Transcript for Video “How to reduce stemflow in a macadamia orchard”

Narrator: The Australian macadamia industry is located on the New South Wales north coast and southern and Central Queensland. These districts can experience high rainfall intensity at certain times of the year. On bare soil, this can result in the movement of soil and severe soil erosion if it’s not managed well.

Recent research funded by the macadamia industry has revealed that some of the most significant soil losses can occur from around the base of the tree as a result of water flowing down tree stems. This process is called stemflow.

John Pretorius, (macadamia grower): For every hectare, which is 10,000 square metres, every event of 100 millimetres is a Meg of water, in other words a million litres, that has to come down. The bare soil or grassed area can absorb a lot of it, but on the stem it’s concentrated and it saturates the soil around the stem and then just has to run off.

Narrator: Stemflow can lead to severe root exposure over time and this can also lead to nuts being trapped in roots and unable to be harvested. Soil scientist Justine Cox at the Centre for Tropical Horticulture was keen to discover where the sources of soil erosion were. In particular she wanted to measure the effects of stemflow.

Justine Cox (NSW DPI): 	Here at the Centre for Tropical Horticulture we built this device to capture all the water pouring down the macadamia trees. It gets funnelled into this tube and into the data logger, which then tips and all of the data is measured automatically.

Over a 32 hour storm event in May 2009, we measured an average of 1,100 litres of water flowing down each trunk. We calculated that up to 28% of the water falling onto an orchard was funnelled down the tree trunks. On average, 5-10% of rainfall was converted to stemflow. This high level of stemflow is thought to be due to the architecture of the macadamia tree and its smooth bark. 

We wanted to find out how much soil was lost at the base of each macadamia tree, so with this purpose built frame, we measured the distance of the frame to the soil surface with a laser meter and we measured the soil topography. After 18 months, soil was lost from the whole measured surface area around the tree. Roots were exposed, and soil was lost around these roots, exposing them more.

We found that soil movement from around each tree was 13 millimetres a year. So that meant that around the vulnerable base of the tree over a centimetre of soil was getting lost. On average spacing of 8 by 4 metres, that means the soil loss translated to 3.8 tonnes of soil moved around the base of the tree per hectare per year, and that’s a substantial soil movement.

Narrator: The issue with soil movement due to stemflow is that the moved topsoil is very likely to be transported further down the slope in the next intense rainfall event, and therefore add to the rate of soil loss from an orchard. OK, what are the keys to reducing soil movement from around the base of the tree? There are three main elements. 

The first is to reduce stemflow. The second is to protect the soil at the base of the tree, and the third is to maintain a groundcover on as much of the orchard floor as possible. 

So firstly, how to reduce stemflow?  While not yet fully researched, grower experience suggests that limb removal or hedging could help reduce stemflow. Stemflow is related to canopy size and the tree architecture, so a reduced canopy size could reduce stemflow.

John Pretorius: and it’s related to variety. There’s a difference between the different varieties, some have a lot more runoff, especially the 344’s that are more upright, where as the A16s that are more rounded canopy tend not to have the same volume running down the stem. So that has now become a farming practise of ours, is that we’ll chip in June July, we’ll put our compost down and then we’ll profile.

Narrator: Now the second key, protecting the soil at the base of the tree. Mulch is an effective way to protect the soil and coarse woodchip from prunings can eliminate the movement of soil from under a tree row. Grass clippings from the inter-row are also useful.

Noah Seccombe (macadamia grower): When we do the large limb removal, we feed the branches through a wood chipper and they get thrown straight back under the tree on the ground. If you have a look into the woodchip after 18 months (this stuff’s been down about two months) after 18 months you start to see the fungal hyphae through the woodchip, it really binds it down and becomes quite a stable product under the tree. It really helps to build the soil back up.

Narrator: And the final key. Planting groundcovers such as smothergrass is probably the most effective measure to prevent soil erosion in a macadamia orchard. Previous research found that erosion can be reduced by 99% where a good groundcover exists. 

Noah Seccombe: OK, so this is our original erosion control fence, trying to slow the water flow down through the gully. We’ve been through another step of creating erosion control sausages out of hessian sacks filled with mulch and dirt and whatever we can get our hands on. But now we’ve gone to planting actual smothergrass strips, cut our own strips of turf, and planted them through the gullies. 

There’s one here that’s been planted for probably about a week and a half, just starting to root down a bit. Another important part of it’s getting the sunlight in, probably the most important part.