Taxonomic and ecological aspects of Myrceugenia mesomischa ( Myrtaceae ) , an endemic tree from southern Brazil

Myrceugenia mesomischa (Burret) D. Legrand & Kausel (Myrtaceae), a tree species poorly cited in floristic and phytosociological surveys in its area of known occurrence, was recorded as one of the species with the highest importance value in a fragment of Araucaria forest in São Francisco de Paula municipality, Rio Grande do Sul state, Brazil. The species was abundant in the understory strata of the forest, showing the highest frequency and density of the survey. Considering the paucity of data on the species, its degree of rarity and endemism (occurring only in the states of Rio Grande do Sul and Santa Catarina, Brazil), more studies are needed to evaluate the true current state of conservation of populations of this species.

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Legrand & Kausel, Myrtaceae, was described by Burret (1941) as Luma mesomischa based on type material collected by Herter (26250) in São Francisco de Paula, Rio Grande do Sul (RS), Brazil.In the following decade, Legrand (1953) proposed a new taxonomic combination for the species, transferring it to genus Myrceugenia.Some years later, Legrand (1957) published a study of Neotropical Myrceugenia O. Berg, which also cited M. mesomischa.In these three studies, the authors made no reference to the species habit or growth form and the descriptions have been conducted solely through analysis of original type material, which was deposited in the herbarium of Berlin and has probably been destroyed (Sobral 2003).The specific epithet refers to the median length of the petioles when compared to other species described in the same source that show longer petioles (Sobral 2003).
According to Landrum (1981), Myrceugenia presents two species occurring in the Fernandéz Islands, 12 other species in Chile and adjacent Argentina, and a majority of 25 species occurring in eastern South America.Most of these are found in the eastern border of the Brazilian coast and on the plateau (Planalto), from Rio de Janeiro to Porto Alegre.Legrand & Klein This paper has additional data published in its electronic version.Setubal, R.B et al. (1970) studying Myrceugenia in Santa Catarina state (SC), cited 24 native species without mentioning the occurrence of M. mesomischa.The extensive work of Reitz et al. (1978Reitz et al. ( , 1983) ) listed the occurrence of 713 and 515 species of native trees in SC and RS, respectively.In these studies, the significant contribution of Myrtaceae was evidenced by the high number of species obtained for the family, 190 in SC (26.6% of total) and 103 in RS (20%), none citing M. mesomischa.Furthermore, Klein (1984) also highlighted the sociological importance of Myrtaceae in the forests of Brazil. Between 2000 and2003, the RS Continuous Forest Inventory (SEMA/UFSM-RS 2001) reported the occurrence of 469 species of trees and shrubs with perimeter at breast height > 3 cm, without citing M. mesomischa.Sobral (2003), in a biogeographical and taxonomic study of Myrtaceae from RS based on material examined in collection tours and herbaria, made the first citation of species occurrence after its description and new combination, describing it as a shrub up to 3 m tall.The author highlighted its restricted occurrence in the Araucaria forest of RS and SC and in the riparian forests of Serra do Sudeste physiographic region of RS.However, the author did not specifically comment on this record, which was until then unnoticed in other works.Sobral et al. (2006) presented a new review of native tree species of RS with a list of 519 species, also not mentioning M. mesomischa.Soon after, Mattei (2007), in a study involving patterns of landscape ecology, species richness and diversity of the Araucaria forest on the Planalto Médio region (RS), cited the record of an individual tree of M. mesomischa in one of the 16 fragments analyzed.Sonego et al. (2007) presented a structural description of the tree component of an Araucaria forest fragment in the Floresta Nacional de São Francisco de Paula (FLONA), citing M. mesomischa among the five species with the highest importance value in the survey, related to the great number of individuals.More recently, Avila (2010) cited M. mesomischa as one of the main species for the natural regeneration of tree species of the Araucaria forest at FLONA.Jeske-Pieruska et al. (2010) also reported the occurrence of the species in a forest fragment in São José dos Ausentes (RS), a neighboring municipality of São Francisco de Paula.In spite of the importance of these works, they do not provide profound taxonomic and ecological information on M. mesomischa.
The aim of this paper is to present new data on M. mesomischa, obtained from a floristic and phytosociological survey in a fragment of Araucaria forest in São Francisco de Paula, RS.The importance of this work is due to: (i) the addition of new taxonomic and ecological information on M. mesomischa, given the current lack of data available on this species, (ii) provide phytosociological data obtained for the species in the study area; (iii) the enrichment of knowledge about a species with geographic distribution restricted to southern Brazil, rarely mentioned in floristic and phytosociological surveys in its area of known occurrence, besides increasing data about its current conservation status.

Study area
The study was conducted in São Francisco de Paula Municipality, located in Campos de Cima da Serra physiographic region, northeastern RS (Fortes 1959).The predominant lithology of the region is represented by acidic and basic volcanic rocks belonging to the Serra Geral Formation, constituting the Planalto Meridional (White 1908).The landscape is marked by low hills separated by wide valleys extended by successive steps of dissection that left slope ruptures and small differences in topographic levels.The climate in the region is Köppen's Cfb, considered subtropical mesothermic, although temperate climate periods also occur (Nimer 1990).The mean annual temperature is 16º C and average annual rainfall stands at around 1800 mm (São Francisco de Paula 2008).Most of the region has Bruno Cambisol soils, the main features being humic and alic properties, low to median depth, high accumulation of organic matter, sharp acidity and low fertility (São Francisco de Paula 2008).
The study area is located in the western quadrant of the municipality (29°22'33"S 50°30'38"W; altitude max.875 m), accessed by highway RS-235.The area is part of the Guirra hydrographic microbasin, belonging to the Santa Cruz River Basin, tributary of the Caí River.In a study of the vegetation, the RADAMBRASIL project recognized Taxonomic and ecological aspects of Myrceugenia mesomischa three phytoecological formations occurring in the region: Steppe (subdivided into park-steppe and grassy-woody steppe with gallery forest), Araucaria forest and Atlantic Forest, with the former predominating (Teixeira et al. 1986).In the study area, the vegetation was represented by a mosaic composed of grassland (steppe) and Araucaria forest, the first occurring preferentially on the top and upper third of the hills and the second on hillside areas and watercourse margins.The municipality belongs to the Atlantic Forest Biome (IBGE 2004).
In the study area, extensive cattle ranching predominates with frequent entry of cattle inside the forest which promotes trampling and grazing of herbaceous and woody strata at some points.Moreover, the absence of valuable timber species, such as Lauraceae and centenarian individuals of Araucaria angustifolia (Bertol.)Kuntze are evidences of selective logging in the recent past.Despite these disturbances, a large part of the forest is currently in an advanced stage of regeneration, presenting an ample canopy and complex stratification.

Species description
The morphological description of M. mesomischa was based on material collected during fieldwork and also on the review of material deposited in the herbarium collections of ICN, HAS, HUCS, MBM and PACA (Thiers 2010).Terminology is according to Radford (1986), Sobral (2003) and Stearn (1973).

Vegetation survey
The qualitative and quantitative survey of the tree component present in a fragment of Araucaria forest (about 26 ha) was conducted in May 2010.The floristic survey was performed through a visual sampling method (Filgueiras et al. 1994), which consisted of walking through the fragment with the aid of maps.Concurrently, we described the different communities present in the fragment that were recognized by comparative analysis of the parameters relief, drainage, geomorphology, flora and physiognomy.The species were arranged in families according to APG III (2009).
The phytosociological survey was carried out using the plot method, with the sampling units (SUs) measuring 10 × 10 m (Müller-Dombois & Ellenberg 1974).The arrangement of the SUs was done in a systematic preferential way in four transects distributed according to local relief toposequences, covering the following geomorphological compartments: top, slope (upper, middle and bottom) and lowland.The distance between plots was c. 40 m.Two transects had southern exposure and two had southeastern exposure, the predominant directions of the local relief.Each transect contained one SU per geomorphological compartment, that is, four SUs per compartment and 20 SUs in the total survey (total sampled area: 2,000 m 2 ).
Two inclusion levels traditionally used in phytosociological surveys were considered, one comprising individuals with diameter at breast height (DBH) between 5 and 10 cm and other with DBH over 10 cm.This criterium aimed to evaluate structural and compositional differences between the regenerative and canopy strata, allowing the comparison of the data obtained with other similar surveys.Besides collecting phytosociological data from each specimen, standardized sampling sheets were used to collect qualitative information on environmental factors such as relief position, soil moisture and sunlight, seeking to verify possible influence on the composition and distribution of species along the geomorphological compartments, and visual quantitative evaluations of exposed soil, litter, density of shrubby strata and density of lianas (low = 0-25%; medium = 25-50%; high > 50%).For each individual were recorded: scientific name, height (meters) and circumference at breast height (CBHcm), and then calculated the value of DBH according to the formula: CBH/π.Bifurcated individuals had all trunks with DBH > 5 cm measured and then used to calculate the basal area per individual.The phytosociological parameters calculated as descriptors of vegetation were: Ni = number of individuals of the species; Np = number of plots where the species occurred; AF = absolute frequency (%); RF = relative frequency (%); AD = absolute density (%); RD = relative density (%); TBA = total basal area by species (m 2 ); ADo = absolute dominance (%); RDo = relative dominance (%); IVI = Importance Value Index (%).Besides the above-mentioned parameters, the diversity index of Shannon-Wiener (H') and Evenness (J') were also calculated (Krebs 1999).Sampling sufficiency was estimated using the species-area curve (adjusted for logarithmic regression).The recognition of forest structural features was based on analysis of height distribution (intervals of one meter), DBH distribution (intervals of 5 cm) and field observations.Setubal, R.B et al.
The species occurs in Brazil, only in the states of SC and RS (Sobral 2003).Myrceugenia mesomischa is located in Araucaria forests of the Planalto Meridional (SC and RS) and riparian forests (seasonal semideciduous forest) of Serra do Sudeste physiographic region (RS).Landrum (1981) considered Luma mesomischa as a possible earlier name of Myrceugenia cucullata D. Legrand suggesting that the description of Burret (1941) was ambiguous.However, Sobral (2003) did not regard Myrceugenia mesomischa as an ambiguous species, differentiating it from M. cucullata by the presence of the calyx lobes shortly cucullate, never with a tuft of trichomes at the apex; abaxial leaf surface irregularly discolored, grayish or paler-green, secondary nerves evident, generally on the abaxial surface; petioles 2-5 mm, leaf length/ petiole length ratio 16-20:1; pedicels 2-5 mm, petiole length/ pedicel length ratio 0.6-1:1.Myrceugenia cucullata has markedly cucullate calyx lobes , sometimes with a tuft of trichomes at the apex; abaxial surface of the leaves uniformly discolored, grayish or whitish, secondary nerves barely evident; petioles 4-8 mm, leaf length/ petiole length ratio 9-11:1; pedicels 1-2.6 mm, petiole length/ pedicel length ratio 3-4:1.

Família Espécie
Myrceugenia mesomischa had AH = 8.8 m (SD = 2.4 m) and ADBH = 9.9 cm (SD = 6.4 cm) occupying both understory levels (Tab.3).In contrast, A. angustifolia was first in importance in the general survey, registering AH = 18.2 m (SD = 5.1 m) and ADBH = 25.2 cm (SD = 15.5 cm), forming the emergent strata (Fig. 7-8).M. mesomischa showed the highest abundance of the survey, reaching 17% (59 individuals) of the entire sample (A.angustifolia had 9.6%).Of this total, 39 individuals (66.1%) had DBH values from 5 to 10 cm, 15 (25.4%) had values from 10 to 15 cm and only five individuals (8.5%) had values over 15 cm.The M. mesomischa individual that had the highest DBH (49.4 cm) recorded a height of 7 m, while the AH of this SU was 15.4 m.Considering the heights of the species, we observed that 52 individuals (88.1%) had values ranging from 6 to 12 m.In the SU where we recorded the highest species height (16 m) the canopy was 20 to 22 m tall.
The diversity indices were 3.07 nats (H') and 0.81 (J') for the total sample.For the tree component with DBH from 5 to 10 cm the values were 2.63 nats (H') and 0.78 (J') and for all species with DBH over 10 cm the values were 2.96 nats (H') and 0.84 (J').

Discussion
Myrceugenia mesomischa stood out as the species with highest frequency in the general survey, occurring in 80% of the SUs.The species was absent mainly on the plains along the lower third of the slopes where there is greater water accumulation, which results in marshy, waterlogged soils most of the year (hydrophilous communities).In these areas, M. mesomischa was replaced by Myrcia lajeana and Myrceugenia glaucescens occupying structural niches similar to those described for the species.In other environmental conditions, such as well drained to moderately drained areas, the species remained common throughout the environmental gradient variation of the slope profile, occurring successfully in the other described communities.Despite the high RDo values of M. mesomischa, this parameter is influenced by RD, related to the large number of individuals and not by the significant size of its basal area.Among the Myrtaceae species recorded in the study, M. mesomischa corresponded to 35% of individuals of this family whereas second-place Myrcia lajeana obtained 13.9%.Sonego et al. (2007) also reported M. mesomischa as one of the six species with higher IVI among 41 species sampled in one of the remnants of Araucaria forest at FLONA.Even though there was no detailed data about M. mesomischa, the authors also highlighted the species because of its high number of individuals and high frequency, which indicates a regular distribution of these populations in the two study areas.
The structural data obtained for M. mesomischa when compared with A. angustifolia, made explicit the different ecological niches occupied by these species in the study area.While the latter is known to be the main climax species of emergent and canopy strata of the Araucaria forest, M. mesomischa occupied the upper and   The sum of relevant data obtained for M. mesomischa indicates that it is an important understory constituent in the forest fragment evaluated, which has already been reported for other similar Myrtaceae species of RS occurring in the study region.Among the ten species with highest IVI in the general survey, five were representatives of Myrtaceae, while in DBH values ranging from 5 to 10 cm there were six representatives.Klein (1984)   altitudes 900 and 1200 m, where some species reach values of frequency between 20 to 30% of the communities.Of the 52 tree species of the family listed by the author for the region, 23 were described as exclusive to the Araucaria forest (44.2%).Therefore, this forest took second place in species richness and diversity of this family in the RS, only behind the Atlantic Forest with 41 species, summing 28 exclusive species (68.3%).On the other hand, in the Serra do Sudeste region, of the 14 species found by the author, none is exclusive, which remains true due to the occurrence of M. mesomischa in both regions mentioned.
The recognized importance of the family in the understory colonization of Araucaria forest and the large number of individuals suggests an evolutionary success of this group in the occupation of this ecological niche.One reason that may be linked to the success of dispersal of these species is the berry fruit type that attracts numerous agents of the fauna.In the case of M. mesomischa, successful colonization can be further enhanced by fruiting between September and November, a period of the year when virtually no other zoochoric species are fruiting in the Araucaria forest (Seger 2010).Also according to the author, since the seeds of the species have a membranous testa and do not show dormancy, seed germination can occur soon after dispersal or even inside the fruit.The period of the year also supports this feature, being the optimal time for germination and for seedling development and establishment.Avila (2010), evaluating natural regeneration mechanisms of the Araucaria forest in the FLONA, highlighted M. mesomischa as one of the main species forming the local seedling bank.However, the confirmation of these strategies still needs more accurate evaluation of the dispersal and colonization strategies displayed by the species.
Regardless the fact that Landrum (1981) cited M. mesomischa as an ambiguous species, we believe that according to the material examined, M. mesomischa is clearly different from M. cucculata (for more details see Species description section and Sobral 2003).The recognition of the tree habit in M. mesomischa brings to 517 the number of native tree species occurring in the RS by adding it to the 516 species already described by Sobral et al. (2006).Moreover, we are also providing a detailed description of fruit morphology and fruiting phenology according to recent data verified in Seger (2010).Nevertheless, future studies should check other taxonomic important aspects of M. mesomischa, for example, the possible need for experts to propose a lectotype or a neotype for this species.
Despite the high number of individuals founded in this study, similar to that reported by Sonego et al. (2007), M. mesomischa seems to be a rare occurrence in the Araucaria forest and even in the Serra do Sudeste region since few studies recorded it.This fact deserves attention because besides its possible natural rarity, there is a recognized level of destruction and fragmentation of its habitat, which shows the need to reevaluate the species' current, delicate conservation status.

Figure 1 -
Figure1-Myrceugenia mesomischa -a.abaxial surface of the leaf blade with hyalin glands, mucronulate apex and fruit in initial phase of maturation; b. adaxial surface of leaves with grooved central nerve; c,d.ripe fruits (squares of the scale 0.5 cm); e. branches with leaves presenting dark green adaxial surfaces; f. fruits in final phase of maturation; g. flower; h.abaxial surface of leaves with conspicuous venation.

Figure 3
Figure 3 -a.Myrceugenia mesomischa with dense, dark-green foliage; b. fragment of Araucaria forest studied; c. subxerophilous community; d. mesophilous community; e. hygrophilous community; f. hydrophilous community.All communities recorded in the phytosociological survey of an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.

Figure 5 -
Figure 5 -Histogram of heights of the tree component recorded in the phytosociological survey of an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.

Figure 4 -
Figure 4 -Species-area curve registered in the phytosociological survey of an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.
Figure 7 -Comparative histogram of heights of Myrceugenia mesomischa (59 individuals) and Araucaria angustifolia (43 individuals) evaluated in the phytosociological survey of an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.

Figure 6 -
Figure 6 -Histogram of DBH classes of the tree component recorded in the phytosociological survey of an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.

Table 1 -
Floristic list of tree species found in an Araucaria forest fragment, São Francisco de Paula, RS, Brazil.

Table 2 -
Description of four tree communities according to their characteristics of topography, drainage, geomorphology and indicator species registered in an Araucaria forest fragment, in São Francisco de Paula city, RS, Brazil.

Table 3 -
Description of vertical strata through their height intervals and common species found in an Araucaria forest fragment, in São Francisco de Paula city, RS, Brazil.