Vascular Epiphyte Assemblages on Isolated Trees Along an Elevational Gradient in Southwest Panama

diversity

Article Vascular Epiphyte Assemblages on Isolated Trees along an Elevational Gradient in Southwest Panama

Calixto Rodríguez Quiel 1,2,* and Gerhard Zotz 1,3

1 Department of Biology and Environmental Sciences, Carl von Ossietzky University of Oldenburg, Ammerländer Heerstraße 114-118, 26129 Oldenburg, Germany; [email protected] 2 Programa de Maestría en Biología Vegetal, Universidad Autónoma de Chiriquí, El Cabrero, David, Chiriquí P.O. Box 0427, Panama 3 Smithsonian Tropical Research Institute, Balboa, Ancon, Panama City 0843-03092, Panama * Correspondence: [email protected]

Abstract: Ongoing destruction of tropical forests makes isolated pasture trees potentially important for the persistence of original forest dwellers such as many vascular epiphytes. We studied epiphyte assemblages on 100 isolated trees at ten pasture sites in southwest Panama along an elevational gradient ranging from 140 to 1240 m a.s.l. We analysed epiphyte species composition (richness, similarity) and registered climate and host trait variables of potential influence on their occurrence. We found a total of 5876 epiphyte individuals belonging to 148 species. Epiphyte abundance, species richness and diversity all varied about 4-fold among the 10 sites, with a high similarity of epiphyte assemblages among sites. Two sites at 870 and 1050 m a.s.l. did not fit into the overall elevational trend of increased abundance, species richness and diversity. However, all three measures were significantly correlated with humidity as the independent variable. This highlights that a gradient in

humidity, and not elevation as such, is responsible for the typical elevational changes in epiphyte assemblages, so that special local conditions may lead to deviations from expected patterns. Our

Citation: Rodríguez Quiel, C.; Zotz, documentation of current elevational diversity patterns also provides a baseline for the study of G. Vascular Epiphyte Assemblages on long-term changes in epiphyte assemblages in anthropogenically modiﬁed landscapes. Isolated Trees along an Elevational Gradient in Southwest Panama. Keywords: diversity; humidity; local climate; modiﬁed landscape; pastures; temperature Diversity 2021, 13, 49. https://doi.org /10.3390/d13020049

Academic Editors: Michael Wink and 1. Introduction José M. García del Barrio Despite their important role as the habitat of at least half of the global terrestrial Received: 18 December 2020 biodiversity, tropical forests keep decreasing in cover due to human activities [1,2]. As Accepted: 20 January 2021 human populations grow, there is an increasing need for land to provide food, living Published: 28 January 2021 space and other resources, which currently represents the principal threat to tropical biodiversity [3–5]. Resolving the conflict between environmental conservation and eco- Publisher’s Note: MDPI stays neutral nomic globalisation represents a big challenge [6]. In the tropics, land use change rather with regard to jurisdictional claims in than climate change or invasive species is the most important cause of decreasing biodiver- published maps and institutional affil- sity [7]. Knowledge of the current status of tropical biodiversity in modified landscapes iations. will allow us to establish management plans for conservation and sustainable development beyond primary forests [8]. In many pristine tropical forests, vascular epiphytes are one of the most species-rich plant groups, with major impacts on nutrient and hydrological cycles in the ecosystem [9–11]. Copyright: © 2021 by the authors. Epiphyte diversity can be impressive, e.g., a single tree may harbour almost 200 vascular Licensee MDPI, Basel, Switzerland. epiphyte species [12]. However, epiphyte richness in human-modified landscapes is usually This article is an open access article substantially reduced (see [13,14]). In the case of pastures, isolated trees kept to offer shade distributed under the terms and for livestock can provide connectivity and allow gene flow among epiphyte populations in conditions of the Creative Commons Attribution (CC BY) license (https:// pastures and surrounding forest patches [15–17]. creativecommons.org/licenses/by/ Knowledge of the structure and dynamics of vascular epiphyte assemblages on iso- 4.0/). lated trees is still relatively limited. Traditionally, studies with epiphytes were mainly

Diversity 2021, 13, 49. https://doi.org/10.3390/d13020049 https://www.mdpi.com/journal/diversity Diversity 2021, 13, 49 2 of 12

undertaken in relatively pristine forests and, to a lesser degree, in secondary forests (e.g., [18–22]). More recently, isolated trees in pastures have been included in a number of studies that compared vascular epiphyte assemblages in primary forests, fragmented forests and isolated pasture trees [13,23–27]. Epiphyte communities and their relationship with large-scale environmental gradients has been studied for decades mainly in forests [28] and, lately, in isolated pasture trees in the lowlands (e.g., [11,13,17]). The elevational gradients and the “mid-elevation bulge” of vascular epiphytes are well known. However, diversity changes in vascular epiphyte assemblages in pastures along an elevational gradient in a single region have not been documented. Epiphytes do not show an exceptional elevational pattern: many groups of organisms show an increase in richness from the lowlands until a maximum is reached at intermediate elevations [29]. Maxima of vascular epiphyte diversity are typically reached at c. 1400–1600 m, e.g., Küper, Kreft, Nieder, Köster and Barthlott ([20], at 1500 m a.s.l., in a range from 0 to 3200 m a.s.l. in Ecuador), Krömer, Gradstein and Acebey [19], Krömer, Kessler, Gradstein and Acebey ([10], at 1500 m a.s.l., from 350 to 4000 m a.s.l. in northern Bolivia) and Hietz and Hietz-Seifert ([30], at 1400 m a.s.l., from 720 to 2370 m a.s.l. in Mexico). The current study focused on epiphyte assemblages on isolated pasture trees along such an elevational gradient in western Panama to test whether elevational trends were comparable to those documented in previous studies in forest settings (see [19–21,28,30]). Since the studied elevational gradient ended at c. 1200 m, we expected a steep and continuous increase in species numbers with elevation. We also describe the variation of β-diversity in the elevational gradient and the effects of some important biotic and abiotic variables that typically have a direct effect on epiphyte assemblages [14,23]. Since epiphyte communities on isolated pasture trees tend to have low β-diversity [24], we expected the same, simple pattern along the gradient. Further, we expected clear relationships between changes in local climate and changes in epiphyte assemblages.

2. Materials and Methods This study was conducted in southwest Panama, Gualaca district (between 8◦32027.3600 N, 82◦32027.3600 W from the lowlands at 140 m a.s.l. and 8◦41027.4200 N, 82◦13047.2700 W at 1240 m a.s.l, near the Cordillera Central). We studied 100 trees distributed in 10 1-ha pasture plots at elevation intervals from 70 to 190 m (Figure1). According to Tosi Jr [ 31], the surrounding vegetation type represented patches of tropical lowland forest in the lower regions and pristine montane forest in the highlands. There is a dry season between January and April that governs lowland and highland pastures [17,32]. The pasture trees at 1240 m a.s.l. were mostly remnants of the cleared forest in close proximity to the Fortuna Forest Reserve, whereas most other trees along the gradient were planted, common pasture trees or had established spontaneously after clearance. Most of the pastures were established more than 60 years ago (personal communication of local farmers). Typical tree species were Tabebuia rosea (Bertol.) DC. (Bignoniaceae), Byrsonima crassifolia (L.) Kunth (Malpighiaceae) and Guazuma ulmifolia Lam. (Malvaceae). Within each pasture plot, we randomly selected ten trees with a diameter at breast height (DBH) ≥ 10 cm [following 17] and their geographical coordinates were recorded with a GPS. We recorded the DBH of each host tree. We monitored air temperature and relative humidity at each elevation with Hobo U23 Pro V2 data loggers (Onset Computer Corporation, MA, USA). The devices were installed in one haphazardly chosen tree per site at its ﬁrst inner branches at approximately 3 m height. Data were logged every 30 min for 186 days (August 2017 to February 2018) and loggers were checked for consistent readings before and after the measurement campaign. We quantiﬁed tree density in each 1-ha pasture plot and noted the surrounding forest cover (expressed as area percentage) in a radius of 100 m of the plots using the measuring tools of Google Earth Pro 7.1.7.2606 software and satellite images from 2018. Surrounding forest cover area was studied because it represents a potential source pool of epiphytes for neighbouring pastures. Diversity 2021, 13, x FOR PEER REVIEW 3 of 12

Diversity 2021, 13, 49 3 of 12 software and satellite images from 2018. Surrounding forest cover area was studied because it represents a potential source pool of epiphytes for neighbouring pastures.

Figure 1. StudyStudy area area in Gualaca district in southwest Panama. Ten Ten plots were establis establishedhed along the elevational gradient from 100 to 12401240 mm a.sl.a.sl. InIn eacheach plot,plot, tenten treestrees werewere selected,selected, andand theirtheir epiphyte epiphyteassemblages assemblages were studied. were studied.

We recordedrecorded allall vascular vascular epiphytes epiphytes in in the the selected selected trees. trees. Following Following Poltz Poltz and and Zotz Zotz [17], [17]small, small seedlings seedlings were were excluded excluded because because of of (1) (1) difficulties difficulties in in species species determination determination and because of of (2) (2) their their high high mortality mortality rates. rates. When When growing growing close together, close together, individual individual epiphytes epiphytescannot always cannot be distinguishedalways be distinguished from neighbours. from neighbours. In these cases, In these we defined cases, individualswe defined individualsfollowing Sanford following [33 ]Sanford as a “stand”, [33] as i.e., a “stand”, a cluster i.e., of individuals a cluster of or individuals stems belonging or stems to belongingthe same species to the separated same species from separated other clusters. from Inother our clusters. analyses, In we our only analyses, considered we only true consideredepiphytes as true defined epiphytes by Zotz as [ 34defined], but hemi-epiphytes,by Zotz [34], but mistletoes hemi-epiphytes, and vines mistletoe were noteds and to vinescontribute were to noted a general to contribute distributional to a database. general distributionalTall trees were database. accessed with Tall single-rope trees were accessedclimbing with techniques single- [rope35]. climbing techniques [35]. Species identification identification of vascular epiphytes was mostly conducted in the field.field. When this was not possible, ferti fertilele and sterile individuals were sampled and processed in the Herbario de la Universidad Autónoma Autónoma de Chiriquí Chiriquí (UCH). To identify species, we used several keys of vascular epiphytes from from Panama, Costa Costa Rica Rica and and Mesoamerica Mesoamerica [36 [36––3939]].. Some problematic specimens were sent to specialists for identification.identification. In In many cases, reproductive structures structures were were lacking lacking and and we hadwe had to name to name morphospecies. morphospecies. Scientific Scientific names nameswere standardised were standardised according according to The Plant to The List Plant [40]. List Vouchers [40]. Vouchers are deposited are deposited in the Chiriqu in theí ChiriquíHerbarium, Herbarium, UCH. UCH. A profile profile of the local climate was produced from temperature and relative humidity data. We We calculated site averages of both variables. Differences among sites in regard to climate and surrounding forest cover were assessed with regression analyses and Pearson correlation tests. To compare epiphyte richness per elevation and evaluate sampling effort, we used epiphyte richness (α-diversity) per tree at each site and estimated the richness per elevation

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with the Chao 1 richness estimator [41]. Further, we produced species accumulation curves of the trees sampled in every pasture by randomising the trees with 100 permutations. We also calculated the abundance, richness and diversity per tree (here as the Shannon– Wiener index) as the average per elevation (a-diversity). With these data, changes in diversity components along the elevational gradient were assessed with regression analyses and Pearson correlation tests. Additionally, we tested if local climate variables that may influence epiphyte assemblages were correlated with the elevational gradient (relative air humidity and temperature). Furthermore, to test whether differences among host trees could influence epiphyte assemblages along the elevational gradient, we performed Kruskal–Wallis tests (KW) on tree size (DBH) and also tree density between the pasture plots. Finally, the relationship between tree size and epiphyte richness was analysed with a Pearson correlation test with the aim to find a positive correlation on the trees hosting epiphytes. Diversity types (α, β, γ) are also graphically defined in Figure S1. To analyse variation in vascular epiphyte assemblage composition along the elevational gradient (β-diversity), we used a general multiple assemblage abundance-based N overlap measure (Cq )[42,43], in which we produced similarity profiles at the tree scale for every pasture [43,44]. This analysis allows comparisons of communities by transforming the occurrence and abundance data to Hill numbers, which facilitates the use of numerous diversity parameters. The resulting graph relates similarity as a function of the sensitivity parameter “q” (x-axis). Values on the y-axis range from 0 to 1, indicating how similar the studied assemblages are, with 0 being completely distinct and 1 identical. When q = 0, similarity is calculated as the Sorensen index, which takes into account all species from the database and abundance is irrelevant; when q = 1, similarity is calculated as the Horn overlap index that weighs species proportionally to their frequency; and when q = 2, the measure is based on the Morisita–Horn index that weighs abundant species more than rare species [45]. For clarity, only data of five plots, but spanning the entire elevational range, were considered. The composition of epiphyte assemblages on every studied tree along the gradient was analysed with non-metric multidimensional scaling (NMDS) using Euclidean distance. The stress value for NMDS was 0.16. To test differences within epiphyte assemblages at each elevation and according to local relative humidity, a permutational analysis of variance (PERMANOVA) was conducted. All tests were run with R v.3.3.2 software [46], where the package “vegan” [47] was used to calculate species accumulative curves, similarity profiles, NMDS and PERMANOVA and the package “vegetarian” was used to elaborate similarity profiles [48].

3. Results 3.1. Climatic Variables and Vegetation along the Elevational Gradient The average temperature decreased by 0.6 ◦C with each increase of 100 m in elevation (Figure2A, R 2 = 0.99, Pearson correlation test, t = −25.58, df = 8, p = 0.01). Increases in relative humidity were not as tightly coupled with elevation but still signiﬁcantly related (Figure2B, R 2 = 0.57, Pearson correlation test, t = 3.25, df = 8, p = 0.01). Surrounding forest cover increased more than 5-fold within the studied elevational range (R2= 0.76, Pearson correlation test, t = 4.98, df = 8, p = 0.01), i.e., the proportion of modiﬁed vegetation strongly decreased towards the mountain range (Figure2C). Tree density varied from 12 to 37 tree ha−1 in the pastures but did not vary with elevation (KW = 9, df = 9, p > 0.05, Table1). Tree size did not vary with elevation either (KW = 15.89, df = 9, p > 0.05). Diversity 2021, 13, x FOR PEER REVIEW 5 of 12

Table 1. Characteristics of study sites along an elevational gradient in southwest Panama. Columns show the abiotic characteristics such as temperature (T) and relative humidity (RH), the biotic characteristics such as surrounding forest cover, diameter at breast height (DBH) and tree density and characteristics of epiphyte assemblages at the landscape scale (total richness, Chao richness and total abundance) and at the tree scale (abundance per host, richness per host, Shannon index per host). Temperature and relative humidity values are the average of 186 days of recording. Relative humidity, DBH, Chao richness, abundance per host, richness per host and Shannon index per host are expressed as the average ± standard deviation (x ±SD).

Surrounding Tree Shannon Elevation Total Chao Total Abundance Richness T (°C) RH (%) Forest Cover DBH (cm) Density Index Per (m a.s.l.) Richness Richness Abundance Per host Per Host (%) (Trees ha−1) Host 140 28.3 87 ± 10 4 41.9 ± 18.6 22 28 48 ± 15 610 61 ± 53 7 ± 5 1.0 ± 0.7 240 27.6 87 ± 10 3 50.9 ± 20.9 23 25 36 ± 12 416 42 ± 35 9 ± 5 1.5 ± 0.9 390 26.6 89 ± 9 14 36.4 ± 14.7 20 21 35 ± 15 404 40 ± 52 7 ± 4 1.4 ± 0.6 510 26.3 91 ± 9 6 30.8 ± 6.7 37 24 26 ± 2 814 81 ± 27 10 ± 2 1.8 ± 0.2 590 26.3 91 ± 9 7 48.0 ± 28.6 25 31 65 ± 27 758 76 ± 61 9 ± 5 1.6 ± 0.5 730 24.1 92 ± 9 8 37.9 ± 13.8 17 30 39 ± 7 554 55 ± 42 10 ± 4 1.8 ± 0.3 800 23.7 92 ± 8 21 38.9 ± 19.2 20 36 59 ± 16 728 73 ± 98 9 ± 6 1.6 ± 0.5 Diversity870 2021, 13 23.3, 49 89 ± 9 30 26.8 ± 19.7 17 16 22 ± 5 184 18 ± 25 3 ± 3 0.75 ± of 0.6 12 1050 22.4 90 ± 11 27 33.8 ± 10.0 19 28 41 ± 9 497 50 ± 55 6 ± 5 0.9 ± 0.7 1240 21.2 97 ± 3 51 35.9 ± 16.8 12 82 107 ± 11 911 91 ± 38 21 ± 7 2.7 ± 0.3

FigureFigure 2. 2. ChangesChanges inin abioticabiotic andand bioticbiotic featuresfeatures thatthat mightmight inﬂuence influence epiphyte epiphyte assemblages assemblages ((((AA)) temperature,temperature, (B(B)) relativerelative humidityhumidity andand (C(C)) surrounding surrounding forest forest coverage) coverage) along along the the elevational elevational gradientgradient (100–1200 (100–1200 mm a.s.l.)a.s.l.) inin Gualaca district, Panama Panama.. In In graph graph B, B, humidity humidity is is shown shown to to be be low low in intwo two pastures, pastures, highlighted highlighted with with asterisks asterisks (at (at 800 800 and and 890 890 m ma.s.l.). a.s.l.). Surrounding Surrounding forest forest coverage coverage is a ismeasure a measure for the for size the sizeof the of potential the potential epiphyte epiphyte species species pool that pool acts that as actsa seed as source a seed for source the epiphyte for the assemblages on the pasture trees. Equations and coefficient of determination are given for each epiphyte assemblages on the pasture trees. Equations and coefﬁcient of determination are given for regression. each regression. 3.2. Taxonomic Composition of Vascular Epiphyte Assemblages Table 1. Characteristics of study sites along an elevational gradient in southwest Panama. Columns show the abiotic characteristics such as temperatureWe (T) registered and relative humidity6027 structurally (RH), the bioticdepend characteristicsent plants, such representing as surrounding 167 forest species. cover, diameter at breast heightApproximately (DBH) and tree97% density of all and individuals characteristics and of 89% epiphyte of the assemblages species were at the true landscape epiphytes, scale (total richness, Chao richnessrepresenting and total 148 abundance) species and and 58 at76 the individuals, tree scale (abundance distributed per in host,49 genera richness and per 13 host, families Shannon index per host). Temperature(Table S1). andHemi-epiphytes, relative humidity mistletoes values are and the vines average made of 186up days2% of of the recording. plants and Relative 11% of the humidity, DBH, Chao richness,species. abundance per host, richness per host and Shannon index per host are expressed as the average ± standard deviation (x ±OrchidaceaeSD). was the most important family accounting for more than 50% (75 taxa) of the epiphyte species and 49% (2905) of the individuals, Polypodiaceae represented 17% Surrounding Tree Shannon Elevation of the species (841/14% individuals)Total andChao BromeliaceaeTotal representedAbundance 16%Richness of the species T(◦C) RH (%) Forest DBH (cm) Density Index Per (m a.s.l.) Richness Richness Abundance Per host Per Host Cover(1809/31% (%) individuals).(Trees ha −These1) three families were abundant over the entire gradient.Host 140 28.3 87 ± 10 4 41.9 ± 18.6 22 28 48 ± 15 610 61 ± 53 7 ± 5 1.0 ± 0.7 240 27.6 87 ± 10 3 50.9 ± 20.9 23 25 36 ± 12 416 42 ± 35 9 ± 5 1.5 ± 0.9 390 26.6 89 ± 9 14 36.4 ± 14.7 20 21 35 ± 15 404 40 ± 52 7 ± 4 1.4 ± 0.6 510 26.3 91 ± 9 6 30.8 ± 6.7 37 24 26 ± 2 814 81 ± 27 10 ± 2 1.8 ± 0.2 590 26.3 91 ± 9 7 48.0 ± 28.6 25 31 65 ± 27 758 76 ± 61 9 ± 5 1.6 ± 0.5 730 24.1 92 ± 9 8 37.9 ± 13.8 17 30 39 ± 7 554 55 ± 42 10 ± 4 1.8 ± 0.3 800 23.7 92 ± 8 21 38.9 ± 19.2 20 36 59 ± 16 728 73 ± 98 9 ± 6 1.6 ± 0.5 870 23.3 89 ± 9 30 26.8 ± 19.7 17 16 22 ± 5 184 18 ± 25 3 ± 3 0.7 ± 0.6 1050 22.4 90 ± 11 27 33.8 ± 10.0 19 28 41 ± 9 497 50 ± 55 6 ± 5 0.9 ± 0.7 1240 21.2 97 ± 3 51 35.9 ± 16.8 12 82 107 ± 11 911 91 ± 38 21 ± 7 2.7 ± 0.3

3.2. Taxonomic Composition of Vascular Epiphyte Assemblages We registered 6027 structurally dependent plants, representing 167 species. Approx- imately 97% of all individuals and 89% of the species were true epiphytes, representing 148 species and 5876 individuals, distributed in 49 genera and 13 families (Table S1). Hemi-epiphytes, mistletoes and vines made up 2% of the plants and 11% of the species. Orchidaceae was the most important family accounting for more than 50% (75 taxa) of the epiphyte species and 49% (2905) of the individuals, Polypodiaceae represented 17% of the species (841/14% individuals) and Bromeliaceae represented 16% of the species (1809/31% individuals). These three families were abundant over the entire gradient. Tillandsia fasciculata was the single most abundant species, with 569 individuals at eight sites. Diversity 2021, 13, x FOR PEER REVIEW 6 of 12

Tillandsia fasciculata was the single most abundant species, with 569 individuals at eight sites. Other common species were Vriesea sanguinolenta (563 individuals), Polypodium polypodioides (387 individuals), Catopsis nutans (360 individuals) and Epidendrum difforme sensu lato (348 individuals), which were all found at nine sites.

3.3. α-Diversity along the Elevational Gradient Regarding the sampling effort, the observed species represented between 45 and 95% of the estimated richness per site (Table 1). Species richness at 1240 m a.s.l. was Diversity 2021, 13, 49 substantially higher than at all other elevations (Figure 3). Not surprisingly, we found6 of 12 richness per tree was correlated with tree size (R2 = 0.11, t = 3.59, df = 98, p = 0.01). However, neither epiphyte abundance (Pearson correlation R2 = 0.05, t = 0.60, df = 8, p > 0.05,), richness (R2= 0.17, t = 1.42, df = 8, p > 0.05) nor diversity per host (R2 = 0.10, t = 0.95, df = 8, Otherp > 0.05) common correlated species significantly were Vriesea with sanguinolenta elevation along(563 the individuals), entire gradientPolypodium (Figurepolypodioides 4). (387 individuals),This unexpectedCatopsis finding nutans was driven(360 individuals) by the low values and Epidendrum of the two study difforme sitessensu at 870 lato (348and individuals),1050 m a.s.l. Noteworthy which were is all that found relative at nine humidity sites. was very low at these two sites given the elevation (Figure 2B). Excluding these two plots from the analysis yielded 3.3.significantα-Diversity increases along with the Elevational elevation Gradientin all three assemblage attributes (Figure 4). These significantRegarding relationships the sampling were strongly effort, the influenc observeded, but species not entirely represented driven between by, the site 45 and with 95% ofthe the highest estimated values richness in all three per site measures (Table1 ).at Species1240 m. richness Including at 1240only mthe a.s.l. seven was lowest substantially plots higherin a third than analysis at allother still yielded elevations a significant (Figure3 ).relationship Not surprisingly, for diversity we found (R2 = richness 0.57, p < per0.05) tree wasand a correlated trend for abundance with tree size (R2 = (R 0.39,2 = 0.11,p = 0.13). t = 3.59,Alternatively, df = 98, pwhen= 0.01). relating However, abundance, neither epiphyterichness and abundance diversity (Pearsonto relative correlation humidity, all R2 three= 0.05, relationships t = 0.60, df were = 8, highlyp > 0.05,), significant richness ((PearsonR2= 0.17 ,correlation, t = 1.42, df abundance, = 8, p > 0.05) t nor= 2.45, diversity df = 8, p per = 0.04; host richness, (R2 = 0.10, t =t 3.85, = 0.95, df df= 8, = p 8, = p0.01;> 0.05) correlateddiversity, t signiﬁcantly= 3.78, df = 8, withp = 0.01, elevation Figure along 5). the entire gradient (Figure4).

FigureFigure 3. SpeciesSpecies accumulation accumulation curves curves of epiphyte of epiphyte assemblages assemblages on host on trees host from trees pasture from pasture sites sites distributeddistributed along along an an elevational elevational gradient gradient in inGual Gualacaaca district, district, Panama. Panama. Data Data show show the mean the mean ± the ± the standard deviation of species in every pasture. standard deviation of species in every pasture. 4.

Figure5. 4. Variation in the average of (A) abundance, (B) richness and (C) diversity (as the Shannon index) of epiphyte assemblages in pasture trees (n = 10 per plot) along the elevational gradient in Gualaca district. Dotted lines represent linear regressions including all plots. Solid lines represent the correlation without the plots at 870 and 1050 m a.s.l. (highlighted with red symbols). Coefﬁcients of determination and probability values are given in each plot.

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This unexpected finding was driven by the low values of the two study sites at 870 and 1050 m a.s.l. Noteworthy is that relative humidity was very low at these two sites given the elevation (Figure2B). Excluding these two plots from the analysis yielded significant increases with elevation in all three assemblage attributes (Figure4). These significant relationships were strongly influenced, but not entirely driven by, the site with the highest values in all three measures at 1240 m. Including only the seven lowest plots in a third analysis still yielded a significant relationship for diversity (R2 = 0.57, p < 0.05) and a trend for abundance (R2 = 0.39, p = 0.13). Alternatively, when relating abundance, richness and diversity to relative humidity, all three relationships were highly significant (Pearson correlation, abundance, t = 2.45, df = 8, p = 0.04; richness, t = 3.85, df = 8, p = 0.01; diversity,

t = 3.78, df = 8, p = 0.01, Figure5). 5.

Figure 5. Variation in the average of (A) abundance, (B) richness and (C) diversity (as the Shannon index) of epiphyte assemblages in pasture trees (n = 10 per plot) as a function of relative humidity along an elevational gradient in Gualaca district. Pastures at 870 and 1050 m a.s.l. are highlighted with red symbols. Coefﬁcients of determination and p-values of regression analyses are given in each plot.

3.4. β-Diversity along the Elevational Gradient The among-tree similarity of the epiphyte assemblages ranged between 65 and 85% (q = 0; Figure6). Among-tree similarity in regard to the most common species (q = 1) ranged between 45 and 75% along the gradient, while variation regarding very abundant species1 was largest with 15–70% (q = 2). In brief, among-tree similarity of epiphyte assemblages varied little between elevations considering species richness (q = 0), but considering common and also abundant species (q = 1 and q = 2, Figure6), the structure of epiphyte assemblages varied strongly between the different elevations. Diversity 2021, 13, x FOR PEER REVIEW 8 of 12

DiversityDiversity 20212021, 13, 13, x, 49FOR PEER REVIEW 8 of 12 8 of 12

Figure 6. Multiple-assemblage similarity profile of epiphyte assemblages growing along the elevational gradient in Gualaca district. The graph illustrates the compositional differences in the Figure 6. Multiple-assemblage similarity proﬁle of epiphyte assemblages growing along the eleva- Figureepiphyte 6. assemblages Multiple-assemblage from different similarity elevations profile using of similarity epiphyte indices assemblages that differ growing in their sensitivity along the tional gradient in Gualaca district. The graph illustrates the compositional differences in the epiphyte elevationalto relative species gradient abundance. in Gualaca The district. x-axis The shows graph the illustratesorders of qthe (sensitivity compositional parameter) differences and the in ythe- assemblages from different elevations using similarity indices that differ in their sensitivity to relative epiphyteaxis shows assemblages the values fromof mean different similarity, elevations from using0 = assemblages similarity indices being completelythat differ in distinct their sensitivity to 1.0 = x- y speciestoassemblages relative abundance. species being The abundance.identical.axis shows The the x-axis orders shows of q (sensitivity the orders parameter) of q (sensitivity and the parameter)-axis shows and the y- theaxis values shows of the mean values similarity, of mean from 0similarity, = assemblages from being 0 = assemblages completely distinct being to completely 1.0 = assemblages distinct to 1.0 = being identical. assemblages3.5. Ordination being Analyses identical. 3.5. OrdinationConsistent Analyses with the relatively high similarity of epiphyte assemblages among 3.5. Ordination Analyses elevations,Consistent the withNMDS the relativelyindicated high a substantial similarity ofoverlap epiphyte of assemblagesepiphyte assemblages among eleva- along the tions,gradientConsistent the NMDS (Figure indicated with7), which the a substantial relativelywas further overlap high supported of si epiphytemilarity by assemblages theof epiphyteresults along of assemblages the gradientPERMANOVA among (Figure7), which was further supported by the results of the PERMANOVA regarding the elevations,regarding the the elevation NMDS indicated (F = 6.69, a R substantial2 = 0.06, p = overlap 0.01) and of theepiphyte local relative assemblages humidity along (F the= elevation (F = 6.69, R2 = 0.06, p = 0.01) and the local relative humidity (F = 4.20, R2 = 0.04, gradient2 (Figure 7), which was further supported by the results of the PERMANOVA p4.20,= 0.01). R = 0.04, p = 0.01). regarding the elevation (F = 6.69, R2 = 0.06, p = 0.01) and the local relative humidity (F = 4.20, R2 = 0.04, p = 0.01).

FigureFigure 7. 7.Non-metric Non-metric multidimensional multidimensional scaling scaling (NMDS) (NMD ordinationS) ordination analyses ofanalyses epiphyte of assemblages epiphyte onassemblages trees along theon elevationaltrees along gradient the elevational in Gualaca gradient district, in Panama Gualaca (n = district, 100). Stress Panama value for(n = NMDS 100). Stress isFigurevalue 0.16. for 7. Non-metricNMDS is 0.16. multidimensional scaling (NMDS) ordination analyses of epiphyte 4.assemblages Discussion on trees along the elevational gradient in Gualaca district, Panama (n = 100). Stress 4. Discussion valueAs for epiphytes NMDS is lack 0.16. access to soil, they respond more than other life forms such as trees or terrestrialAs epiphytes herbs to lack variation access in to humidity soil, they [28 respon]. Thus,d themore typically than other observed life forms increase such in as trees 4.or Discussionterrestrial herbs to variation in humidity [28]. Thus, the typically observed increase in epiphyteAs epiphytes abundance lack and access species to soil,richness they with respon elevationd more up than to aother maximum life forms at intermediate such as trees orelevations terrestrial [19–21,28,30] herbs to variation is usually in humidity explained [28]. by Thus, an theincrease typically in observedwater availability. increase in epiphyteSurprisingly, abundance our results and did species not fulfil richness the expewithctation elevation of a up steady to a maximum increase because at intermediate neither elevations [19–21,28,30] is usually explained by an increase in water availability. Surprisingly, our results did not fulfil the expectation of a steady increase because neither

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epiphyte abundance and species richness with elevation up to a maximum at intermediate elevations [19–21,28,30] is usually explained by an increase in water availability. Surpris- ingly, our results did not fulfil the expectation of a steady increase because neither epiphyte richness, abundance nor diversity showed significant trends (Figure4). However, when these parameters were correlated directly with the hypothesised driver, i.e., with local relative humidity, a significant relationship emerged (Figure5). This suggests that the two sites at 870 and 1050 m are drier than expected for their elevation and that this deviation causes the unexpected result. Since any elevational trend in diversity is not due to elevation as such, but rather due to spatial constraints (e.g., [49]) or co-varying abiotic factors such as temperature or moisture availability (Figure2,[ 50,51]), our results actually lend support to the general mechanistic explanation for differences in epiphyte community structure. Together with local climate, variables such as tree density, surrounding forest coverage, time since original disturbance and presence of nearby human settlements all represent local factors that can modify general patterns. For example, we found that isolated trees at 1240 m a.s.l. were remnants of the primary forest with much forest in the vicinity (Figure3 ), which provides a straightforward explanation for the high α-diversity, while most trees at lower elevations established after clearance and the remaining forest as source areas are scarce. A high degree of local variability in epiphyte communities related to a varying number of remnant trees and newly established ones was also reported for pastures in lowland Panama [11]. Tree density did not differ much among sites, similar to observations in pastures in the lowlands of Panama [17]. Previous studies emphasised the role of the spacing of isolated trees for the connectivity and genetic restoration of isolated plant populations [15]. Maintaining or even increasing tree density may be an important factor for the long-term persistence of vascular epiphyte assemblages in pastures. Further, the positive correlation of epiphyte abundance and host DBH found in this study supports the general notion that large and old trees are of disproportionate importance for epiphytes [15,23]. As expected, assemblages varied little in species composition (Figures6 and7), al- though there was variation in assemblage structure (i.e., a change in the most abundant species) along the elevational gradient. Moreover, the overlap of epiphyte assemblages in the NMDS and PERMANOVA analyses supports the notion of small differences among elevations or by comparing them with their local relative humidity. Despite that, similarity profiles suggest that heterogeneity in the structure of epiphyte assemblages was mainly caused by changes in the most abundant species at the highest elevation where humidity is highest (Figure6). Werner, Homeier and Gradstein [ 27] also reported a high species richness on isolated remnant trees in Ecuador, which was still substantially reduced compared to a forest habitat. Noteworthy is that all long-term studies (e.g., [17,27,52]) suggest that epiphyte assemblages are currently not saturated (for a theoretical treatise, see [53]). Even on isolated trees in pastures, epiphytes tend to increase in abundance and species numbers over time: Einzmann and Zotz [54] reported a 3-fold increase in abundance over just eight years. Our study provides the basis to test whether such increases are really a universal phenomenon by repeating the census on the same pasture trees in a few years.

5. Conclusions We documented the occurrence of vascular epiphytes in pasture trees along an elevational gradient in western Panama. Surprisingly, diversity was not significantly correlated with elevation, but variations in abundance, species richness and diversity were significantly related to differences in relative humidity. The proximity of surrounding forest and land use history influenced the number of potential species that can reach isolated trees. Both introduce a high level of local idiosyncrasy. We emphasise that the value of this study is not restricted to the analysis of the status quo but also lends itself as a starting point for the investigation of long-term changes in epiphyte assemblages in this human-modified landscape. Diversity 2021, 13, 49 10 of 12

Supplementary Materials: The following are available online at https://www.mdpi.com/1424-281 8/13/2/49/s1: Figure S1: Analytical design. Alpha (α-) diversity represents the number of species in each individual tree. At each elevation, beta (β-) diversity represents the turnover between trees and gamma (γ-) represents the total diversity of a plot. We calculated the average diversity at the tree scale (a-diversity) to perform regression analyses (n = 10) and we compared the β-diversity of each plot. Table S1: Epiphyte abundance and composition in isolated pasture trees along an elevational gradient in southwest Panama. Author Contributions: Conceptualisation G.Z.; investigation, C.R.Q. and G.Z.; methodology, C.R.Q. and G.Z.; data collection, C.R.Q.; resources, C.R.Q. and G.Z.; funding acquisition, C.R.Q. and G.Z.; data curation and formal analysis, C.R.Q.; data interpretation, C.R.Q. and G.Z.; writing—review and editing, C.R.Q. and G.Z. All authors have read and agreed to the published version of the manuscript. Funding: This investigation was funded by the project APY-NI-2017-11 of Secretaría Nacional de Ciencia, Tecnología e Innovación (SENACYT) of the Republic of Panama Government and the Programa de Maestría de Biología Vegetal from SENACYT and Universidad Autónoma de Chiriquí. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in the supplementary material. Acknowledgments: We are grateful to Helena Einzmann, Katrin Wagner (both Oldenburg) and Iris Fossatti (Panama) for their help in field work, to the pasture owners of the study areas, specialists in taxonomic determination, Zabdy Samudio, Zuleika Serracín, Rodolfo Flores (UCH) and other collaborators in this investigation. We also acknowledge the support of governmental institutions, especially the Ministerio de Ambiente (Scientific Permit SE/P-23-18), SENACYT and the Herbario UCH of the Universidad Autónoma de Chiriquí. Conflicts of Interest: The authors declare no conflict of interest.

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