Trees and shrubs of the tropical dry forest of the Magdalena river upper watershed (Colombia)

Abstract Background We describe the database of trees and shrubs of tropical dry forest patches of the Magdalena upper river basin in Colombia, preserved in the Herbarium of Universidad de Ciencias Aplicadas y Ambientales. The dataset includes 211 taxa, from which 156 were identified to species. We reported 48 families and 137 genera. The most species rich and abundant families were Fabaceae and Rubiaceae and the most abundant species was Talisia stricta (Sapindaceae). We found differences in diversity between north and south zones of the study area. New information The Magdalena river upper watershed region is an important tropical dry forest conservation area. Twenty nine species and 4 genera recorded in this study have not been reported in previous reviews of the region. Additionally, Oxandra espintana is reported in literature as critically endangered and Aspidosperma polyneuron is reported as endangered, but there are no studies about their conservation status in the region. Our results suggest the strong need to develop additional inventories of plants that contribute to the knowledge of the plant diversity of this ecosystem in the region and studies of their conservation status.


Introduction
Tropical dry forests (TDF) correspond to a complex and fragile ecosystem. This complexity is due to evaporation exceeding precipitation and there being one or two periods of drought which may last between 4 and 6 months/year, resulting in the defoliation of part of the vegetation by water stress (Janzen 1988, Murphy andLugo 1986). Therefore, TDF has a great diversity of life forms, mixing deciduous and evergreen species with complex ecophysiological patterns (Dirzo et al. 2011a, Pennington et al. 2009). TDF has higher species richness in Mexico, Bolivia, Paraguay and Argentina (Gentry 1995, Olson and Dinerstein 1988), countries located in sites which challenge the pattern of increasing diversity when approaching the Ecuadorian line (Chazdon and Denslow 2002). Additionally, TDF has greater species richness thedrier they are, as in Mexico and Bolivia (Gentry 1995). Moreover, TDF has a high number of endemic species (Linares-Palomino et al. 2011) and provides a wide range of ecosystem services to human beings (Balvanera et al. 2012, Maass et al. 2005. TDF is one of the most threatened ecosystems by human activity around the world (Hoekstra et al. 2004, Miles et al. 2006. There have been reports of strong erosional processes and loss of natural cover of TDF (Portillo-Quintero and Smith 2018), including its disappearance from some regions of Central and South America (Mares et al. 1985, Janzen 1988. This is because the TDF distribution area coincides with regions suitable for livestock and agriculture (Hoekstra et al. 2004) and where firewood and wood removal activities are practised. Portillo-Quintero and Sánchez-Azofeifa (2010) reported that 72% of TDF has been lost from North and Central America, whereas in South America, 60% has been lost.
Recently, research has paid more attention to TDF. However, studies on its structure and diversity are not evenly distributed in the Neotropics. Most of the studies have been concentrated in a few countries such as Mexico (e.g. López-Martínez et al. 2013, Dzib-Castillo et al. 2014, Palacios-Wassenaar et al. 2018, Silva Aparicio et al. 2018, Silva-Aparicio et al. 2018 and Brazil (e.g. Barbosa et al. 2012, Apgaua et al. 2014, Silva et al. 2014, Lima and Coelho 2015, de Queiroz et al. 2017, Rocha et al. 2017. For larger scales, Dirzo et al. (2011b) developed a regional synthesis for Latin America addressing aspects of the TDF ecology.
Colombia is one of those countries where TDF is the most threatened and least studied. Only 8% of TDF original distribution in the country remains . In their review of diversity and conservation status of TDF in Colombia, Pizano and García (2014) state that the available literature in Colombia consists of studies on local scale, concentrated mostly on the Caribbean coast (north of the country) and the Chicamocha (north-west), Cauca and Patia watersheds (south-west). Moreover, the authors state the largest number of samplings has taken place in the Caribbean and in the valley of Cauca river. The Magdalena river upper watershed has been less sampled and most of the samplings have been concentrated in the north area (e.g. Mendoza-C 1999, Figueroa-C and Galeano 2007, Frenández-Méndez et al. 2013, Villanueva et al. 2015, Melo-Cruz et al. 2016. Additionally, in the Magdalena river upper watershed, only 13% of its potential distribution remains (Romero-Duque et al., data not published). Our goal was to contribute to the knowledge of plant diversity of TDF of the Magdalena river upper watershed. This paper provides a large dataset for occurrences of trees and shrubs.

Sampling methods
Sampling description: We selected twelve TDF patches according to their size, accessibility and owner's permission. Half of the patches were in Tolima and Cundinamarca departments (north zone of the study area) and the other half was in Huila department (south zone of the study area). In each site, we established ten 50 m x 2 m transects (0.1 ha), at least 7.5 m apart from each other at each site.
Quality control: All the materials were processed following the standardised procedures for herbaria described by Forman and Bridson (1989). Taxonomic  Step description: According to the criteria of Font Quer (1979), two life forms were recognised: shrub (woody individual less than 5 m tall that branches from the base at 1.5 to 4.9 m) and tree (woody individual that had a shaft of ≥ 5 m in height), all the individuals rooted within the transect and having ≥ 1 cm of DAP, were measured (DBH, height) and were identified as fully as possible to species. We registered data as common name, form of growth, vegetative and reproductive characteristics of aroma, colour, exudate, indument and glands and made their respective photographic records. We packed the material in plastic bags for easy handling and then we put them in botanical presses. We collected flowers (when possible) and stored them in bottles with glycerine. We entered field data with the Darwin Core format and, with the advisory team of SiB Colombia, the database was published.

Taxonomic coverage
Description: The dataset contains a total of 655 tagged individuals. We found 211 taxa (48 families and 137 genera), from which 156 were identified to species (see data resource). Some individuals of 44 genera remained unidentified. This is mainly related to the lack of appropriate material (e.g. flowers) to provide a definite determination. Three species of Cactaceae family were included in the database.
For the total study area, Fabaceae and Rubiaceae were the most species rich and the most abundant (individuals sampled) families (Table 2). Talisia stricta (Sapindaceae) was the most abundant species recorded (35 individuals). The north zone was the most diverse (species number) ( Table 2). We found 177 species, 120 genera and 43 families, whereas, in the south zone, we found 65 species, 47 genera and 26 families (Table 3). These differences could be due to the precipitation being higher in the north zone than in the south zone. Moreover, the north zone is a transition zone between tropical dry forest and tropical humid forest, which would explain the greater diversity, as well as the presence of some species representative of wet and moist forests in tropical dry forests ( The frequency distribution of the number of species amongst study sites was very skewed (Fig. 2). Sixty four percent of the species appeared in a single site, whereas 18% of the species appeared in two of the 12 sites. No species appeared in more than 5 sites. Astronium graveolens Jacq., Casearia corymbosa Kunth and Randia armata DC. were present in five sites (Table 4, Fig. 2). Only one species of the sampled species with a  higher importance value index (relative abundance, relative density, relative frequency) was shared between north and south zones of the study area (