Chromolaena congesta (Hooker & Arnott) King & Robinson (1970: 200)

Christ, Anderson Luiz, Nakajima, Jimi Naoki & Ritter, Mara Rejane, 2025, Towards a better understanding of the Chromolaena congesta complex (Asteraceae, Eupatorieae): updated synopsis with taxonomic novelties, Phytotaxa 689 (2), pp. 163-184 : 167-169

3. Chromolaena congesta (Hooker & Arnott) King & Robinson (1970: 200) . Eupatorium congestum Hooker & Arnott (1836: 239) .

Type :— URUGUAY. s.l., s.d., J. Tweedie s.n. (lectotype: E00385502 !, designated by Freire & Ariza Espinar (2014: 339); isolectotypes: E00385501 !, GH00007606 !, K0004868261 ) .

( Figs. 1G–I , 3G–I )

Distribution: — Brazil (Paraná, Rio Grande do Sul, Santa Catarina and São Paulo states) and Uruguay (Lavalleja, Maldonado, Rivera and Tacuarembó provinces). Previous uncertainties about the occurrence of C. congesta in Argentina and Paraguay were solved in the recent research, as no specimens from these countries were found during herbaria revisions. The geographical distribution model produced by Christ et al. (2023b) suggested suitable habitats in Argentina (Misiones province), but further fieldwork needs to be conducted to confirm if there are populations of C. congesta there. Furthermore, contrary to Christ & Rebouças (2020), this species was not found to occur in the Brazilian states of Mato Grosso do Sul and Minas Gerais. This species is common both in highland grasslands of the Mata Atlântica domain in Southern Brazil and in dry grasslands of the Pampa domain in the Rio Grande do Sul state and Uruguay.

Flowering period: —Flowers mostly from February to April.

Conservation status: —Least Concern (LC). The value of AOO suggests a classification of C. congesta as Endangered but given that more than five populations are currently known and that no extreme fluctuations in EOO, AOO or number of individuals per population are known to be taking place, we decided to classify this species in the LC category. Chromolaena congesta is common throughout its range, and most populations are composed of numerous mature individuals. Despite that, we believe that habitat loss and fragmentation of grassland environments in southern Brazil could potentially pose a risk of extinction to many populations, particularly those located outside conservation units, thus reducing this species AOO and elevating the rate of fragmentation of known populations soon.

Comments: — Chromolaena congesta was previously thought to be difficult to circumscribe due to the absence of stable morphological traits, which resulted in its treatment as a highly polymorphic taxon. However, we have recently found that the shape (usually apiculate) and length (shorter than 2.5 mm long.) of the outer phyllaries ( Fig. 1G ) are useful morphological traits to delimitate and identify this species, especially when comparing it to C. elliptica ( Christ et al. 2023a) . Other potentially informative traits are the erect habit ( Fig. 1H ); deltate leaves with truncate or rounded bases and emergent venation on the abaxial surface ( Fig. 1I ); capitula with 7–13 florets; and cypselae almost always glandular and setulliferous (e.g. Fig. 2C ). The most similar species to C. congesta is C. diversifolia , from which it differs principally through the indument of stems (usually strigose to puberulous in C. congesta vs. sericeous in C. diversifolia ) ( Fig. 3G ) and leaves (mostly strigose on the adaxial surface and strigose to tomentose on the abaxial surface in C. congesta vs. sericeous on the adaxial surface and lanate on the abaxial surface in C. diversifolia ) ( Fig. 3H–I ). Contrary to what was reported by Christ & Rebouças (2020), this species does not have glabrescent leaves and the outer phyllaries are only rarely longer than 2 mm long.

Through herbaria revisions and fieldwork expeditions, we observed a set of populations from the Brazilian states of Paraná and Santa Catarina with apparently intermediary traits between C. ascendens and C. congesta , i.e., typical villose indument of C. ascendens on the stems and abaxial surface of the leaves, but with involucres, number of florets, size of pappus and cypselae more akin to C. congesta . Our morphometric survey indicated that many of these populations are indeed more similar to this species than to C. ascendens ( Christ et al. 2023a) . Also, considering our SDM analysis ( Christ et al. 2023b) indicated few suitable habitats in southern Brazil for C. ascendens (which is known to occur only in southeastern Brazil; see discussion above), we decided to treat these atypical populations as anomalous individuals of C. congesta until further studies can be conducted to better clarify their identity and status.

Despite authors such as Cabrera et al. (1996), Freire & Ariza Espinar (2014) and Perez (2019) treating this species as a synonym of C. squarrulosa ( Table 1), our studies with integrative taxonomy support the recognition of C. congesta as a separate species. These studies also pointed to an apparent proximity of this species to C. elliptica , particularly due to the existence of intermediary populations, which should be further studied in the future. Our studies also failed to support a particularly close relationship between C. congesta and C. hirsuta , which was suggested by Matzenbacher (1979) and Cabrera & Klein (1989), who treated the latter as a variety of the former.

Selected specimens: — BRAZIL. Paraná: Balsa Nova , Ponte dos Arcos, 25°31’44”S, 49°43’54”W, 02 April 2019, A. L. Christ 638 ( ICN) ; Curitiba, Jardim Botânico , 28 February 2002, J. Cordeiro 2031 ( FURB) ; Jaguariaíva , PR-151, 24°21’17”S, 49°48’22”W, 05 April 2019, A. L. Christ 682 ( ICN) ; Tibagi, Parque Estadual do Guartelá, 21 February 1995, J. R. Stehmann 2255 ( BHCB). Rio Grande do Sul: Cambará do Sul , 29°0’11.9”S, 50°17’17.2”W, 01 March 2010 E. K. O. Hattori 1108 ( BHCB) ; Caxias do Sul, Criúva , - 28°58’06.7”S, - 50°59’32.3”W, 13 April 2013, J. Gaio 338 ( HUCS) ; Erechim , BR-153, 25 January 1995, A. Butzke s.n. ( HUCS 11488 ) ; Giruá, Granja Sodal , March 1964, K. Hagelund 1790 ( ICN) ; Jaquirana, Parque Estadual do Tainhas , 29°05’39”S, 50°21’55”W, 04 March 2018, A. L. Christ 503 ( ICN) ; Porto Alegre, Morro Santana , 30°03’31”S, 51°07’29”W, 21 March 2017, A. L. Christ 394 ( ICN) ; Santa Maria , CISM, 29°44’17.8”S, 53°49’42.6”W, 14 April 2015, A. L. Christ 104 ( SMDB) ; Santana do Livramento, Cerro Palomas , 30°49’29.8”S, 55°21’00.7”W, 23 February 2010, R. Záchia 6037b ( SMDB) ; Vacaria, BR-116, 28°18’50”S, 50°48’12”W, 28 February 2020, A. L. Christ 709 ( ICN). Santa Catarina: Bom Retiro, Campos Novos do Sul , 09 March 2005, G. Hatschbach 78958 ( FURB) ; Capão Alto , 28°03’43.1”S, 50°35’16.7”W, 07 March 2019, A. L. Christ 574 ( ICN) ; Garuva, Alto Quiriri , 20 February 2015, L. A. Funez 3820 ( FURB) ; Painel , 28°04’45”S, 50°07’29”W, 28 February 2020, A. L. Christ 703 ( ICN) . URUGUAY. Lavalleja: Sierra de la Lorencita, 33º54’27’’S, 54º45’15’’W, 26 March 1996, D. Bayce s.n. ( MVFA 25509 ). Maldonado: Aiguá, Cerro de la Salamanca , 26 March 1939, Rosengurtt B-2900 (US) . Rivera: Rivera, 21 March 1964, T. M. Pedersen 13877 ( MBM). Tacuarembó : Tacuarembó , March 1944, J. Chebataroff 10141 ( LP) .

Figures

FIGURE 1. A–C. Chromolaena ascendens. A. Detail of stem. B. Habit, stem and leaves. C. Primary capitulescence. D–F. Chromolaena caaguazuensis. D. Stem and leaves. E. Detail of adaxial surface of a leaf. F. Capitulum. G–I. Chromolaena congesta. G. Capitulum. H. Growing habit. I. Details of leaves. J–L. Chromolaena elliptica. J. Growing habit. K. Leaves. L. Capitulum. Scale bar of A = 5 mm; scale bars of G and I = 2 mm. Image credits: Anderson Christ (A, G-H, K-L), Cauê Paiva Vidigal Martins (B-C), Camilo Perez (D–F), Claudenice Dalastra (J) and Sérgio Bordignon (I).

FIGURE 2. Representative cypselae of species of the C. congesta complex portraying morphological variation in diagnostic traits. A. glabrous cypsela (i.e. setae completely or nearly completely absent). B. setulliferous cypsela (i.e. setae visibly present on the ribs and/on between them). C. glandular cypsela (i.e. glandular hairs located at the tips of the arrowheads). Scale bars: 2 mm. Further details on the structure of the cypselae of this species complex are described in Queiroz et al. (2025).

FIGURE 3.A–F. Chromolaena diversifolia. A. Specimen growing in its natural habitat. B. Detail of the fertile part of the plant. C. Details of the stem and leaves. D. Sericeous indument of the stem. E. Sericeous indument of the adaxial surface of a leaf. F. Lanate indument of the adaxial surface of a leaf. G–I. Details of morphological traits of C. congesta, for comparison with C. diversifolia. G. Strigose indument of the stem. H. Strigose indument of the adaxial surface of a leaf. I. Strigose indument of the abaxial surface of a leaf. Scale bars of D and G = 2 mm; scale bars of E, F, H and I = 5 mm. Image credits: Claudenice Dalastra (A–C) and Anderson Christ (D–I).