Macrolobium Schreb. is the second most species-rich genus within the legume subfamily Detarioideae (Govaerts et al., 2021). This subfamily comprises 79 genera and ca. 760 species, occurring in African, American, and Asian tropical forests (Estrella et al., 2017). Recent molecular studies have demonstrated the monophyly of the genus and placed it together with the genera Brownea Jacq., Browneopsis Huber, Brachycylix (Harms) R.S. Cowan, Ecuadendron D.A. Neill, Heterostemon Desf., and Paloue Aubl, all of which collectively comprise the Brownea clade (Murphy et al., 2018; Redden et al., 2018; Schley et al., 2018). Macrolobium can be differentiated from its phylogenetically closely related genera by having flowers with a single petal, lacking vasculature in occasional vestigial petals, three fertile stamens, staminodia absent, fruit smooth, and densely striate pollen exine (Cowan, 1953). The genus currently comprises 75 species of shrubs and trees, distributed from Central to South America (Ducke, 1941; Cowan, 1953; Govaerts et al., 2021).

Macrolobium is morphologically and ecologically diverse, with its largest center of diversification in the Negro river basin (Ducke, 1941). Species of Macrolobium are found mainly in scrublands and lowland rain forests, seasonally flooded forests and white-sand vegetation (Ducke, 1941; Cowan, 1953). In the Amazon basin, Macrolobium is an important forest element, being one of the most species diverse plant lineages with 55 species currently recorded (Cardoso et al., 2017), some of which are considered as hyperdominant trees (ter Steege et al., 2013) and as a source of food for the Amazonian fauna (Barnett et al., 2015). In Brazil, the genus is represented by 37 species and 13 varieties, of which 15 species are endemic (Morim & Gurgel, 2020). It occurs in both the Amazon and the Atlantic Forest biomes, and is especially well represented in Amazonian forests (35 spp.), with most species growing mainly on oligotrophic soils of white-sand vegetation, and seasonally inundated black-water floodplain forests known as igapó (Cowan, 1953; Kubitzki, 1989; Morim & Gurgel, 2020). In addition to its ecological importance, the genus has been the focus of phytochemical studies, which have identified triterpenes, steroids, flavonoids, and a rare chromone glycoside called macrolobin (Nascimento et al., 2020) in leaves and stems of M. latifolium Vogel, with therapeutic potential for the treatment of oxidative and infectious disorders (Santos-Ferraz et al., 2021).

Since the last taxonomic revision of Macrolobium (Cowan, 1953), many new species have been described (Cowan, 1955, 1957, 1964, 1973, 1977, 1985; Zarucchi, 1990; Barneby, 1992; Farroñay et al., 2018). New records for countries (Félix-da-Silva et al., 2016a; Aymard & Romero-Gonzáles, 2021) and local taxonomic studies (Félix-da-Silva et al., 2013; Félix-da-Silva et al., 2016b) have also been published. Regardless, the genus still requires more detailed and comprehensive taxonomic studies. During a review of Amazonian Macrolobium species, we found a few specimens that fall outside the morphological species definitions under which they were identified. Because of their unique combination of morphological characters, these specimens are jointly recognized here as a taxonomic novelty at the species rank, which is described, illustrated, and compared with other morphologically similar species. A distribution map and comments on the preliminary conservation status of the species are also provided.

Materials and Methods

The description of the new species is based on morphological characters from specimens of Macrolobium deposited at F, IAC, IAN, INPA, MO, NY, and US herbaria (acronyms follow Thiers, 2022). We also carried out fieldwork in June 2022 in the Manicoré river basin, Amazonas state, Brazil, where we made new collections of fruiting individuals of this new species. Specimens were analyzed with a dissecting stereomicroscope and morphological characters measured with digital calipers. In the species description, measurements are represented by the range of minimum and maximum values, followed in brackets by the average and standard deviation. Specific morphological terms follow Cowan (1953) and Barneby (1992). For the habitat description, we used the terminology of Junk & Piedade (2010) and Adeney et al. (2016). Field photographs were taken using a Nikon D5300 camera, and photographs of dry structures were taken with a Leica M205 C stereomicroscope.

We used georeferenced data from the cited collections as input to GEOCAT (Bachman et al., 2011) to calculate the extent of occurrence (EOO) and area of occupancy (AOO) of the species. The preliminary conservation status is based on the IUCN guidelines and criteria (IUCN, 2012; IUCN Standards and Petitions Committee, 2019). The map was generated in the software R (R Core Team, 2018).

Taxonomic Treatment

Macrolobium paulobocae Farroñay, sp. nov.—Type: Brazil, Amazonas: Mun. Manicoré, Rio Marmelos, mata de igapó, solo argiloso úmido, 15 Abr 1985 [fl], C. A. C. Ferreira 5558 (holotype: INPA accession 127,022 [!]; isotypes: F accession 2,020,310 [image!], MO accession 3,256,416 [image!], NY barcode 1,162,461 [image!], US barcode 3,181,734 [image!]). (Figs. 1, 2)

Fig. 1
figure 1

Macrolobium paulobocae. A. Fertile branch with fruit. B. Detail of persistent stipule. C. Apex of inflorescence with persistent bracts. D. Adaxial and abaxial surfaces of a bract. E. Flower bud. F. Adaxial and abaxial surfaces of a bracteole. G. Adaxial surface of a sepal. H. Petal. I. Pedicel (u), hypanthium (v), stipe (w), ovary (x), style (y), and stigma (z). J. Detail of ovary with two ovules. Illustrated by Maria Thamiris de Sousa Macedo. [A–B from Vicentini 2920 (INPA); C–E, H from Prance 3671 (INPA); F–G, I–J from Ferreira 5558 (INPA)].

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Fig. 2
figure 2

Macrolobium paulobocae. A. Fertile branch with fruit. B. Persistent stipules (arrows). C–D. Adaxial and abaxial surfaces of leaflets. E. Detail of abaxial surface of leaflet. F. Winged leaf rachis (arrow). G. Fruit. H. Detail of the fruit stipe. [A from Vicentini 2920 (INPA); B–F from Farroñay 1868 (INPA); all photos by Francisco Farroñay].

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Diagnosis.—Macrolobium paulobocae differs from other congeners by the combination of bijugate leaves, leaflets densely pilosulose and punctate abaxially, persistent puberulous stipules, pilosulose inflorescence, and marginally pilosulose ovary. It is morphologically and ecologically most similar to M. molle (Fig. 3), from which it differs by the 2-jugate leaves (vs. 4–10-jugate), persistent stipules (vs. caducous), glabrous filaments (vs. villosulose basally), and oblong ovary (vs. ovate).

Fig. 3
figure 3

Macrolobium molle. A. Fertile branch with fruit. B–C. Adaxial and abaxial surfaces of leaflets. D–E. Close up of adaxial and abaxial surfaces of leaflets. F. Detail of abaxial surface of leaflet. G. Canaliculate leaf rachis (arrow). H. Part of inflorescence and flowers. I. Fruit. J. Detail of the fruit stipe. [A–G, I, J from Farroñay 1834 (INPA); H from Vicentini 2905 (INPA); all photos by Francisco Farroñay].

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Tree 5–15 m tall, 10–20 cm DBH; branchlets densely pilose. Stipules 4–7.6 [5.9 ± 1.2] mm long, persistent, subulate or falcate-linear, acuminate, densely puberulous on both surfaces. Leaves with petiole 2–4.3 [3.2 ± 0.7] mm; slightly canaliculate on the upper surface, the rachis 6–18 [10.8 ± 3.4] mm long, alate, plane, pilosulose; 2-jugate (with two pairs of leaflets, rarely 3-jugate), the leaflets discolorous, upper surface minutely puberulous on the midvein, lower surface densely pilosulose and punctate, midvein slightly impressed above, prominent beneath, margin ciliate, the basal leaflet pair smaller, 1.4–3.6 × 1–2.8 [2.5 ± 0.4 × 1.7± 0.4] cm, orbicular to oval, the base inequilateral, apex truncate to retuse, the upper leaflet pair 2.2–6.1 × 1.1–3.8 [4.5 ± 0.9 × 2.5 ± 0.6] cm, oval to oblong to oblong-obovate, the base inequilateral, apex retuse to emarginate. Inflorescences sessile racemes, 3.7–6.5 [5.2 ± 0.9] cm long, terminal to axillary, the axis densely pilosulose; bracts 2.5–2.6 × 1–1.2 mm, persistent, triangular, glabrous on the inner surface, puberulous externally. Pedicels 4–6.6 [5.6 ± 0.9] mm long, pilosulose. Bracteoles 6.1–8 × 2.5–3 [6.8 ± 0.6 × 2.7 ± 0.2] mm, elliptic to ovate, sparsely pilosulose within, densely pilosulose externally, free at base. Flowers bilaterally symmetrical; hypanthium 1.8–2.4 [2.1 ± 0.2] mm long, glabrous; sepals 4, glabrous on both surfaces, lanceolate, 2–3.2 × 0.7–1.2 [2.6 ± 0.3 × 1 ± 0.1] mm; petal 1, white apically, red basally, the blade 3–3.2 mm long, glabrous, oval, the claw 5–6 mm long, basally auriculate, glabrous; stamens 3, filaments 11–23.2 [17.4 ± 3.9] mm long, glabrous; gynoecium inserted on the bottom of the hypanthium, the stipe 2.3–3.4 mm long, pilosulose, ovary 2.3–2.8 × 0.8–1.2 mm, oblong, 1–2 ovulate, pilosulose marginally, the lateral surfaces glabrous, style 12.2–14.5 mm long, basally sparsely pilosulose, stigma capitellate. Fruit a dehiscent legume, 6–8.2 × 3.1–3.5 [7.1 ± 0.6 × 3.3 ± 0.1] cm, oblong-oblanceolate, glabrescent at the margins, green when mature, the stipe 0.7–1.2 [1 ± 0.1] cm, sparsely pilosulose. Seeds 1–2 per legume, 1.5–1.7 × 1.1–1.4 cm, suborbicular, smooth.

Distribution and habitat.—Macrolobium paulobocae grows in black-water flooded forests (igapó). It is known from only three river basins in the Central Amazon: along the rivers Urubú, Marmelos, and Manicoré, which are tributaries of two major Amazonian rivers, the Urubú of the Amazon River and the other two of the Madeira River (Fig. 4). The new species grows together with other common igapó trees, such as the legumes Campsiandra angustifolia Spruce ex Benth., Uleanthus erythrinoides Harms, Macrolobium acaciifolium (Benth.) Benth., and M. angustifolium (Benth.) R.S.Cowan, as well as the Apocynaceae species Spongiosperma grandiflorum (Huber) Zarucchi and Malouetia tamaquarina (Aubl.) A.DC.

Fig. 4
figure 4

Distribution map of Macrolobium paulobocae and morphologically similar species.

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Phenology.—Flowering specimens have been collected in April, September, and December along both the Urubú and Marmelos rivers. Fruits were collected in June on the Manicoré River. Both phenophases coincide with the peaks of flowering and fruiting in igapó forests, which in turn are strongly tied to the annual flood pulses of Amazonian river systems (Parolin & Wittmann, 2010).

Preliminary conservation status.—Macrolobium paulobocae is known from eight specimens and three locations, and has an estimated extent of occurrence (EOO) of 1682 km2 and a minimal area of occupancy (AOO) of 12 km2. As such, following the IUCN B1ab(ii,iv) criteria (IUCN, 2012; IUCN Standard and Petitions Committee, 2019), we suggest that the new species should be assessed as Endangered (EN). Populations in the municipality of Manicoré are seriously threatened by various extractive activities (Coelho et al., 2022).

Etymology.—It is a great pleasure to dedicate this new species to the Brazilian botanist Paulo Apóstolo Costa Lima Assunção (1956–2021), better known in the botanical world as “Paulo Boca”, a nick-name he received during his time at the Biological Dynamics of Forest Fragments Project (BDFFP) in the late 1980s and 1990s, where he became an expert in tropical plant identification while conducting plot census work. Paulo was an excellent botanist, collector, and an expert of the Amazonian flora, especially on the family Lecythidaceae, as he worked closely with Scott A. Mori (NY) in the census of a 100-ha Lecythidaceae plot in the late 1980s (Mori & Lepsch-Cunha, 1995), which he revisited again as his last main contribution to the study of Amazonian forests (Milton et al., 2022). Paulo co-authored the Flora of the Ducke Reserve (Ribeiro et al., 1999), one of the most important books for the identification of the Amazonian flora, and also taught and trained countless parataxonomists, botanists, and ecologists. He passed away in January 2021 at the age of 65, a victim of the COVID-19 pandemic.

Additional specimens examined.—BRAZIL. Amazonas: Mun. Itacoatiara, Rio Urubú, ao longo do rio, 13 Sep 1949 [fl], R. L. Fróes 25,205 (IAC no. 14754, IAN no. 51546, NY no. 1162464); Rio Urubú, 19 Sep 1949 [fl], R. L. Fróes 25,309 (IAC no. 14833, IAN no. 51623, NY no. 1162465); Rio Urubú, banks of rio, 15 Dec 1966 [fl], G. T. Prance et al. 3671 (INPA no. 19257, NY no. 1162466, US no. 2579328); Rio Manicoré, floresta de igapó, 09 Jun 2022 [st, st, fr], F. Farroñay et al. 1866, 1867, 1868 (INPA); Rio Manicoré, mata de igapó, 13 Jun 2022 [fr], A. Vicentini et al. 2920 (INPA).

Discussion

Macrolobium paulobocae is morphologically most similar to species of section Macrolobium (sensu Cowan, 1953) because of its cupuliform hypanthium, flowers with 4 sepals, fully opening bracetoles, and clawed petal. The new species falls unambiguously into Cowan’s (1964) ‘series 1b’ of his identification key to the series of Macrolobium, which groups species with 2–4 jugate leaves with sessile leaflets and small bracts. Within ‘series 1b’, M. paulobocae is morphologically similar to M. microcalyx Ducke (Fig. 5) and M. montanum Ducke by the shape and number of leaflets, and to M. molle (Benth.) R.S.Cowan (Fig. 3) in having the abaxial leaflet surface pilosulose and the inflorescence axis pilosusole. However, it differs from all of them in the combination of persistent puberulous stipules, abaxially pilosulose and punctate leaflets, and pilosulose inflorescences (for full comparison of these species, see Table 1 and Fig. 6). Persistent stipules are also a significant taxonomic character for M. huberianum Ducke and M. pendulum Willd. ex Vogel, but M. paulobocae differs from those species by its usually bijugate leaves (vs. multijugate in M. huberianum and unijugate in M. pendulum).

Fig. 5
figure 5

Macrolobium microcalyx. A. Fertile branch with inflorescences. B–C. Adaxial and abaxial surfaces of leaflets. D. Detail of abaxial surface of leaflet. E. Canaliculate leaf rachis. F. Inflorescence apex with one open flower. G. Hirsute ovary. [A–G from Farroñay 1741 (INPA); all photos by Francisco Farroñay].

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Table 1 Morphological and ecological comparison of Macrolobium paulobocae and similar species.*
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Fig. 6
figure 6

Comparison between Macrolobium paulobocae (A–C) and its morphologically similar species M. microcalyx (D–F) and M. molle (G–I). A, D, G. Leaflets. B, E, H. Detail of abaxial surface. C, F, I. Flower structures: pedicel (x), hypanthium (y), and ovary (z). [A, B from Farroñay 1866 (INPA); C from Prance 3671 (INPA); D–F from Farroñay 1741 (INPA); G–I from Vicentini 2905 (INPA)]

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Most of the specimens cited here as Macrolobium paulobocae were previously identified either as M. microcalyx, M. molle, or M. montanum. The first two collections of M. paulobocae were made 73 years ago by the Brazilian botanist Ricardo de Lemos Fróes (R. L. Fróes 25,205; R. L. Fróes 25,309) along the Urubú River. Specimens from these collections were annotated in 1953 by Richard S. Cowan as M. montanum var. montanum, although he did not cite them in his revision of Macrolobium (Cowan, 1953). Moreover, a duplicate sheet of Fróes 25,205 deposited at NY was annotated by him as “var. nov?” and “flowering material is necessary”. Another collection from the same area (G. T. Prance 3671) was also identified as M. montanum by Cowan, but with the annotation “pubescent ovary, etc.”. This specimen also has only one leaf with 3 pairs of leaflets.

Macrolobium paulobocae grows sympatrically and syntopically (same habitat and place) with M. molle on the Manicoré River. It differs morphologically from that species by having 2-jugate (vs. 4–10-jugate) leaves, persistent (vs. caducous) stipules, glabrous (vs. basally villosulose) filaments, and an oblong (vs. ovate) ovary. Despite the coexistence of these two closely related species, we consider M. paulobocae to be sufficiently distinct in morphological and ecological traits to be deserving of specific recognition.