The role of shrub belts and lucerne for intercepting water and reducing deep drainage

Narrow Acacia shrubs belts planted at the break of slope in the Wagga Wagga environment, while using more water than annuals where planted, did not provide recharge (deep drainage) or water-logging control further down slope, but will also not effect production of surrounding pastures.  Lucerne was a more effective option when planted across the entire slope.

Key findings

Lucerne more effective than shrubs for managing recharge and waterlogging

  • The shrubs used more water than annual pastures and therefore created a small dry soil buffer in the area where they were planted.  However, the area sown to shrubs was not large enough to significantly reduce soil moisture at the foot of the slope.
  • Lucerne and the shrubs used similar quantities of water, and because lucerne was sown throughout the paddock, it had a significant effect on soil water at the foot of the slope.
  • Given the lucerne and shrubs used similar amounts of water, the addition of shrubs belts to the lucerne had no impact on soil water surrounding the shrubs, or at the foot of the slope.  That is, lucerne pasture without shrubs would provide recharge control similar to lucerne with shrub belts.
  • Soil moisture in the pasture areas between the shrub belts was largely unaffected and therefore had no impact on pasture growth, but no recharge control either.

Shrubs had no impact on pasture growth

Potential use of shrubs for other purposes

  • The evidence from the EverGraze Proof Site does not support the use of shrub belts for recharge control in environments such as those at Wagga Wagga. However, it does not mean that farmers should not plant shrubs for other reasons such as discharge management, livestock shelter and biodiversity.
Acacia shrub belts planted at the Wagga Wagga Proof Site had no impact on pasture growth
Acacia shrub belts planted at the Wagga Wagga Proof Site had no impact on pasture growth

Read more about this research

The issue

Salinity in the upland regions of eastern Australian states is often due to excess water accumulating in the lower parts of the landscape where it mobilises salt. Preventing the movement of water down slope is one way to mitigate this form of salinity. Deep rooted species such as shrubs and lucerne can dry the soil beneath them significantly more than an annual pasture. This creates a ‘’dry soil buffer’ that can absorb or store moisture that enters this zone either as water infiltrating down from the soil surface or moving sideways down the slope through the soil. Only once the dry soil buffer is filled water will continue to move down the slope. Soil acidity and topography severely limit the areas suitable for deep rooted perennial pasture species such as lucerne, which could use soil water before it accumulates at the bottom of the slope. Belts of shrubs planted on the contour could potentially create dry soil buffers that are capable of absorbing water flowing down slope as either surface flow or sub surface flow across the soil A/B horizon interface.

Considering shrubs and lucerne as potential options

At the EverGraze Proof Site at Wagga Wagga, the shrubs component aimed to examine the impact of shrub belts planted across the slope on recharge and subsequent soil moisture conditions at the bottom of the slope.

Because shrub belts may potentially compete with pasture for water and nutrients, measurements to assess the degree of competition between shrubs and annual pasture or lucerne were made once the shrubs had become large enough to compete with the neighbouring pasture for resources. The spacing of the shrub belts, and the type of shrubs planted, is comparable with the component study investigating lamb survival, but the requirement to have a uniform slope meant this study was best conducted on a smaller, more uniform site.

What was done

A replicated field experiment was conducted to test if planting of shrub belts on the contour could;

  1. make the hydrology of a hillslope planted to annual pasture equivalent to that of one growing lucerne;
  2. increase the water use of a hillslope planted to lucerne;
  3. impact the adjacent pasture growth.

Acacia shrub belts were planted in rows along the contours
Acacia shrub belts were planted in rows along the contours

The experiment was established on the west facing slope (10% grade) of a commercial farm. The soil type changed from a tenosol with no recognisable B horizon at the top of the slope to a deep red chromosol at the bottom. Twelve plots (140 m down slope x 25 m across slope) were established in 2005 with six plots each of lucerne and annual pasture. Two belts of acacia shrubs were planted on three lucerne plots and three annual pasture plots in 2005. These were designed as three-row wind breaks with a single row of taller species of acacia (A. deanei, A. decurrens) as the centre row and a mix of shorter, denser A. decora and A. iteaphilla as the outer rows; each row contained 15 shrubs, each 2m apart within the row (Figure 1).

The water content of the top 250cm of soil was measured with a neutron moisture meter every month from January 2008. These measurements were made within the shrub belts, in the pasture at the same elevation as the shrub belts and at the foot of the slope, below the lower shrub belt. The neutron probe was calibrated at two locations in a “sacrifice” area adjacent to the experimental plots; these locations were at the same elevation/landscape position as the shrub belts.

Sheep grazed the pasture as needed to maintain pasture biomass between 750 kg/ha and 6000 kg/ha. Green and dead feed on offer (FOO) were assessed each time the experiment was grazed (short duration high stocking rates) and the assessment made immediately before sheep were introduced to the experiment.

Figure 1. The hillslope hydrology experiment (areas shaded green are lucerne treatments).
Figure 1. The hillslope hydrology experiment (areas shaded green are lucerne treatments).

What was found

Seasonal conditions

Over the course of the experiment the site experienced a series of drought years followed by a very wet year in 2010-2011 (Figure 2).  The wet year provided a good test for recharge management.

Figure 2. Wagga Wagga average monthly rainfall 2006 – 2011 and long term average 1941-2011
Figure 2. Wagga Wagga average monthly rainfall 2006 – 2011 and long term average 1941-2011

The drying effect of shrubs

Shrub belts dried the soil more than adjacent annual pasture, but not more than adjacent lucerne.  This is likely due to better initial colonization of the root zone by the lucerne.  Shrubs were more effective when planted lower on the slope than higher on the slope – the annual pastures stored on average 150mm water than the adjacent lower shrub belt, but only 100mm more than the adjacent upper shrub belts. The drying of soil by the shrubs relative to annual pasture increased with time at both the upper and lower positions.

Water storage at the foot of the slope

The drying effect of the shrub belts in the annual pasture did not to translate into drier soil at the bottom of the slope (Figure 3). There was no effect of planting shrub belts on the lucerne slopes. However, the soil of the lucerne treatments was much drier than the annual pasture. Adding shrub belts to annual pasture does not appear to make them hydrologically equivalent to lucerne and the hillslope hydrology of lucerne is unaffected by the addition of shrub belts.

Figure 3: Soil water stored to 250 cm at the foot of the slopes sown to annual pasture and lucerne with the presence or absence of shrub belts. Differences smaller than the size of the standard error bar are not statistically significant (ie we cannot confidently say the difference is real).
Figure 3: Soil water stored to 250 cm at the foot of the slopes sown to annual pasture and lucerne with the presence or absence of shrub belts. Differences smaller than the size of the standard error bar are not statistically significant (ie we cannot confidently say the difference is real).

Impact of shrub belts on feed on offer

The presence of shrub belts did not significantly depress the amount of feed on offer (FOO) in the adjacent pasture. However, there were significant differences between the FOO from lucerne and annual pasture and where the pasture was being grown with most pasture generally available higher up the slope. The quantity of lucerne and annual pasture FOO was similar in the top and bottom pasture areas.

What it all means

Shrub belts cost approximately $180/ha (plus fencing costs) to establish and occupy 15% of the grazing area. These costs need to be recovered from improved productivity in areas of the farm below the shrubs where water-logging or salinity effects could be reduced. While including shrub belts in the annual pasture made a significant difference in water use and recharge control under the shrubs, at only 15% of the paddock area, it had no statistically significant impact on soil moisture at the foot of the slope. There would consequently be no significant reduction of the area affected by salinity or water logging to cover the cost of shrub belt establishment or the ongoing loss of grazing area.

Lucerne pasture was the most effective management option. It occupied the entire area so no land was removed from production and dried the soil at least as well as the shrubs. Lucerne made a bigger difference in soil moisture reduction at the foot of the slope than either the annual pasture or the annual pasture with shrub belts. Investment in perennial pastures over large areas is costly and must be appropriate for the livestock systems. Perennial pasture plantings will provide some salinity and waterlogging benefits, but these may not cover the cost of establishing pasture. Consequently, new pastures should be sown primarily to provide additional benefit to the grazing system. Any benefit in recharge control may reinforce the decision to establish pastures. The use of lucerne in the grazing systems at the Wagga Wagga Proof Site is discussed in the message More lucerne increases production and profit.

While the evidence from the EverGraze Proof Site does not support the use of shrub belts for recharge control in environments such as those at Wagga Wagga, it does not mean that farmers should not plant shrubs for other reasons. If shrub belts are established for other valid reasons there will likely be no impact on adjacent pasture growth.

Acknowledgements

EverGraze is a Future Farm Industries CRC research and delivery partnership. The Wagga Wagga EverGraze Proof Site team was led by Dr. Michael Friend at Charles Sturt University. Dr. Iain Hume led the shrubs component of the Proof Site. An enthusiastic Regional Advisory Group consisting of farmers, consultants, extension officers and CMA staff provided significant input into the direction, management practices and interpretation of outcomes from the experiment and modelling to make them relevant to farmers. Andrew Bathgate conducted some of the modelling.

Contributors

Dr. Iain Hume, NSW Department of Primary Industries

Dr. Jason Condon, Charles Sturt University, Wagga Wagga

Dr. Phil Eberbach, Charles Sturt University

Research

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