A new species of dragon’s blood Croton (Euphorbiaceae) from Serra dos Órgãos (Rio de Janeiro, Brazil)

Abstract Crotonrizzinii Farias & Riina, sp. nov. is a new species from Serra dos Órgãos National Park in the Atlantic Rain Forest domain (Rio de Janeiro state, Brazil). It is known from the municipalities of Guapimirim, Teresópolis and Petrópolis, where it grows in montane ombrophilous dense forest, between 500 and 1500 m elevation. This arborescent species belongs to CrotonsectionCyclostigma Griseb., a Neotropical lineage distributed in forest habitats from Mexico to northern Argentina. It is mainly characterised by its laciniate-glandular stipules, bracts with two inconspicuous glands (colleters) at the base and campanulate pistillate flowers with sepals covering the ovary. We describe and illustrate the new species, and compare it with close relatives occurring in the Atlantic Rain Forest. We also provide a distribution map, habitat information and suggestions for the assessment of its conservation status.


Introduction
The Atlantic Rain Forest (ARF) is the richest Brazilian domain for flowering plants (BFG 2018) and one of the world hotspots of biodiversity (Myers et al. 2000). Not surprisingly, the megadiverse genus Croton L., with around 1200 spp. (Berry et al. 2005), is well represented in this region. Of the 305 species of Croton known for Brazil, 96 occur in the ARF (Flora do Brasil in prep), of which 21 species are trees (Santos et al. 2017). Amongst these arborescent lineages, C. sect. Cyclostigma is the most diverse in the ARF, having nine species (Santos et al. 2017).
While working on Croton for the Flora do Brasil project, we came across several specimens from Serra dos Órgãos that we could not place to species, all of which were collected within the limits of the eponymous national park (PARNASO). These specimens were from small trees and detailed examination of their morphology showed a clear affinity to species in section Cyclostigma.
Croton section Cyclostigma, which includes species commonly known as dragon's blood, comprises 46 species distributed from Mexico to northern Argentina (Feio et al. 2018a). The new species, described here, increases the total number in the section to 47 and to ten species in the ARF. This section is characterised by fast-growing trees, generally occurring in secondary vegetation, roadsides, river banks and landslide areas of wet forests (Riina et al. 2009), but there are a few species also occurring in dry forests (Riina et al. 2007, Feio et al. 2018a). The common name, dragon's blood, comes from the presence of abundant and often reddish latex, which is used by indigenous and rural populations for wound-healing, preventing infections and other ailments (Ubillas et al. 1994, Jones 2003, Smith 2006. The Serra dos Órgãos mountain range is part of the Serra do Mar mountain system in Rio de Janeiro state. The PARNASO was created in 1939 and it covers the municipalities of Guapimirim, Magé, Petrópolis and Teresópolis (Cronemberger and Castro 2007). Most of the park's vegetation consists of dense ombrophilous forests, but there are also sparse areas with open vegetation or "campos de altitude" (Veloso et al. 1991). The great elevational gradient (ca. 100 to 2285 m) in a relatively small area has been suggested as the main factor in the formation of this unique and highly diverse environment (Cronemberger and Castro 2007).
Our study contributes to the botanical knowledge of the ARF region by describing a new tree species, Croton rizzinii. We provide a complete morphological description, detailed illustrations and a distribution map. We also compare the new taxon with the morphologically most similar species in section Cyclostigma occurring in the ARF area.

Materials and methods
We carried out fieldwork between 2015 and 2017 in different areas of the PARNASO, focusing on Croton species. Species were randomly sampled at each locality and the in-dividuals were revisited during the following two years after the initial collections. The description of the new species was based on the analysis of dry plant material deposited in herbaria, including recently collected specimens, as well as field observations. The comparison with related species was undertaken using specimens from the herbaria R and RB, as well as photographs of specimens, including types, from herbaria F, G, K, NY, P, US and W (acronyms follow Index Herbariorum, available at https://sweetgum.nybg.org/science/ih/). We also used protologues of Croton species known from the ARF and recent floristic studies, particularly that of Santos et al. (2017). Morphological terms follow Hickey and King (2000) and Radford et al. (1974) for leaf morphology, Webster et al. (1996) and Feio et al. (2018b) for trichomes and Vitarelli et al. (2015) and Feio et al. (2016) for secretory structures. The classification of habitat (vegetation type) follows Veloso et al. (1991). The distribution map was produced using the software QGIS version 2.14 (QGIS Development Team 2019), based on geographic coordinates obtained during the collection expeditions and from herbarium specimens. Our suggestion for conservation status was based on field observations, IUCN Red List Categories and Criteria (IUCN 2017) and geospatial conservation assessment (GeoCAT) (Bachman et al. 2011). The extension of occurrence (EOO) was calculated using the GeoCAT online tool (http://geocat.kew.org).
Phenology. The new species has been collected in flower in January, March, April, June, November and December. The flowering peak is in December and January, with fruits between January and April.
Etymology. The specific epithet is given in homage to Carlos Toledo Rizzini, a Brazilian botanist, for his dedication to the study of the flora of the Serra dos Órgãos mountain range.
Conservation status. Croton rizzinii is known from a few collections in three municipalities of the PARNASO. It has an estimated Extent of Occurrence (EOO) of 19,653 km 2 . Although the species occurs within a conservation unit, it suffers considerably from anthropic pressure caused by tourism, urbanisation, agriculture and grazing. Based on our data, Croton rizzinii could be assessed either as Critically Endangered (CR B1b[iii]) or Data Deficient (DD). The latter category is based on the lack of sufficient information regarding the distribution and size of populations.
Discussion. Croton rizzinii can be recognised in the field by its yellowish latex, laciniate-glandular stipules that are cream to green-yellowish in colour, bracts with two inconspicuous glands (colleters) at the base, campanulate pistillate flowers with sepals covering the entire ovary and the presence of maculate glands on the adaxial surface. We are assigning the species to Croton sect. Cyclostigma (Van Ee et al. 2011) due to its arborescent habit, yellowish latex, stellate trichomes, triplinerved to palmate venation, acropetiolar glands, basal bisexual cymules, pistillate flowers with vestigial petals and staminate flowers with numerous stamens. However, further testing using molecular characters would be desirable to confirm its phylogenetic position. The finding of a new species in this section in a relatively well explored area is surprising and shows that botanical exploration continues to be relevant in highly diverse Neotropical areas and especially in biodiversity hotspots (Myers et al. 2000, Joppa et al. 2011 regardless of their current level of floristic inventory and the taxonomic knowledge of the focal group (Riina et al. 2018).
Croton rizzinii can be distinguished from other Brazilian Cyclostigma species by its laciniate-glandular stipules, bracts with glands (colleters) at the base, sepals of the pistillate flowers with glands (colleters) at the apex and, sometimes, macular glands on the adaxial surface. Another character distinguishing it from its most similar species in the ARF is the size of seeds, which are the smallest in size amongst them (Table 1). Amongst the ARF Cyclostigma species, Croton rizzinii appears to be most similar to C. celtidifolius (Santos et al. 2017;Caruzo and Cordeiro 2007). Both species occur sympatrically and syntopically and can be confused by their similar ovate to cordate and pubescent leaves, trinerved at the base to palminerved venation, short-stipitate to stipitate petiolar glands on the abaxial side of the petiole and subglobose fruits. However, they can be readily distinguished by several characters, mainly by those related to latex colour, type of trichomes on branches, number of acropetiolar glands, stipules, bracteoles, stamen number and pistillate flowers (see Table 1 and identification key). Due to their resemblance, specimens of C. rizzinii have often misidentified as C. celtidifolius in herbaria.
Croton rizzinii is also similar to C. vulnerarius Baill. and C. alchorneicarpus Croizat, mostly regarding the young pistillate flowers. Croton vulnerarius and C. rizzinii occur sympatrically within PARNASO, but so far, they have not been found in the same locality. These species share an arborescent habit, pubescent leaves, conspicuous stipules, sessile to subsessile campanulate pistillate flowers, valvate to slightly imbricate sepals and styles connate at the base. Nevertheless, they can be distinguished by several characters (see Table 1 and identification key). In relation to C. alchorneicarpus, both species present ovate to cordate leaves, trinerved at the base to palminerved venation, conspicuous stipules, sessile to subsessile pistillate flowers with styles connate at the base, rotate staminate flowers and subglobose fruits. However, they too can be separated by several vegetative and reproductive characters (see Table 1 and identification key). The overall aspect of the plant and the floral morphology are more similar to C. alchorneicarpus, but the stipules are more similar to those of C. rizzinii (laciniate-glandular). Further studies and additional sampling of Croton trees from the ARF are needed to determine the identity of these specimens.
Croton lagoensis Müll. Arg. is also similar to C. rizzinii, but it occurs in deciduous forest and in transitional areas between the Cerrado and the ARF in the state of Minas Gerais (Santos et al. 2017). Both species have ovate to cordate and pubescent leaves, two acropetiolar glands and campanulate pistillate flowers with sepals covering the en-tire ovary. However, C. rizzinii differs from C. lagoensis mainly by its arborescent habit and other features listed in Table 1 and in the identification key.
The description of C. rizzinii adds to the number of species with laciniate stipules in Croton and in section Cyclostigma in particular. Laciniate stipules are found in seven species in Cyclostigma (C. charaguensis Standl., C. churutensis Riina & Cornejo, C. medusae Müll. Arg., C. perspeciosus Croizat, C. purdiei Müll. Arg., C. rizzinii and C. speciosus Müll. Arg.), but they can be present in other sections of Croton (sect. Adenophylli Griseb., sect. Barhamia (Klotzsch) Baill., sect. Medea (Klotzsch) Pax, amongst others) (Van Ee et al. 2011). However, given the inconsistencies found in Croton taxonomic treatments regarding the terms used to describe stipules, we suggest further studies to standardise this terminology across the genus.
Identification key for Croton rizzinii and morphologically similar species in the ARF