Data from: Local-scale tree and shrub diversity improves pollination services to shea trees in tropical West African parklands

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.

2.1 研究物种 乳油木（Vitellaria paradoxa）是山榄科（Sapotaceae）该属唯一物种。其分布范围仅限非洲西部和中部的萨赫勒-苏丹过渡带（Sudano-Sahelian zone，图1），包含两个亚种：分布于东部的尼罗特乳油木亚种（Vitellaria paradoxa ssp. nilotica），以及分布于其分布区中部和西部的模式亚种（Vitellaria paradoxa ssp. paradoxa）（Naughton等，2015）。
乳油木在旱季（1月至4月间）开花，盛果期为7月末，不过传闻证据显示其开花和结果周期每年存在差异。花着生于枝条顶端的花序（inflorescences）上，花期较短，但在较长的开花周期内，花苞、可授粉花朵以及盛放过的花朵会同时存在（Stout等，2018）。该植物为雌性先熟（protogynous），柱头会从花苞中伸出并提前具备可授性，随后花萼才会张开以显露花瓣与雄蕊（Hall、Aebischer、Tomlinson、Osei-Amaning & Hindle，1996）。尽管这一特性降低了花朵自花授粉的概率，但同一花序上存在不同成熟阶段的花朵，这表明花序内授粉是有可能发生的。目前对于花序内授粉的成功率尚不清楚。乳油木结出的果实带有可食用的甜美果肉，内部包裹着大型种子（即乳木果“坚果”），偶尔单个果实中会产出不止一颗坚果。
2.2 样地选择 本研究在布基纳法索中南部地区开展，该区域位于卡博雷·坦比国家公园（Kaboré Tambi National Park, KTNP）以南、纳青加狩猎牧场（Nazinga Game Ranch）以东（图1）。所有数据均于2017年2月至6月间记录。我们在既往雨季已开垦的农田中选取了10块1公顷（100m×100m）的样地。样地被划分为5组样地对，每组间距2~3.5km，样地对之间的最小距离为4km。这种配对设计可最大限度减少外部因素的影响，例如距离大型城镇或自然保护区的远近。在每组样地对中，我们分别选取一块木本物种多样性较低的样地，以及一块木本物种多样性较高的样地。
对每块样地内所有高度超过3m的乔木和灌木（包括乳油木）进行物种鉴定与数量统计，并计算平均逆辛普森指数（Simpson’s index，1/D）以表征样地多样性。我们未纳入高度低于3m的小乔木和灌木，因为当地经验表明这类植株大概率不会开花（后续观测也证实了这一点）。草本植物也未纳入样地多样性计算，因为我们的研究区域为翻耕过的牧场，草本植物盖度极低（<0.01%）。
我们使用ArcMap 10.4软件，结合ESRI世界地图（ESRI world maps）及谷歌地球（Google Earth，2017年9月获取，影像拍摄日期为2013年）提供的卫星影像，对每块样地中心半径1km范围内的半自然生境面积进行了测绘与计算。将半径限定为1km可避免样地间的生境测绘范围重叠。尽管西方蜜蜂（Apis mellifera）的觅食范围远超该距离，但基于摆尾舞（waggle-dances）和标记重捕（mark-recapture）的研究表明，非洲蜜蜂亚种（包括非洲蜜蜂阿当松亚种Apis mellifera adansonii）的大部分觅食活动集中在蜂巢或先前投喂点周边1km范围内（Roubik，1999；Schneider，1989）。
本研究的自变量为10块样地的木本物种多样性（即样地多样性，1/D）以及半自然生境面积（详见数据分析部分）。我们在每块样地内选取10株符合以下标准的乳油木以收集授粉相关数据：植株具有单一主干、高度超过3m、长势健康且带有发育中的花苞。若某块样地内符合标准的植株超过10株，则需将所选的10株均匀分布在样地内。
2.3 授粉限制 在开花期内，我们对每块样地进行了3次走访，期间开展补充人工授粉（supplemental hand-pollination）。每棵树上选取最多3对花序，每对花序上具有数量相近的带有可授柱头的花朵。每对花序中的其中一个花序上的带可授柱头花朵会被标记，以便后续季末统计坐果率（fruit set），该组花朵保持开放以接受自然授粉（即对照组花朵）。每对花序中的另一个花序同样会被标记并保持开放，此外还会用另一株树的花粉对其进行人工授粉（即花粉补充组）。由于并非每棵树都能找到3对符合要求的花序，不同样地的植株和花序数量存在差异（表1）。本次分析共纳入2248朵花：其中1096朵为对照组花朵，1152朵为人工授粉补充组花朵。每块样地内人工授粉花朵与对照组花朵的数量平均差异为2.4%，最大差异为8.6%。所有与花朵和坐果率相关的结果均来自上述选定的花朵。
2.4 访花昆虫 我们在开花季（2017年1月至3月）的3个日期内，对每棵树进行两次10分钟的访花昆虫观测：一次为早上6点至8点，另一次为下午4点至6点，这两个时段是昆虫访花活动最为活跃的时刻（Stout等，2018）。若某棵树上无可授粉花朵，则不进行观测记录（表1）。对观测期间访花的所有蜜蜂进行捕捉。本土蜜蜂为非洲蜜蜂阿当松亚种（Apis mellifera adansonii Latreille），可直接识别；其余蜜蜂则被置于70%酒精溶液中，送往专业机构进行物种鉴定。
2.5 坐果率统计 我们于2017年5月初统计坐果率，此时选定花朵已发育为小型幼果，但尚未成熟脱落。2017年6月至8月间，我们收集成熟果实并在野外称量其坚果重量。最初我们采用人工采摘的方式采集果实（n=35），但发现果实尚未成熟。剩余果实（n=133）则通过套袋收集，以便果实自然成熟脱落且不会丢失。仅将套袋收集的成熟果实纳入果实重量分析。