Ethnobotany and conservation biology of Warburgia salutaris (G.Bertol.) Chiov., a threatened medicinal plant in southern Mozambique
Authors:   , Senkoro Annae Maria 
Published: April 2022

Warburgia salutaris, locally known as “chibaha” (Ronga, Tsonga), “isiBhaha” (Zulu) and “siBhaha” (Siswati), is one of the most highly-valued medicinal plant species in southern Africa. Its bark has long been employed to treat bacterial and fungal infections, and it is often a front line herbal treatment for malaria. The species is popular in rural and urban areas due to its lower price compared to commercial pharmaceuticals and because of its long-standing spiritual and symbolic significance. Consequently, W. salutaris is threatened in Malawi, South Africa and Swaziland and is believed to be extincted in the wild in Zimbabwe. In Mozambique, where the tree is confined to the southern part of the country, it is considered vulnerable due to high local demand and cross-border export. Although there is limited research, further work is urgently needed to provide the basis for options and strategies to achieve desired outcomes. This study aimed to explore different approaches for the conservation of W. salutaris in the Lebombo Mountains (LM), the Tembe River (TR) and the Futi Corridor (FC) areas, in southern Mozambique. For this, potential distribution, socio cultural factors that influenced sustainable management of the species, demography, harvest impact, genetic diversity and population differentiation were considered. The kuenm package in R, 2 846 occurrence and 11 environmental data were used to model the potential current and future distribution in 2050 and 2070 using four general circulation models (CESM1(BGC), ACCESS1.3, FIO-ESM and IPSL-CM5A-MR) under representative concentration pathways (RCPs) 4.5 and 8.5. The extent of occurrence (EOO) and habitat suitability (HS) were also determined. Stratified random semi-structured interviews with 182 heads of households were carried out to explore cultural and socio incentives of use, compared knowledge distribution, determine local management practices and explore local ecological knowledge related to the species in the three study areas. In addition, 17 focus group discussion were conducted to clarify issues raised during household interviews. One hundred and twenty four quadrants were used to determine the abundance and population structure. The Simpson index of dominance, permutation index and quotient between consecutive classes were used to assess population stability, and an eight point scale was used to evaluate the extent of bark damages. Genomic deoxyribonucleic acid (gDNA) (n=48) was used to assessed genetic diversity and population structure through the newly developed single sequence repeats (SSR) molecular markers. Model selected was statistical significant for prediction of potential distribution of W. salutaris in southern Mozambique (partial ROC =0.00, omission rate = 0.04 and delta AICc = 0.00). The current environmental conditions mostly explained by climatic predictor, indicated in general, the potential distribution on areas of occurrence observed during the field work in southern Mozambique. Future project suggested an increase of climatic suitable areas in all the general circulation models in both conservative and pessimistic climatic scenarios (RCPs 4.5 and 8.5, respectively) in 2050 and 2070, and consequently the increase of HS and the EOO. Results of this work suggests W. salutaris would not extinct at least in the coming 50 years due to climate change but other threats such as overexploitation of the species need to be addressed. With increase of climatic suitable areas, conservation and management of this species could focus on reintroduction and cultivation for recovery of the species, with the help of time. W. salutaris was used for medicinal purposes to treat 12 health problems, with the bark being the most commonly used part (92% of informants). Knowledge of the species uses differed significantly between the three study areas but not with respondent gender or age. Informants also mentioned a considerable number of alternative species of W. salutaris (37 species in LM, 33 species in TR and 17 species in FC). Local ecological knowledge on the ecological niche of the species also varied substantial in the three areas with both types of knowledge generally higher in the Tembe River area. Local management practises were revealed mostly on harvesting through vertical bark stripping cited by 100% of informants in TR, 61.6% FC and 51.7 LM (71% of all informants). Yet most (64.3% of all informants, corresponding to 98.4% in TR, 51.7% in Fc and 41.7% LM) used non-traditional harvesting tools (machete) bark removal. For sustainable use of the species, consideration should be given to combined conservation strategies such as the use of alternative species with the same application, substitution of bark by leaves, and increases in alternative sources of plant material through cultivation. The densities of alive stems were not statistically different in the three areas but was higher in LM (mean of 4500.3±9209.8 stems/ha). Equally the mean densities of recruits (diameter ≤2 cm) were not considerable different but was higher in the FC (3296.8±9116.7 stems/ha). The regression analyses for the three study area revealed negative slopes, indicating more recruits and fewer individuals in larger classes. The Simpson indexes of dominance, permutation indexes and quotients between the consecutive size classes suggested some levels of population instability in the three areas. Harvesting took place in the eight point scale, all size classes were harvested but preference was higher on intermediate (15.1-20 cm) or larger (>20 cm) size classes in all the study areas but gatherers avoided harvesting bark from recruits. Although the mean densities of harvested stem was not significant this work indicated higher levels of stem destruction in FC. Harvested dead stems ware significant in the three areas and FC had the higher of (39.5±85.9 stems/ha) compared to 7.6±19.1 and 2.1±4.3 in TR and LM, respectively. Equally, the density of physiologically stressed stems differed substantially in the three areas with higher in FC 20.5± 75.5 stems/ha compared to 18.7±82.9 stems/ha in TR and 3.8±11.8 stems/ha in LM. This results suggests implementation of protection measures such as the inclusion of the species in the national CITES list to regulated cross-border trade and strengthening of the law enforcement. Genetic diversity, structure and gene flow in the exiting populations of southern Mozambique revealed that despite fragmentation and overexploitation, this species maintains a relatively high level of genetic diversity supporting the existence of random mating. Two genetic groups were found corresponding to the northern (LM and TR) and southern (FC) areas. The study suggests that, if local extinctions occurred in Mozambique, the pepper-bark tree persisted in sufficient numbers to retain a large proportion of genetic diversity. Management plans should concentrate on maintaining this high level of genetic variability through both in and ex-situ conservation actions.