Leucaena leucocephala ( Lam . ) de Wit in Brazil : history of an invasive plant

Recebido em: 07 mai. 2019. Aceito em: 11 set. 2019. Publicado em: 22 abr. 2020. ABSTRACT: The purpose of this text is to provide a long-term record of occurrences of the species Leucaena leucocephala (Lam.) de Wit. in the Brazilian territory, in order to (i) determine if its introduction occurred before the earliest official record, (ii) identify points of occurrence in Brazil, and (iii) assess its status as a useful although invasive plant. The text is partly based on research done on the confirmed presence of this plant in the Brasília National Park, located in Brazil’s Federal District. Three databases available on the Internet containing information from several herbariums were accessed to obtain the location and the dates of the relevant records of the plant in Brazilian territory. We found that the species was rather widely present in the Brazilian territory before the official record of its earliest introduction. In addition, we found that the plant’s current geographic distribution indicates that it continues to have a strong invasive potential in Brazil, especially because there are social and technical incentives to cultivate it in rural properties.


Introduction
The species Leucaena leucocephala (Lam.) de Wit. is native to the current territory of Mexico.
The exact date and the circumstances of its arrival in Brazil are unknown. Vilela and Pedreira (1976) reported that the earliest introduction occurred In this article we try to answer the following question: Are there records of the presence of the species in the Brazilian territory prior to 1940?
We sought to answer this question in order to aid the understanding of its invasive traits and its current wide distribution throughout the Brazilian territory. We used records of the occurrence of the species present in three databases, which

Exotic and invasive species as a topic for environmental historians.
The study of exotic or invasive species (plants, animals and microorganisms) is a relatively recent, but challenging and controversial field for ecologists, conservationists, geneticists, public health researchers, biogeographers and environmental historians. The standard initial statement on the subject was made in the 1950s by an ecologist, Charles Elton (ELTON, 2000[1958). Understanding the role of human societies in the deliberate or accidental movement of organisms beyond their natural range reveals important dimensions of the interplay between human culture and the natural environment and helps understand the myriads of consequences ("good" or "bad", desired and undesired) of these events. Our study of Leucaena seeks to make a contribution to this field, combining the precepts of botanical research and classification with the perspective of environmental history (which focuses on the interplay between human culture and nature). The Leucaena case is propitious because the "good" and desired effects of this deliberately introduced plant go hand in hand with its "bad" and undesired effects.
In the cases of deliberate introductions or transfers of exotic plants and animals, humans have been motivated by a long list of needs and values. The past tense used in the following paragraphs could be substituted appropriately by the present tense, because introductions or transfers of organisms to and between societies, regions, countries and continents continue to be common and are expected to increase rapidly on a global scale (SIMBERLOFF, 2013). We will not discuss cases of unintentional introductions and transfers, even though they too may be highly relevant for environmental history research. have sometimes led to the establishment of populations of transferred organisms intended to control these "pests". In many cases these new organisms engaged in behaviors that created other types of problems affecting human interests and the natural environments into which they were introduced as "solutions" (CROSBY, 1973(CROSBY, , 1986TODD, 2001;LOW, 2001;BURDICK, 2005;JOHNSON, 2010;MORETTO, 2017).

Ranking environmental historians have dealt
with intentionally transferred organisms. Alfred Crosby (CROSBY, 1973, particularly in chapters 3 and 5; see also Crosby, 1986)   In an influential methodological paper aimed at environmental history practitioners, Donald Worster (WORSTER, 1990, p. 1090-1091 proposed three analytical "levels" for the emerging discipline: (i) the "structure and distribution of natural environments of the past"; (ii) the "productive technology as it interacts with the [natural] environment"; and (iii) the "perceptions, ethics, laws and myths" generated by the human understanding of natural settings.
Although Worster did not place special emphasis on organisms transferred from one natural environment to another, the three proposed analytical levels are neatly relevant to their study.
First, these organisms will always originate in a native setting of their own, which must be properly understood by the historian in order to understand their "native" ecological traits and evaluate the new traits that they may adopt in the distinct natural setting into which they are moved. Second, Worster opens this text with a short allusion to an episode involving a transferred organism -in this case it was an unintentional introduction (p.

The species Leucaena leucocephala
The genus Leucaena belongs to the family Leguminosae, subfamily Mimosoidae, and Mimosae tribe. It comprises 22 species, 4 subspecies, 2 varieties, and 2 hybrid taxa (HUGHES, 1998). They include individuals with shrub and tree sizes, varying between 5 and 18 m in height.
They are perennial, fast growing, and adapted to tropical dry regions (BREWBAKER, 1978). They require average temperatures between 25º and 30º C for optimal growth (HUGHES, 1998).
Before spreading around the planet, the genus was domesticated in its area of origin (EVANS, 1993), in the current territory of Mexico. It is native to the region of Chiapas and the Yucatan peninsula Within the Leucaena genus, Leucaena leucocephala is the best-known species, because it is widespread and useful to humans.

Morphological differences between subspecies
were noticed in early agronomic evaluations. The species was classified into three major types: (i) the "Common" type is an intensely branched shrub, has slow growth, high seed production, and invasive potential; (ii) the "Giant" type has an arboreal size, can reach a height of up to 20 m, grows vigorously and produces much wood and fodder; (iii) the "Peru" type is erect and has many branches, with a height of up to 15 m; it produces much forage with high protein content.  In Brazil, the species has been regularly used over the last decades for economic purposes in many areas. It is cultivated especially as a forage plant (SOUSA, 2001;COSTA, N. et al., 2004).
Cattle, sheep and goats (SOUSA, 2001) and wild native animals such as the paca (Cuniculus paca) appreciate its foliage (MATTOS, 2015). It may be offered to animals young or mature, green, dry or ensiled (NAS, 1977). The species is currently found, cultivated or not, in Brazil's five official geographical regions and in almost all Brazilian states, since it grows well in both the northeastern warm and semiarid region and the colder and humid southern areas (LIMA, 2005).
However, its appraisal as an important forage tree and the widespread incentives for its use in farmlands have been accompanied by a growing concern about its behavior as an exotic species.

Data sources
Records of the occurrences of Leucaena leucocephala in Brazilian territory were extracted from three databases: speciesLink, Flora do Brasil 2020 and GBIF. We proceeded through four steps. First, we searched records that had collection dates prior to 1940 (date of the earliest registered introduction of the species in Brazilian territory). Second, these records were compared in each database, in order to exclude possible duplicates (same location, same collector, and same year). Third, we tried to identify which records had exsicatae images with collection dates. Fourth, we classified the data set considering all collection points recorded in Brazil. Records were divided into two groups: one containing records that are geo-referenced, the other containing non geo-referenced records. The second set was considered only to tally the total number of records in the Brazilian territory. The first group was used to build maps; the second group was used to generate graphs that allow the visualization of records by decade of collection.

Image 18 depicts the sample collected in 1937
by the Brazilian soil scientist Carlos Arnaldo Krug

Records by year and date of collection
Discarding duplicate records found in the three databases, we tallied a total of 536 individual records. They were plotted according to the years and decades in which respective collections occurred. Images 21 and 22 display the graphs illustrating these distributions.

Discussion
The three consulted databases are valuable sources of data on the occurrence of many plant  The data allowed us to illustrate (Image 22) the duration of the gaps between collection records.
The reasons that led to these gaps may be many,

17/20
The scarcity of data on Leucaena in the early stages of its history in Brazil is visible in the first period of collection (1830 -1920), which yields only 19 records in 90 years. The richer records of the second (1930 -1970) and third (1980 -2010) periods can be attributed to (i) the spreading of Additionally, results allow us to build timelines of potential invasions (COUSENS et al., 2013).
These time lines are useful to determine the species' circulation flow (FUENTES et al., 2008) and to estimate propagation rates in different environments (PYŠEK; HULME, 2005) over time.
This of course may inform strategies designed to reduce the species' chances of establishment (MacDOUGALL et al., 1998;KRICSFALUSY;TREVISAN, 2014) and contain its dissemination.