After only two years the coffee pulp treated area had 80 per cent canopy cover compared to 20 per cent in the control area. The canopy in the coffee pulp area was also four times taller than that of the control area.
The addition of the half-metre thick layer of coffee pulp eliminated the invasive pasture grasses which dominated the land. These grasses are often a barrier to forest succession and their removal allowed native, pioneer tree species that arrived as seeds through wind and animal dispersal, to recolonise the area quickly.
The researchers also found that after two years, nutrients including carbon, nitrogen and phosphorus were significantly elevated in the coffee pulp treated area compared to the control. This is a promising finding given former tropical agricultural land is often highly degraded and poor soil quality can delay forest succession for decades.
Cole said: "This case study suggests that agricultural by-products can be used to speed up forest recovery on degraded tropical lands. In situations where processing these by-products incurs a cost to agricultural industries, using them for restoration to meet global reforestation objectives can represent a 'win-win' scenario."
As a widely available waste product that's high in nutrients, the coffee pulp can be a cost-effective forest restoration strategy. Such strategies will be important if we are to achieve ambitious global objectives to restore large areas of forest, such as those agreed in the 2015 Paris Accords.
Cole warns that as a case study with two years of data, further research is needed to test the use of coffee pulp to aid forest restoration.
"This study was done at only one large site so more testing is needed to see if this strategy works across a broader range of conditions. The measurements we share are only from the first two years. Longer-term monitoring would show how the coffee pulp affected soil and vegetation over time. Additional testing can also assess whether there are any undesirable effects from the coffee pulp application."
A limitation of using coffee pulp or other agricultural by-products is that its use is mostly limited to relatively flat and accessible areas where the material can be delivered and the risk of the added nutrients being washed into nearby watersheds can be managed.