1. Shea (Vitellaria paradoxa) trees bear fruit and seeds of considerable economic, nutritional and cultural value in the African Sudano-Sahelian zone. In much of West Africa, shea exists within an agroforestry system referred to as “Parkland”, where social changes, including migration, have resulted in expanding areas of crop cultivation, reductions in both the area of fallow land and the duration of fallow periods, and reduced diversity of habitats and woody species. Shea benefits strongly from pollination by bees and the loss of Parkland biodiversity may reduce the availability of pollinators, leading to pollen limitation, and reductions in fruit yields.
2. We investigated whether shea trees in Southern Burkina Faso experienced pollination limitation, and whether local- and landscape- scale diversity was linked to visitation by bees, the degree of limitation observed, and the weight of fruit produced.
3. Honey bees (Apis mellifera) were observed more frequently in diverse sites, whereas non-Apis species were generally widespread but visited trees in greater numbers at diverse sites.
4. We found that shea fruit production was significantly limited due to lack of pollination and that the degree of pollination limitation was greater in sites with lower levels of tree and shrub diversity.
5. Synthesis and applications. Sites with greater diversity of tree and shrub species had more bee visits and less extreme pollination limitation than less diverse sites, indicating that small-scale diversity is associated with more efficient pollination services. Consequently, shea yields are likely to benefit from retention of a range of different tree and shrub species in parklands. We recommend that when fallows are cleared for cultivation, such beneficial plants are retained within cultivated fields, and that measures to conserve pollinators in the region should target both A. mellifera and non-Apis bee species.

2.1 Study species

Shea, or Vitellaria paradoxa, is the only member of its genus in the family Sapotaceae.  Its range is restricted to the Sudano-Sahelian zone of western and central Africa (Fig. 1) and it comprises two sub-species: Vitellaria paradoxa ssp. nilotica in the east and Vitellaria paradoxa ssp. paradoxa in the central and western parts of its range (Naughton et al., 2015).  Shea trees bloom during the dry season, between January and April, with peak fruiting occurring by the end of July, although anecdotal evidence suggests flowering and fruiting periods can vary yearly. Flowers are born on inflorescences located at the tips of branches.  The flowers are short-lived but buds, receptive flowers and older blooms occur simultaneously over a long flowering period (Stout et al. 2018).  The flowers are protogynous, with the stigma emerging from the bud and becoming receptive before the sepals part to reveal the petals and stamens (Hall, Aebischer, Tomlinson, Osei-Amaning, & Hindle, 1996).  Although this reduces the likelihood of a flower self-pollinating, the presence of flowers at different stages of maturity on the same inflorescence indicates that within-inflorescence pollination is possible.  The success of pollination by flowers on the same inflorescence is not well understood.  Shea produces fruit with sweet pulp surrounding a large seed (the shea “nut”), and occasionally more than one nut may be produced in a single fruit.

2.2 Site selection

This study was conducted in the Centre-Sud region of Burkina Faso, south of the Kaboré Tambi National Park (KTNP) and east of Nazinga Game Ranch (Fig. 1).  All data were recorded between February and June 2017.

We selected ten 1 ha sites (100 m x 100 m) in fields which had been cultivated during the previous rainy season. Sites were organised into five pairs located 2 - 3.5 km apart, and the minimum distance between pairs was 4 km. The paired design allowed us to minimise impacts of external factors (e.g. distance to a large town or nature reserve).  Within each pair, we selected one site with low diversity of woody species and one site with higher diversity.  All trees and shrubs, including shea, greater than 3m in height were identified and counted in each site, and the mean inverse Simpson’s index (1/D) was calculated to represent site diversity. We did not include small trees and shrubs (<3 m in height) because local knowledge indicated they were unlikely to flower (an observation that proved true). Herbaceous species were also excluded from site diversity calculations because we worked in ploughed fields that were open to grazing animals, with low cover of herbaceous plants (<0.01% cover). We mapped and calculated the area of semi-natural habitat within a radius of 1 km from the centre of each site in ArcMap 10.4 using satellite imagery provided by ESRI world maps and Google Earth (accessed in September 2017, photos dated to 2013).  Restricting the radius to 1 km avoided an overlap in mapped habitat between sites.  Although the foraging range of A. mellifera can greatly exceed this distance, studies based on waggle-dances and mark-recapture indicate that African honey bee races, including A. mellifera adansonii, focus the majority of their foraging within distances of 1km of the hive or previous feeding stations (Roubik, 1999; Schneider, 1989).  Diversity (1/D) of woody species within our ten sites (site diversity) and area of semi-natural habitats then acted as the independent variables (see data analysis).

We collected data regarding pollination services from 10 shea trees within each site that met the following criteria: selected trees had a single trunk, were greater than 3 metres tall, appeared healthy and had developing flower buds. If more than ten trees in a site met the criteria, the ten selected trees were distributed as evenly as possible through each site.

2.3 Pollination limitation

Supplemental hand-pollination was carried out over the course of three visits to each site during the flowering period.  We selected up to three pairs of inflorescences that had a similar number of flowers with receptive stigmas on each tree.  The flowers with receptive stigmas on one inflorescence per pair were marked for fruit counts later in the season and were left open to natural pollination (untreated flowers).  The other inflorescence per pair was also marked and left open, but in addition was hand-pollinated with pollen from another tree (pollen supplementation).

The number of trees and inflorescences varied between sites because three comparable pairs of inflorescences were not present on every tree (Table 1). In total, 2248 flowers were included in the analysis: 1096 untreated flowers and 1152 flowers with pollen supplementation by hand.  On average, the difference in the number of hand pollinated and control flowers at each site was 2.4%. The greatest difference was 8.6%. All results relating to flowers and fruit set are from these selected flowers. 

2.4 Flower visitors

We surveyed flower visitors during two ten-minute recording episodes per tree, one between 6 and 8am and another between 4 and 6pm, when insect visitors are most active, on each of three dates during the flowering season (January-March 2017) (Stout et al. 2018).   If there were no receptive flowers on a tree, no attempt was made to record visitation (Table 1).  All bees observed visiting inflorescences during recording episodes were caught.  The native honey bee, Apis mellifera adansonii Latreille could be identified easily, but other bees were placed in 70% alcohol solution and sent for expert identification. 

2.5 Fruit set

We counted fruit set in early May 2017, when the selected flowers had developed small fruits, but before fruits ripened and fell.  Ripe fruits were collected and their nuts were weighed in the field in June to August 2017. Initially, fruits were hand harvested (n = 35), but proved to be unripe. The remaining (n=133) fruits were collected by placing bags around them so they could fall naturally as they ripened without being lost. Only the ripe fruits collected in bags were included in the analysis of fruit weight.