Tuesday, April 23, 2024

Trees on-farm help with more than erosion control

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Study looks at how silvopastoral systems can benefit pasture-based farms more widely.
Silvopastoral farm systems, in which trees are planted at wide spacings in pasture with livestock grazing underneath, are common overseas. Photo: Thomas Mackay-Smith/OLAW
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By Delwyn Dickey for Our Land and Water

When Bay of Plenty dairy farmers John and Margret Scrimgeour were looking to diversify on their farm, they decided to plant up a steep 2ha piece of their land with poplars at a 12m by 12m spacing. The north-facing slope gets very hot during warmer months, so along with the poplars’ timber value, the trees offer shade and shelter to grazing animals.

The benefits of tree cover on steep pastural land are well known, with tree root systems helping to bind soil and stabilise slopes. They can also act like sponges in rain events, drawing water into their canopy, where it evaporates.

Silvopastoral farm systems, in which trees are planted at wide spacings in pasture with livestock grazing underneath, are common overseas. In New Zealand, adoption has generally been limited to poplars and willows, planted in erosion-prone gullies and on farms in parts of the North Island for soil conservation and slope stabilisation.

Looking into how silvopastoral systems could benefit pasture-based farm systems more widely was part of a recent study led by Raphael Spiekermann at Manaaki Whenua Landcare Research, with a journal article now submitted for publication. 

“Different tree species will be good for different things on the farm and we wanted to open up the conversation around that,” said Thomas Mackay-Smith, landscape ecologist at Massey University, who was part of the research team. 

Different tree species will be good for different things on the farm and ‘we wanted to open up the conversation around that’, says Massey University landscape ecologist Thomas Mackay-Smith.

The researchers, with funding through Our Land and Water, looked for knowledge gaps in the use of silvopastoral systems on NZ farms, such as the use of native species instead of exotics. While pasture production can be impacted under some trees, it can increase under others, said Mackay-Smith, citing increases found with kānuka in pasture.

Barriers to silvopastoral systems being used more were also looked at. 

A workshop to connect farmers interested in using trees in farm systems to land management advisers, policy advisers and advocates, found most of the farmers already practice some form of silvopasture themselves, and were well aware of possible future benefits. (The researchers acknowledge the make-up of the group didn’t include any farmers who see trees on farms as a nuisance.) 

There were no surprises when the farmers put erosion control at the top of the list of benefits. Being able to provide shade for their animals was also seen as a big plus as the climate warms, to mitigate the risk of heat stress in livestock and its potential to reduce milk production in dairy cows. 

Attracting more biodiversity onto the farm through bird and insect populations was also a popular reason for tree-planting in pasture. Flowering trees will attract bees, helping to pollinate pasture species and providing food for on-farm beehives. The trees could also help create wildlife corridors for birds through farmland, connecting native bush areas, along with birds doing pest control duty and adding a bit of extra fertilizer with their droppings. 

The research found one study that indicated nutrient cycling could be another possible benefit. For the Scrimgeours in the Bay of Plenty, the possibility that their poplar trees may draw nutrients up from deeper in the soil and increase soil nutrients and fertility at the surface through leaf drop was something that fed into their decision to plant. 

The potential of carbon credits prompted a lot of interest, although there was frustration with current rules excluding credits for individual trees, as many silvopastoral designs could be excluded.

The cost of protecting young trees from pests such as possums along with potential survival rates and planting costs were among other issues raised. 

The overriding message from the research was that as well as erosion control, planting a variety of tree species could benefit biodiversity, provide animal shelter, and could be an option to boost farming’s social licence to operate. More pastoral farmers need to give trees a go on their farms to help build up a reservoir of knowledge of which trees, including natives, can add value to New Zealand’s rural landscape.

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