In times of drought, native trees planted as shelter belts can help farmers support crop yields.
A review of the impact of shelter on arable and pasture crops has been undertaken for the Woodland Trust by Louise Donnison at Harper Adams University College. Pulling together studies from UK agriculture and other temperate agriculture systems, its findings are particularly significant today and especially for the farmers in the South of England, an area already officially declared as in a state of drought. Climate change is likely to make this situation more frequent.
Periods of drought during the growing season could mean poor crop germination, reduced growth rates and lower yields. Even where irrigation is available, pressure to maintain domestic water supplies and to protect the ecology of rivers and other water bodies may mean that abstraction is restricted. What irrigation water is available will need to be used efficiently.
How do crops lose water?
Water is lost from crops through evapotranspiration – a combination of evaporation from the soil surface and crop transpiration (where water vapour is lost from leaf surfaces). Evapotranspiration leads to an increase in humidity around the soil or leaf surface until eventually the air is so saturated that the process slows down. However, fast winds can remove the water vapour leading to an increase in evapotranspiration rates – so the crop loses water faster. When soil water levels drop below a certain value the crop becomes water-stressed and this ultimately damages crop yield.
Shelter provides multiple benefits
Trees planted as shelter belts help reduce wind speeds, meaning water loss through evapotranspiration is slowed. Although a crop protected by a shelter belt may use the same amount of water as a non-sheltered crop, the sheltered plant is able to retain its water and use it effectively (rather than having it whipped away by the wind). This means that photosynthesis rates (and therefore growth) increase, and water usage becomes more efficient.
But don’t trees use up water too?
Trees will compete for water and nutrients, reducing crop yields directly adjacent to the shelter belt - however these reductions typically occur up to a distance of one to two tree heights from the shelter belt. These losses are significantly outweighed by the increases in yield represented by more efficient water use in the rest of the crop area.
Windbreaks are common in farming systems worldwide
In parts of the world where droughts are frequent, using tree windbreaks to provide crop shelter is already commonplace. Canada, USA, Australia, New Zealand, China, Argentina and many developing countries use trees to provide shelter. For example, the Agri-Food Canada website states that shelter belts can increase wheat yields by 3.5 per cent compared to unsheltered crops – that figure is greater in drier years.
Looking at these areas, the evidence shows that shelter belts with an optimum porosity (i.e. the degree to which the wind can pass through the trees) of between 40–60 per cent can protect an area up to 30 times the height of the shelter belt.
A study in Italy showed that a shelter belt of 40 per cent porosity increased rain-fed durum wheat yields and reduce evapotranspiration rates up to a distance of 12.7 times the height of the shelter belt. In the UK, where such shelter belts are relatively uncommon, studies using artificial shelters show yield increases of wheat and barley in years when the weather is hot and dry. Whilst more research is needed, it is clear that tree shelter belts could be of real value in the development of sustainable agriculture, especially as we face a changing climate and growing demand for food.
Traditional beliefs abound in the saying ‘a good farmer doesn’t plant trees or own horses’ – I think now is the time to challenge that view.
Mike Townsend, Communications and Evidence Adviser