Kenmcky, Tennessee, and Oklahoma (Cousins, 2005). One especially positive trait of this species for breeding is resistance to phylloxera. It also roots easily, develops a deep root system, and has high vigor, but like V. riparia, it does not tolerate calcareous soils (Einset and Pratt, 1975 Snyder, 1937). St. George root-stock is a selection of V. rupestris (Cousins, 2005). 

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