Can the UAE combat this invasive tree by turning it into fertiliser?

Central American shrub Prosopis juliflora has spread widely and outcompetes native species

An invasive Prosopis juliflora mesquite surrounding a ghaf, the UAE's national tree. Photo: Dr Ali El Keblawy

From carp outcompeting native species, through to zebra mussels multiplying in vast numbers, and knotweed damaging buildings, invasive species have been blamed for ecological and other harms across the planet.

The UAE has also fallen victim, notably with Prosopis juliflora, a Central American mesquite tree introduced to the country around five decades ago as part of afforestation efforts.

It is now found in large numbers across several emirates, particularly in the north-east of the country where it crowds out native plant species.

However, rather than attempt the difficult task of simply eliminating this invasive species, a UAE-based researcher has said that the juliflora could be harvested and then converted into biochar — a charcoal-like substance that is created by burning wood or other biomass at a high temperature without oxygen.

The proposal from Dr Ali El Keblawy, of the University of Sharjah, could bring several benefits. Biochar can improve the nutrient and water-retaining qualities of the soil, help reduce water use by the trees and store carbon in the soil.

“Biochar is organic material that can be [added] to nutrient-poor soil to enrich the soil fertility and enhance the physical, chemical properties. It can make the soil suitable for agriculture,” Dr El Keblawy said.

Biochar can be spread on soil and, unlike wood, is not broken down by microorganisms, remaining stable for tens or even hundreds of years instead of releasing its carbon quickly.

Previous research has suggested that the closely-related ghaf tree, Prosopis cineraria, which in 2008 was named the UAE’s national tree, is suitable for biochar.

Dr El Keblawy would like to secure funding to develop project to use the juliflora tree plant for biochar production.

Juliflora spreads easily because its seeds are eaten by animals and germinate after being released in faeces.

“It is really hard to be controlled. No animal can eat it, just the fruits. This means it is a non-grazeable plant,” said Dr El Keblawy.

“All the trials, not [just] here but globally, have failed to get rid of this species.”

Outcompeting other species

The tree uses a taproot several metres deep to extract large amounts of water — a very limited resource in the UAE. It also has a dense, shallow fibrous layer of roots nearer the surface that absorb nutrients and water from other plants.

Dr El Keblawy has supervised a PhD student who has uncovered further details about how the large roots help the tree to outcompete other species.

Dr Francois Mitterand Tsombou found that the roots of the juliflora extend several metres horizontally, compared to just 40cm or so for the roots of the ghaf tree.

“If there is anything growing from [above the root], the plant … will not have any chance to have humidity, to have nutrients,” Dr Tsombou said.

“The root of the exotic will capture all the water availability, all the nutrients and any native plant will not survive under the canopy of this one.”

Aside from its heavy consumption of water and nutrients, the juliflora tree suppresses the growth of native plants by releasing chemicals — a phenomenon known as allelopathy.

As part of his research, Dr Tsombou collected leaves of juliflora, another non-native tree, Prosopis pallida and the ghaf tree.

Laboratory tests showed that substances in the leaves of the juliflora had a “strongly negative” effect on the growth of other plants, an effect not seen with the leaves of the other two species.

In the juliflora's native territory, these chemicals are used to defend against animals that might eat the plant, but in the UAE they have the effect of suppressing other plant growth.

“The leaves of the Prosopis juliflora contain a lot of inhibitory chemicals that neutralise the growth native plant species,” Dr Tsombou said.

Aside from harming native species, the plant produces large quantities of pollen grains for much of the year, which can cause allergies for sensitive people.

The UAE is not the only country where the juliflora has caused issues. In Kenya, it was credited with creating a green, tree-covered landscape less prone to generating sandstorms.

However, as in the UAE, in Kenya, the plant’s seeds spread in the dung of animals and the tree has become difficult to control, causing the loss of grazing and croplands, the blocking of rivers and even the displacement of people from their homes.