“The roots of plants are the least known, least understood and least appreciated part of the plant.” – Weaver and Bruner (1927)

Abstract

When we look at a pasture very few of us consider what is below the ground and what impact grazing has on this. In terms of water and nutrient absorption, anchorage, storage of carbohydrates, and the production of hormones that regulate root and shoot growth, roots play an important role. In this paper I discuss what influences rooting depth of a range of native grasses, both in pure swards and in grazed mixed swards. When considering roots there are several things that we should consider when managing pastures; these are soil depth, growing season of the species, and the value of rest periods.

Introduction:

Plant roots play an important role in anchorage, absorption of water and nutrients, storage of carbohydrates, and the production of hormones that regulate root and shoot growth (Weinmann 1948, Atkinson 1985). As there is a relationship between herbage growth and underground development a complete understanding of plant growth both above and below ground must be developed. Above ground pasture management affects the roots of all pasture grasses.

In this paper I will discuss the rooting dynamics of a range of native grasses and attempt to draw some links to grazing management

Rooting patterns

As there is a wide range of plant species, there are many different root structures (Savory and Butterfield 1999). Plants can be recognised above ground by their appearance, so underground you can recognise them by their wide variety of rooting patterns. Some plants have abundant surface roots while others probe more deeply into the soil.

We need to develop pasture plants that have rooting depths and architectures that compliment each other and utilise all of the soil profile. In nature this is what would happen. If plants’ root architectures are complimentary then they are not competing for the same resources, water and nutrients, within the same areas of the soil profile.

Table 1 illustrates that the roots of native grass grow at different rates and to different depths. In an ideal pasture situation we would sow or manage a range of species to ensure that the maximum volume of soil is utilised by roots. This would ensure optimum utilisation of water and nutrients.

Table 1: Time taken for roots of eight native grasses and two exotic grasses to reach a depth of 100 cm (monitored from January 1996 to November 1998).

Species	Months to reach 100 cm
Wallaby grass (Austrodanthonia fulva)	Roots failed to reach 100cm
Red grass (Bothriochloa macra)	3
Tall windmill grass (Chloris ventricosa)	17
Cotton panic (Digitaria divaricatissima)	Roots failed to reach 100 cm
Common wheat grass (Elymus scaber)	13
Curly windmill grass (Enteropogon acicularis)	24
Weeping grass (Microlaena stipoides)	18
Kangaroo grass (Themeda triandra)	15
Phalaris cv Sirosa	16
Lovegrass cv Consol	5

Red grass has an ability to explore the soil depth very rapidly, with roots reaching a depth of one metre in three months. Wallaby grass (Austrodanthonia fulva) and weeping grass (Microlaena stipoides) are shallow rooted species; with most of their roots in the top 60 cm of the soil. Harradine and Whalley (1981) found that wallaby grass (Austrodanthonia bipartita) had an extensive root system in the surface 20 cm but the root system was less developed at lower depths.

Cotton panic (Digitaria divaricatissima) and curly windmill grass (Enteropogon acicularis) had a very sparse root system with very few roots. Although kangaroo grass (Themeda triandra) grew roots to one metre in 15 months, similar to that of phalaris, it was very slow to establish an extensive root system.

The location of plants and pastures in the landscape also influences their rooting dynamics and patterns. Research that is being conducted on two grazed pastures near Wagga Wagga demonstrates this. One pasture is dominated by phalaris while the other is a native grass pasture (Bothriochloa and Austrodanthonia are the dominant species). For both pastures, roots are less well developed in the more arid, upper slopes of the catchment than below the break of slope. Roots in the native grass pasture are predominantly in the top 50 cm in the upper slope areas and 100 cm in lower slope areas. In the phalaris catchment, roots extend to 40 cm in the upper slope areas and 100 cm in the lower slope areas.

Timing of root growth:

We need to recognise that the native grasses found in our pastures are a mixture of both C₃ and C₄ species. (See Myers (1999), Understanding C₃ and C₄ Native Grass Species, produced by SA Native Grasses Resources Group, for an excellent explanation of the different photosynthetic pathways used by these two groups of plants.

In brief C₃ grasses are cool season active and C₄ grasses are warm season active.) The seasonal growth patterns differ in plants with these different photosynthetic pathways. As root growth parallels aboveground dry matter production, the C₄ species had most active root growth in the warmer months of the year, whereas the reverse was true for the C3 species (see figure 1). These differences need to be recognised in optimising the grazing management of these pastures.

Figure 1: Differences in root numbers (counted along a 1.8 m minirhizotron tube) over the seasons for a C₃ grass (Microlaena stipoides) and a C₄ grass (Chloris ventricosa).

Figure 1: Differences in root numbers (counted along a 1.8 m minirhizotron tube) over the seasons for a C₃ grass (Microlaena stipoides) and a C₄ grass (Chloris ventricosa).

Figure 1: Differences in root numbers (counted along a 1.8 m minirhizotron tube) over the seasons for a C₃ grass (Microlaena stipoides) and a C₄ grass (Chloris ventricosa).

Research at Rutherglen has found that root numbers increase and decrease over the course of a year. Both C₃ and C₄ grasses had similar root growth patterns but at different times of the year. The most active growth period of the C₄ species was in late spring and early summer. There was then a decline in their root numbers over the cooler months of the year. In contrast the C₃ species had their most active periods of root growth in early spring.

Root dynamics and grazing:

Actively growing roots can improve soil structure through the creation of root channels and the addition of organic matter to soils. The integrity of root systems can be damaged if plants are overgrazed (in any environment) but is enhanced when plants are properly grazed (in any environment) (Savory and Butterfield 1999).

Grazing causes a sudden reduction in leaf area and this will cause an immediate reduction in plant growth rates because of the loss of photosynthetic capacity (Davidson 1969). New leaves are produced from the shoot apex. The shoot apex needs to be protected from grazing animals; otherwise removal of the shoot apex will stop growth as no new leaves can be produced. The shoot apex in phalaris is below the ground and therefore well protected from grazing animals.

The carbohydrate reserves stored in the roots enable grazed plants to produce new leaf material for photosynthesis. Depletion of root reserves by excessive continual defoliation will result in reduced vigour and herbage growth, and in the extreme, death of the plant (Weinman 1948). Plants that have weakened root systems and low reserves of stored carbohydrate are more susceptible to adverse conditions, such as drought, heat and frost (Weinman 1948).

We all know that we need to rest lucerne stands as they will quickly die out if they are overgrazed. But our knowledge and understanding of the best grazing strategies for native grass pastures is less well developed.

In the studies of the roots under the grazed pastures at Wagga Wagga, the phalaris pasture was faster to recover from grazing than the native grass pasture. It appears that recovery time may be due to a combination of factors – intensity and length of grazing and soil moisture.

Conclusions

The main factors we need to consider in developing grazing management practices that take rooting patterns into account are:

• rooting patterns are different in different places in the landscape. In lower slope areas, soils are deeper and therefore the pastures are likely to be more deeply rooted.
• a pasture needs to be a combination of species with different root architectures to ensure the entire soil profile to be utilised.
• the different growth patterns C₃ and C₄ pastures.
• rest periods to allow roots to regrow and store more carbohydrates for the next period of grazing.

Our knowledge about the rooting dynamics of native grass species is very limited. It has only been studied in a few situations in limited environments. To be able to better manage our native grass pasture we really need to have a greater understanding of what is happening below the soil.

Acknowledgements

The results that are reported in this paper came from two native grasses research projects:

• LIGULE, a collaborative project between DNRE and NSW DLWC, and funded by LWRRDC, MLA and the Victorian and NSW Salinity Plans.
• Management of grazed key native grass communities in the Murray-Darling Basin. A collaborative project between DNRE, NSW DLWC, NSW Agriculture and Mt Lofty Ranges Native Grasses Resources Group, funded by MDBC and the state agencies.

References:

Atkinson, D. (1985). Spatial and temporal aspects of root distribution as indicated by the use of root observation laboratory. In ‘Ecological Interactions in Soil. Special Publication Number 4 of the British Ecological Society'. Fitter, A.H., Atkinson, D., Read, D.J., and Usher, M.B. Eds. pp. 43-65.(Blackwell Scientific Publications: Oxford.)

Davidson, J.L. (1969). Growth of grazed plants. Proceedings of the Australian Grassland Conference, Perth, WA. 1968, Vol. 2, pp. 125-37.

Harradine, A.R. and Whalley, R.D.B. (1981). A comparison of the root growth, root morphology and root responses to defoliation of Aristida ramosa and Danthonia linkii. Australian Journal of Agricultural Research 32, 565-74.

Myers, R. Ed. (1999). Species Information Sheet – Understanding C₃ and C₄ Native Grass Species. Native Grasses Resources Group Inc., Mt Lofty Ranges Catchment Centre, Mt Barker, South Australia.

Savory, A. and Butterfield, J. (1999). ‘Holistic Resource Management: A New Framework for decision making.’ (Island Press, Washington, D.C.).

Weaver, J.E. and Bruner, W.E. (1927) Root development of field crops. New York.

Weinmann, H. (1948). Underground development and reserves of grasses. A review. Journal of the British Grassland Society 3, 115-40.