News
In the context of Endangered Species Day, celebrated today, we shine a light on an audacious conservation effort to save the Critically Endangered Zanthoxylum paniculatum (‘bois pasner’), a rare treeof Rodrigues. To do so, the Mauritian Wildlife Foundation (MWF) is turning to another Critically Endangered, but slightly more numerous ally, the Zanthoxylum heterophyllum (‘bois catafaille noir’ or ‘bois de poivre’), a Mascarene endemic (extinct on Rodrigues, and very rare on Mauritius and Réunion). The innovative approach involves grafting the ‘bois pasner’ scion onto the ‘bois catafaille noir’ rootstock in the hope that the latter can help support the survival and growth of the former. It’s a bold experiment, one that has shown promising signs of success and that could very well be the last lifeline for the ‘bois pasner’.
The ‘bois pasner’ is now only found on the windswept plains of Rodrigues. Endemic to Rodrigues, with only three wild individuals currently known, two trees and a natural bonsai, in the Anse Quitor Nature Reserve and François Leguat Reserve repectively. This species once grew more widely on Rodrigues, but habitat degradation caused by invasive animals such as goats Capra hircus and rats Rattus spp., and aggressive alien plants including acacia (Leucaena leucocephala), vieille fille Lantana camara, and coqueluche Pongamia pinnata has pushed it to the brink of extinction. Natural regeneration has been extremely limited. Compounding the problem is the fact that ‘bois pasner’ is a dioecious species; individual trees are either male or female, so reproduction can only occur if both sexes are present and flowering at the same time for pollination to occur. In the wild, this has rarely been observed.
Despite these immense challenges, MWF has been committed to saving this species since 1996, when conservation work began in partnership with the Rodrigues Forestry Service and with the support of international botanists and conservationists. Partners who have helped are the Forestry Service (Mauritius and Rodrigues), National Plant Protection Office (Mauritius), Brest Botanical Gardens (France), National Tropical Botanical Garden (Hawaii), Air Mauritius and the Rodrigues Regional Assembly.
The efforts have been meticulous and long-term. The three remaining trees have been carefully monitored, protected by the construction of stone retaining walls to prevent erosion, and supported with wooden and metal braces to stabilise their leaning trunks of one of the two trees. Over the years, MWF has removed invasive plant species in the Anse Quitor Nature Reserve and improved soil conditions by planting native nitrogen-fixing plants such as Sophora tomentosa. Every flower and immature fruit of the ‘bois pasner’ has been observed and recorded, and attempts have been made to collect viable seeds for propagation, with limited success.
One of the major breakthroughs came a few years ago, when under the guidance of Brest Botanical Gardens, France, successful grafts of ‘bois pasner’ onto the rootstock of Zanthoxylum heterophyllum, flown in from Mauritius, were achieved. This was a significant milestone after many years of unsuccessful attempts to grow the plant from seed or cuttings. We currently have seven grafts of the ‘bois pasner’ in our nursery on Rodrigues, the only ones besides the three wild individuals. The grafting success has opened new doors for conservation of this species, but that is still not secure enough to prevent extinction.
To progress further, and further secure the species, more grafting of ‘bois pasner’ onto ‘bois catafaille noir’ using the same method, is planned.
