Introduction

Bursera is an American genus with around 100 species, most of them native to the Mexican dry tropics on the Pacific slope (^{Espinosa et. al., 2006}; ^{De Nova et al., 2012}). According to ^{Rzedowski (2015)} the current number of Bursera taxa in Mexico is 89. It is a difficult taxonomic group because it is dioecious and mostly deciduous, and the leaves are often absent from flowering or fruiting specimens. As a consequence, field observations and several collections are necessary. Moreover, vegetative structures are difficult to preserve and analyze in voucher specimens since they easily disassemble at drying. Hence, Bursera taxonomy is based on morphological characteristics of leaves, flowers, fruits, and bark when observed mainly in fresh material, at the time of collecting (^{Rzedowski et al., 2004}).

Material and methods

As a result of intensive fieldwork along the Baja California Peninsula during a project to document the floristic composition of the beaches of Mexico (^{Espejel Carbajal et al., 2015}), the eastern shore of the southern Baja California Sur, known as the East Cape Region, was visited by the authors in different locations for plant collecting and vegetation sampling, resulting in the discovery of some interesting plants (^{León de la Luz and Levin, 2012}; ^{León de la Luz and Medel Narváez, 2013}). An additional novelty found was a tree located just beside a beach, on a sandy marine terrace. This tree was considered at first to be a “torote rojo”, Bursera microphylla A. Gray, one of the most common sarcocaulescent trees along most of the Baja California peninsula from sea level up to 1000 m, and whose natural distribution “almost is coincident with the Sonoran Desert province” (^{Turner et al., 1995}). However, a more detailed inspection showed the leaflets to be few in number and broadly obovate-elliptic. These and additional differences of the leaves and flowers warrant recognition of this plant as a new species.

Results

Bursera exequielii León de la Luz, sp. nov. Figs. 1, 2.

Figure 1 Bursera exequielii León de la Luz. A. branchlet with leaves, leaflets, and fruits; B. detail of the leaf and broadly elliptic leaflets; C. detail of the rachis and subsessile leaflet; D., E., F. detail of male cluster of cymes, and 5-merous flower with annular papillose disk; G., H. details of 3-merous female flower with staminodes; I. detail of fruit, and J. seed with a thin pseudoaril. Illustration by Albino Luna. 

Figure 2 Images of Bursera exequielii León de la Luz. A. bark and leaves; B. mature plant; C. bundle of leaves; D. uncommon 4-merous female flower; E. 5-merous male flower; F. 5-merous male flower of B. microphylla A. Gray, note foliole form and synchronous appearing of flowers and leaves. 

TYPE: MEXICO. Baja California Sur, municipio de Los Cabos, rancho Las Lagunas, Punta Arena del Sur, matorral de dunas costeras, 23.54381°N, -109.48435°W, 5 m, 20.VI.2014, J.L. León de la Luz 12073 (holotype: HCIB!, isotypes: IEB!, MEXU!, SD!, more to be issued).

Arbor dioica ad 4 m alta, ramificatione tortuosa, cortice exfolianti flavida. Folia imparipinnata, 7-11 foliolis late obovatis vel late ellipticis, 6-9 mm longis, 5-6 mm latis, glabris. Paniculae laxae 3-5 floribus unisexualibus, pedicellis quam 2 mm brevioribus. Fructus 3-valvi, subglobossi; putamine 5-6 mm diam, pseudoarilo flavido obtecto; hilo depresso.

Dioecious trees to 4 m tall; bark glabrous, orange-yellowish, exfoliating in papery strips as stems thicken; trunk straight but branching tortuous, wood pithy and soft, young stems puberulent with short trichomes; dioecious, exceptionally some male flowers with functional pistils; leaves imparipinnate, produced from brachyblasts, growing gradually during the season, 3-3.5 cm long, oblong in outline; petioles 10-12 mm long, winged, somewhat aromatic when crushed; rachis more narrowly winged than petiole, involute when drying, and appearing quadrangular; leaflets 7-11, 5-6 × 6-9 mm, subsessile, semi-succulent, and broadly obovate to broadly elliptic, yellowish-green, opaque, midvein on abaxial surface prominent; indument spiculate on stems, reproductive structures glabrous, foliage with resinous papilla. Inflorescences produced before leaves, subtended by small bracts on reduced cymes with 2-5 flowers per cluster, flowers rarely solitary; peduncles stout, 2-3 mm long, pedicels slender, <2 mm long, with a basal small and caducous bracteole <1 mm, soon deciduous. Male flowers: to 4-5 mm wide at anthesis; sepals (4)5(6), triangular, red tipped, <1 mm long, petals (4)5(6), broadly lanceolate, 1 mm long, 0.5 mm wide; stamens to 1.5 mm long, twice as much as petals, in two series, opposite and alternate to petals, filaments inserted on edge of annular, papillose disk; pistillode <1 mm long, reduced and rarely functional. Female flowers: to 4-5 mm wide at anthesis; sepals 3(4), <1 mm long, triangular, red tipped; petals 3(4), lanceolate to broadly lanceolate, 1 mm long, 0.5 mm wide; staminodes (5)6, anthers <1 mm long, sterile, filaments 1 mm long, free, inserted on edge of annular, papillose disk; ovary superior, 3-celled, style short, stigma 3-lobed, tiny; fruits subglobose, 3-angles, glabrous; exocarp 3-valved; pyrene one per fruit, covered all by a yellow-reddish pseudoaril; seeds one per fruit, rounded, 5-6 mm diameter, hilum with an irregular depression.

Paratypes: MEXICO. Baja California Sur, municipio de Los Cabos, rancho Las Lagunas, Punta Arena del Sur, matorral de dunas costeras, 23.54788°N, -109. 47352°W, 5 m, 1.XII.2013, J. L. León de la Luz 11912 (HCIB); Punta Arena, ca. 12 km southeast of La Ribera and ca. 18 km north of the village of Cabo Pulmo, 23.5334°N, -109.4829°W, 15.VI.2015, B. T. Wilder 2015-44 (MEXU), 2015-45 (MEXU).

Observations

The exfoliating bark, winged leaf rachis, and 3-valved fruits place this species in section Bursera, the “cuajiote” group in the Mexican mainland vernacular nomenclature (^{McVaugh and Rzedowski, 1965}; ^{Rzedowski et al., 2004}), also known by the bajacalifornians as “torote”.

In addition to belonging to section Bursera, B. exequielii and B. microphylla both have tortuous branching and similar flower and fruit morphology, but the new species differs from B. microphylla by the 7-11 (vs. 7-35) leaflets that measure 7-9 × 5-6 mm (vs. 5-25 × 1-3) and are broadly elliptic to obovate (vs. linear-oblong to elliptic or lanceolate). Moreover, inflorescences are denser in B. microphylla.

Hence, B. exequielii seems to be a local population possibly descendent from the widely distributed B. microphylla. Following the proposal of ^{Becerra (2003)}, who reconstructed a phylogenetic hypothesis of 66 species and varieties of Bursera using parsimony analyses, this new species must be included in the section Bursera and Microphylla group. In addition, ^{De-Nova et al. (2012)} published a Bursera phylogeny at species level by estimating divergence times, temporal diversification heterogeneity, and geographical structure, in order to determine when species could have first appeared in Mexican territory. With respect to B. microphylla they assessed that this species established some 8 My ago; i.e., just when the peninsular territory split off from the Mexican mainland by the opening of the Gulf of California (^{Ferrari et al., 2007}).

Several botanists have noticed that across the broad range of B. microphylla¸ it exhibits variation in leaf and leaflet size, but not in form. ^{Johnson (1992)} illustrated such differences for several populations of Sonora, Mexico (see Fig. 3), which show significant variation in leaflet size but a consistent length/width ratio ≥4, and the leaflets are consistently linear-oblong to narrowly lanceolate. Flower sex of B. microphylla has been barely documented. Available bibliography, such as ^{Shreve and Wiggins (1964)}, relates the species as dioecious, whereas ^{Felger (2000)} stated that in B. microphylla sepals and petals are usually 3 on female flowers and (4)5 on male flowers. In B. exequielii we observed that female flowers have 3(4) sepals and petals, and male flowers (4)5(6) sepals and petals. Moreover, few fruits were seen on male individuals, but due to its low frequency this trait could be considered rather as unusual functional pistils in staminate flowers more than genuine hermaphrodite flowers that would give the polygamodioecious character to the species. The number of cymes per cluster is also a noticeable difference, since in B. exequielii there are commonly 2-3 cymes, whereas in B. microphylla there are 4-6, although both taxa could have 2-3 flowers per cyme.

Figure 3 Image of a leaf of Bursera exequielii León de la Luz with broadly-obovate to broadly-elliptic folioles, and comparison with Bursera microphylla A. Gray leaves variation (modified from ^{Johnson, 1992}). 

Fruit morphology of the two species is similar in shape and size, but the pseudoaril of B. microphylla seems to be more reddish than yellowish. An additional difference could be noticed in the sprouting of the flowers: in B. microphylla leaves and flowers are produced synchronously (^{Johnson, 1992}; see Fig. 2F), while in B. exequielii we observed first the flower development (Fig. 2D, E), and the bud leaves appear a couple of weeks later; both structures continue growing during the rainy season.