Biovera-Epi: A new database on species diversity, community composition and leaf functional traits of vascular epiphytes along gradients of elevation and forest-use intensity in Mexico

Abstract Background This data paper describes a new, comprehensive database (BIOVERA-Epi) on species distributions and leaf functional traits of vascular epiphytes, a poorly studied plant group, along gradients of elevation and forest-use intensity in the central part of Veracruz State, Mexico. The distribution data include frequencies of 271 vascular epiphyte species belonging to 92 genera and 23 families across 120 20 m × 20 m forest plots at eight study sites along an elevational gradient from sea level to 3500 m a.s.l. In addition, BIOVERA-Epi provides information on 1595 measurements of nine morphological and chemical leaf traits from 474 individuals and 102 species. For morphological leaf traits, we provide data on each sampled leaf. For chemical leaf traits, we provide data at the species level per site and land-use type. We also provide complementary information for each of the sampled plots and host trees. BIOVERA-Epi contributes to an emerging body of synthetic epiphytes studies combining functional traits and community composition. New information BIOVERA-Epi includes data on species frequency and leaf traits from 120 forest plots distributed along an elevational gradient, including six different forest types and three levels of forest-use intensity. It will expand the breadth of studies on epiphyte diversity, conservation and functional plant ecology in the Neotropics and will contribute to future synthetic studies on the ecology and diversity of tropical epiphyte assemblages.


Introduction
Elevational gradients provide a wide range of opportunities for studying the effects of different ecological and evolutionary factors on biodiversity patterns. Steep elevational gradients in temperature, precipitation and other climatic variables usually play a fundamental role in shaping plant diversity (McCain and Grytnes 2010, Peters et al. 2019) and also contribute to linkages between plant traits and environmental conditions (Bruelheide et al. 2018, Keddy 1992. They are also used as proxies for understanding diversity patterns across latitudinal gradients (McCain and Grytnes 2010), while controlling for species pools and biogeographic history (Ricklefs 2004). Nevertheless, anthropogenic forest disturbance may modify climatic conditions at local and regional scales which, in turn, may affect the response of species, causing upward shifts in the treeline (Cazzolla Gatti et al. 2019), shifting the distribution of plants and animals (McCain et al. 2016) and might be especially threatening for canopy-dwelling life forms, such as vascular epiphytes that are sensitive to changes in air humidity and temperature (Larrea and Werner 2010, Werner and Gradstein 2009, Zotz and Bader 2009. Furthermore, while a growing number of studies shows that climate change affects a wide range of species and ecosystems (Peters et al. 2019, Root et al. 2003, Trisos et al. 2020, Walther et al. 2002, few studies focus on vascular epiphytes and their composition, diversity and functional traits, especially comparing different levels of forest-use intensity.
Functional traits are measurable characteristics of individual plants impacting their growth, reproduction and survival (Violle et al. 2007) and reflect how species interact with their environment (Vesk 2013). Functional traits are widely used to elucidate mechanisms that underpin many ecological processes along vertical and horizontal environmental gradients (e.g. Petter et al. 2015, Bruelheide et al. 2018), but also evolutionary patterns associated with variation in plant form and function, such as geographic distributions of woody and non-woody species . Despite recent progress (e.g. Agudelo et al. 2019, Petter et al. 2015, studies in the field of functional traits of vascular epiphytes are rare, suggesting that our knowledge of the factors that determine the distribution of vascular epiphytes along environmental gradients is similarly limited.
Mexico is a country with high floristic diversity and endemism. Almost 50% of its 23,114 native species of vascular plants are endemic. However, Mexico has lost approximately half of its forest cover in the past 50 years (Barsimantov and Kendall 2012). Furthemore, it has been estimated that about 7.8% of the Mexican vascular flora are epiphytes, 750 of which (569 angiosperms and 181 pteridophytes) are native to Veracruz (Krömer et al. 2020). Vascular epiphytes usually reach their highest diversity in humid tropical forests at mid-elevations (Guzmán-Jacob et al. 2019,Küper et al. 2004, Körmer et al. 2005, Cardelús et al. 2006), (Fig. 3). Moreover, they contribute significantly to ecosystem functioning through biotic interactions and by providing microhabitats for other organisms (Nadkarni 1984, Veneklaas et al. 1990, Zotz 2016. Even when epiphytes represent about 9% of all vascular plant species (Zotz 2016), they are strongly under-represented in global traits datasets. With this study, we aim at contributing to the percentage of epiphyte species represented in global datasets. We believe that the assemblage of local information in global databases covering species occurrences and functional traits can help to validate ecological theories at larger scales. In particular, the inclusion of an increasing number of studies on functional ecology can foster new frameworks and theories to better understand how biodiversity responds to an increasingly fragmented natural world. Here, we describe a new database on species  distributions and leaf functional traits of vascular epiphytes along gradients of elevation and levels of forest-use intensity.

General description
Purpose: BIOVERA-Epi includes plot data from an elevational gradient located in the central part of the State of Veracruz, Mexico. Specifically, it contains two distinct, but related datasets: the first dataset includes distribution and plot level frequency information (frequency.subplot) for 271 vascular epiphyte species, sampled in 120 20 m × 20 m plots along the elevational gradient, ranging from 0 to 3500 m a.s.l. The second dataset includes measurements of nine morphological and chemical leaf traits for 102 species, 474 individuals and a total of 1595 leaves, which were sampled in 45 plots at three sites along the same elevational gradient. The leaf traits studied were: leaf area, leaf density, specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen content, leaf phosphorus content, leaf carbon content, nitrogen isotope ratio (d N) and carbon isotope ratio (d C). For each plot, we also provide geographical coordinates, forest-use intensity (old-growth, degraded, secondary) and elevation. For the surveyed host trees, we report diameter at breast height (DBH), total height (H) and species identity (see Data collection).

Conclusion:
The species distribution dataset shows the value of old-growth forest for epiphyte diversity, but also show that degraded and secondary forest, depending on the elevation, may maintain a high species diversity and thus play an important role in conservation planning. Across our 120 study plots, Orchidaceae was the family with more species within the Angiosperms and Polypodiaceae within the Pteridophytes (Fig. 4). Furthermore, the leaf trait dataset shows the leaf trait variation among families, where some families show larger variation than others for both morphological and chemical traits (Figs 5, 6).

Sampling description: Sampling design
The elevational gradient spanned from sea level to 3500 m on the eastern slopes of Cofre de Perote, a 4282 m extinct volcano located in the central part of Veracruz State, Mexico ( Fig. 1). In this region, the Trans-Mexican volcanic belt and the Sierra Madre Oriental converge, creating complex geological conditions and combining floristic elements from the Nearctic and Neotropics. The climate in the study region ranges from dry and hot in the lowlands (mean annual temperature ( We investigated three levels of forest-use intensity (FUI) that could be consistently found along the entire gradient (  plots for each of the three FUI levels, respectively, yielding a total of 120 plots (Suppl. material 1). We used a Garmin® GPSMAP 60Cx device (Garmin International, Inc. Kansas, USA) to record geographical coordinates and elevation for all plots. Vascular epiphytes were surveyed between July 2014 and May 2015 following the sampling protocol of Gradstein et al. 2003. First, ground-based surveys were conducted in four 10 m × 10 m subplots nested within each plot, to represent epiphyte assemblages in the forest understorey up to a height of ~ 6 m (Krömer et al. 2006, Krömer and using collecting poles and binoculars (Flores-Palacios and Garca-Franco 2001). We selected one mature host tree per plot, based on size, vigour and crown structure for safe canopy access. We climbed from the base to the outer portion of the tree crown using the singlerope climbing technique (Perry 1978) and recorded the presence of vascular epiphyte species in each of the five vertical tree zones according to Johansson (1974), (Fig. 2). Johansson zones are a frequently-used scheme to record and describe the spatial distribution of vascular epiphytes within tree trunks and canopies (Gradstein et al. 2003, Sanger andKirkpatrick 2016). We recorded diameter at breast height (DBH) and total height for each climbed tree. We recorded the frequency of each species as the sum of incidences in the four nested subplots (frequency.subplot, maximum frequency per plot = 4) (Suppl. material 2, Figs 3, 4). We also recorded the frequency of each species as the sum of incidences in the five Johansson zones of the central host tree (frequency.J.zones, maximum frequency = 5).

Data collection leaf trait dataset
In a separate sampling campaign from June to September 2016, leaf trait sampling took place at three of our studied elevational sites (0, 500 and 1500 m a.s.l.). In this field campaign, we aimed to resample as many vascular epiphyte species from the first survey as possible. At each elevation, epiphytes were sampled up to a height of 20 m on one or more trees using the single-rope climbing technique. Epiphytes below 6 m were sampled from the ground using a collecting pole. Functional traits were collected for all vascular epiphyte species classified as holoepiphytes (epiphytes in the strict sense, i.e. living their whole life cycle as epiphytes). In this dataset, we excluded nomadic vines because of their contact with the ground (Zotz 2013). Additionally, we excluded species of the family Cactaceae from trait measurements because stems are their main photosynthetic organs. This dataset differs in the sampling resolution between morphological and chemical traits; morphological traits include leaf measurements per individual at each study site and chemical traits include one measurement (from pooled samples) per species from each study site.

Leaf trait measurements
We collected between one and three leaves per adult individual from three individuals to obtain, if possible, a maximum of 10 leaves per species. We sampled fully expanded leaves without visible signs of herbivory or disease. Collected leaves were rehydrated in a sealed plastic bag and kept cool in a refrigerator at 7°C for a minimum of 8 hours before taking measurements. , ii) specific leaf area (SLA = leaf area/dry weight; mm/mg) , iii) leaf density (LD = SLA/leaf thickness; g/cm) and iv) leaf dry matter content (LDMC = dry weight/fresh weight; g/g) (Suppl. material 3, Fig. 5). We measured the following leaf chemical traits: i) leaf nitrogen content (leaf nitrogen; %), ii) leaf carbon content (leaf carbon; %), iii) leaf phosphorus content (leaf phosphorus; %), iv) nitrogen isotope ratio (d N; ‰), and v) carbon isotope ratio (d C; ‰) (Suppl. material 4, Fig. 6). Dried leaf samples were ground and homogenised using a ball mill. To quantify leaf nitrogen content, leaf carbon content, d N and d C, we used an elemental analyser-isotope ratio mass spectrometer (Carlo Erba 1110 EA coupled via a Conflo III to a Delta ; Thermo Electron, Bremen, Germany). We used an internal standard, which is a solution of proline and sucrose with a C:N ratio of 8.8, d N of 0.16 (+/i 0.15) and d C of -10.20 (+/-0.13). We tested standards every ten samples, after which the IRMS was recalibrated using five certified isotope standards, i.e. IAEA-600, IAEA-N-1, IAEA-N2 and USGS-25. Atmospheric air (AIR) was used for d N and the Vienna Pee Dee Belemnite (VPDB) for d C as standards. Leaf phosphorus concentrations were determined colourimetrically (Murphy and Riley 1962). After digestion, 770 μl distilled water was added and the absorption by the molybdenum-phosphorus complex was measured at 710 nm using a UV-VIS spectrophotometer (Specord 50, Analytik Jena, Jena, Germany). Chemical analyses of samples were performed at the University of Oldenburg for phosphorus and at the University of Vienna, Department of Microbiology and Ecosystem Science for nitrogen, d N and d C.

Species identification
Vouchers from the first field campaign were collected, if possible, in triplicate for preservation as herbarium specimens. These specimens were identified using relevant literature (Croat and Acebey 2015, Espejo-Serna et al. 2005,Hietz and Hietz-Seifert 1994, Mickel and Smith 2004 (14) and Araceae (12). A total of 72.2% of the sampled epiphyte individuals could be identified to species level, while another 26.1% were identified to genus level and 1.7% to family level. The trait dataset includes measurements for 1595 leaves from 474 individuals belonging to 102 species in 10 families. In total, most species were orchids (42.7%), followed by ferns (28.1%) and bromeliads (20.4%).
2) Phorophytes: The 120 climbed host trees belong to 32 tree species distributed in 25 genera and 21 families. Tree identification to the species level was possible in 53% of the cases, while another 44% were identified to genus level and 3% to family level.

Data set name: Distribution table
Description: Distribution data of 271 vascular epiphyte species at each plot along the elevational gradient and three levels of forest-use intensity (n = 5 plots per forest-use intensity within each elevation) (Suppl. material 2).

Additional information
We provide the description of the content and structure of each supplementary material in Table 1, with the source of standardisation for each term used according to Darwin Core glossary and the Thesaurus of Plant Characteristics.

Standardised Term Term in this study Definition Unit
Darwin Core Family Family The full scientific name of the family in which the taxon is classified.
Darwin Core Habitat Vegetation A category or description of the habitat in which the Event occurred.
Darwin Core locationID Plot_ID An identifier for the set of location information (data associated with dcterms: Location). May be a global unique identifier or an identifier specific to the dataset.
Darwin Core Locality Site The specific description of the place. Less specific geographic information can be provided in other geographic terms (higherGeography, continent, country, stateProvince, county, municipality, waterBody, island, islandGroup). This term may contain information modified from the original to correct perceived errors or standardise the description.
Darwin Core organismID Sp.code An identifier for the Organism instance (as opposed to a particular digital record of the Organism). May be a globally unique identifier or an identifier specific to the dataset.