Tree-planting drones could help restore the world’s forests

Forestry engineers have mastered the job of harvesting commercial forests efficiently, but replanting those forests or establishing new ones is still largely people-powered, using a spade and a bag of seedlings. Using drones to deliver seed packages may change all that and become a standard tool in the forester’s toolkit, alleviating the perennial shortage of labour for the back-breaking job of manually planting trees in often difficult and remote terrain.

Using drones to plant seeds could help to cool the planet by rapidly establishing new forests, replanting timber-harvested areas, reseeding in fire-devastated zones more quickly, and accessing difficult-to-reach areas. Several young start-ups have developed drones to rapidly plant seeds from the sky, many claiming headline-grabbing promises to plant a billion trees or more in the next decade, which has not hurt their ability to raise capital.

On paper, there’s no doubt that drones can get the job done. But how effective is seeding from drones? California-based DroneSeed claims that its unmanned aerial vehicles (UAVs) can reforest an area six times faster than human planters, while competitor Canadian start-up Flash Forest claims to scatter 100,000 seeds a day, but these eye-catching metrics are not what counts.

The all-important statistic is the initial germination rate from the dropped pods combined with one- and two-year survival and growth rates of the young trees. One study found that fewer than 20 per cent of seeds dropped by drone take root and grow into trees.

A US Forest Service study has found that “survival and costs [of drone planting] have not been optimal compared to hand planting”, but their report stated that the technique might be suitable for replanting in fire-damaged areas prone to mudslides and instability, that are difficult to access on foot.

Before aerial seeding can start, seed must be collected, cleaned, and sorted – by trained humans. Each species’ seeds may require a different pre-treatment to allow them to successfully germinate. Some need to undergo two periods of sub-zero temperatures followed by warming of spring-like weather; others will not germinate until they have been in a forest fire, or travelled through the gut of an animal or bird.

Pre-treated seeds have to be encapsulated in a protective shell so that they can survive being fired pneumatically into the ground from a height by a drone. Clay pellets or gel packets are often used to surround the seed. Some companies add chilli to the capsule to make it distasteful to clever foragers, like squirrels, or passing birds who might swiftly end the seed’s chance of ever growing up.

The planting operation uses heavy-lift payload drones that can carry a substantial payload of the pre-primed encapsulated seeds, which may include pre-germinated seed, and an initial nutrient package to assist early growth. Those used by DroneSeed can carry a payload of 25kg each (enough to plant 3,000m2 per flight). These UAVs can operate in swarms to deliver pucks of seed to the planting site under the supervision of a couple of skilled operators. Aftercare can involve drones equipped with tanks of herbicides to kill competing weeds and grasses that might overgrow the developing tree seedling, or deliver liquid fertiliser to boost the young trees’ growth and establishment. The subsequent germination success and survival rates must be measured by camera-equipped drones in the months and years after the initial planting.

Standardisation and modularisation of the drones themselves is key to a wider adoption of drone planting. Currently, most companies use different drones for each stage of the operation. Landowners are much more likely to invest in the technology if the same drones can be repurposed for each stage – much as they would with a tractor, equipping it with different add-ons for seed drilling, ploughing, harvesting etc. Initial surveys are undertaken by lightweight lidar-equipped mapping machines. The results of initial surveys are fed into algorithms that assess the hydrology, slope, soil conditions and underlying geology to calculate the best areas for planting or replanting, producing a route map of a planting grid that the seeding drones will follow.

Rather than the state of drone technology, it is the availability of quality tree seeds that might be a key limiting factor in our ability to increase the planet’s forest cover. The new drone companies recognise this, and five-year-old DroneSeed recently gained Silvaseed, who have been collecting and processing cones and seeds from wild forests in the Western US for over a century. DroneSeed plans to double capacity at Silvaseed to help address the shortage of tree seeds. Government agencies in the US are also planning to double the capacity of their seed-harvesting operations.

Taking an alternative approach to flying robots is an Estonian company Milrem Robotics, which is building on its work developing autonomous tanks to design tree clearing and planting robots suitable for working in the rough terrain of a harvested forest, stumps and all; but although components of its tracked vehicle have been tested as an autonomous tree planter, the complete design is still at the concept stage.

Speeding up the process of reforestation is important. Because of the compounding crises of climate change, wildfires and forest loss, accelerating the pace and scale of reforestation is essential by whatever means. According to the IPCC report in 2018, limiting global warming will need one billion hectares of new forested area – about the area of the US.

Amazingly, our planet has space for it, mostly in the wilder parts of China, Russia and Canada. If allowed to mature, some estimates say those future forests could absorb 205 gigatonnes of carbon, or two-thirds of the carbon that humankind has emitted since the 1800s, and would be potentially one of the cheapest natural solutions to buy us time to tackle the climate emergency.