Strawberry guava – the plant behind the death of pristine Mauritian forests
Impenetrable thickets of strawberry guava!
Strawberry guava (Psidium cattleianum), sometimes erroneously called Chinese guava, is one of the most invasive plants on tropical islands. Originally native to Brazil, it now dominates wide areas of native forests on tropical islands. The upland forests of Mauritius have been extensively invaded by strawberry guava, threatening the native flora and fauna ever since they were introduced. Feral pigs, frugivorous birds and humans aid in the seed dispersal of strawberry guava. Strawberry guava is able to propagate very quickly due to copious fruiting, high shade tolerance and its ability to form dense stands, all of which have allowed it to outcompete endemic flora and dominate the native forests of Mauritius. Strawberry guava lowers the water level from aquifers, soils, streams and groundwater systems which limits water supply to native species and local communities.
Our germination experiment has now confirmed that strawberry guava releases allelochemicals that inhibit the growth of native plants. The allelopathic effect of strawberry guava reduces the ability of slow-growing native plants to compete with this faster-growing exotic species.
If you’ve ever tried to get rid of unwanted weeds in your backyard, you know that it’s pretty much impossible to eliminate them entirely. Sooner or later, they will come back and constant removal is needed to manage the weeds. Mechanical removal is used to reduce the occurrence of strawberry guava in the native forest remnants of Mauritius which has a positive effect on native plants. However, due to the high cost of mechanical tending, this operation has only been carried out on 1% of the remnants of native forests. The expansion of mechanical tending to cover a significantly wider area of native forests would be too costly.
Cheaper long-term alternatives to control strawberry guava involve biological control agents, such as its natural enemies, Tectococcus ovatus and an undescribed species of Eurytoma. Tectococcus ovatus produces leaf gall while Eurytoma sp. produces stem gall to reduce or prevent the growth of strawberry guava. While both Tectococcus ovatus and Eurytoma sp. have a high specificity for strawberry guava, there are risks associated with the introduction of any non-native biological control agents since they might themselves become invasive and attack native species. If biological control agents are introduced on a small island such as Mauritius, where less than 1% of the native forests remain, the effects of an insect outbreak will be devastating, potentially exterminating most or all of the native forest remnants and the endemic fauna that will be left without a habitat.
Biochar has great potential to remove strawberry guava from the native upland forests of Mauritius with more efficiency and less risks to the ecosystems than mechanical and biological control methods. Biochar can also be produced at very low costs in a low-tech pyrolysis system and can therefore be applied at a larger scale. Our project may hold the key to the eradication of strawberry guava from native Mauritian forests and other tropical forests that have been severely degraded by this invasive plant, such as the Hawaiian rainforests.
