Analysis of the Diversity of Corticolous Lichens Associated with Tree Trunks in the Understories of Four Tropical Dry Forests of the Atlántico Department in Colombia

Abstract

Tropical dry forests (TDFs) are unique ecosystems with high biodiversity, including a rich variety of lichen species. Lichens are sensitive to environmental changes and can serve as bioindicators of ecosystem health. This study examined the diversity of lichen communities at four TDF sites in the Atlántico Department of Colombia. More than 700 tree lichen specimens were collected and identified at the four sites. A total of 135 species of lichens were identified, of which 19 are possibly undescribed. The most diverse sites were Usiacurí and Repelón, both protected areas with relatively well-preserved forests. The findings of this study demonstrate that the Atlántico TDFs host a large diversity of lichens, with a significant number of records of new species. The observed differences in species composition between sites highlight the importance of habitat heterogeneity and anthropogenic pressures on lichen communities. The results emphasize the need for conservation strategies to protect these ecologically valuable lichen communities within the Atlántico TDFs.

1. Introduction

Tropical dry forests (TDFs) are characterized by an extreme environment due to their high variability and long periods of drought and are among the most threatened ecosystems in the world [1]. In Colombia, up to 92% of the TDFs have been lost, due mainly to deforestation and climate change [2,3]. In the Department of Atlántico (located in the northern region of Colombia), TDFs are the second most predominant ecosystem, with 20% of the vegetation coverage (ca. 677.6 km²), and represent the largest forest cover, although they are highly fragmented due to anthropogenic pressure resulting from population growth (ca. 2835509 inhabitants, the fourth most populated in Colombia) and other significant threats, such as deforestation, natural resource exploitation (i.e., quarrying) and induced wildfires [4,5,6]. One of the strategies to mitigate these impacts on TDFs has been the creation of protected areas.

In Atlántico, there are different protected areas that are crucial for the recovery of tropical dry forests (TDFs). These areas are reservoirs for the conservation of species, especially those that are endemic, native and resistant to drought, such as lichens. These organisms are a symbiotic association between a fungus and an alga or a cyanobacterium. The fungus provides structure and protection, while the photosynthetic partner produces nutrients through photosynthesis [7]. Crustose lichens are characterized by their flattened, crust-like growth form that adheres tightly to the substrate. Lichens are also bioindicators of an ecosystem’s environmental health, and they play a crucial role in nitrogen fixation, soil formation and climate regulation [8,9]. Furthermore, lichens provide habitat and resources for a wide variety of organisms, fostering a mutually beneficial relationship, as defined by Hawksworth and Grube [10] in their redefinition of lichens. As reported by Moncada et al. [11], Colombian TDFs are rich in lichens. In Atlántico, more than 100 lichen species have been reported, including new endemic species such as Fissurina linoana Lücking, Moncada & G. Rodr., Graphis lurizana Lücking, Moncada & Celis, G. mokanarum Lücking, Moncada & M.C. Martínez and Phaeographis galeanoae Lücking, Moncada & B. Salgado-N. [12]. However, reports by Bhagarathi et al. [13], Chuquimarca et al. [14], Ellis et al. [15] and Miranda-González [16] indicate that the diversity and distribution of lichens in TDFs are negatively affected by climate change (e.g., increased temperatures, altered rainfall patterns) and human activities (e.g., deforestation), leading to the reduced availability of water and nutrients, which affects their growth and reproduction. Therefore, biodiversity studies provide fundamental knowledge about species and their habitats, interrelationships and potential threats. Thus, it is necessary to continue to conduct biodiversity studies that include analyses of the gamma (i.e., regional diversity), alpha (i.e., local diversity) and beta (e.g., species change or turnover among places) diversity [17].

Lichens, with their wide distribution, longevity and habitat preferences (rocks, trees, etc.), have emerged as indicators of forests’ health and conservation status. Their sensitivity allows them to respond to even subtle environmental changes, making them a valuable tool for researchers studying environmental impacts on forest ecosystems [18,19,20]. In TDF ecosystems, disturbances that reduce canopy cover and tree species richness have been shown to impact the epiphytic lichen richness [21]. Furthermore, Trindade [22] demonstrated how the biomass and diversity of epiphytic lichens are directly influenced by environmental heterogeneity, light availability and other habitat structural factors. All of these findings demonstrate the need to document the lichen diversity in the Atlántico TDF. Doing so will contribute to a more comprehensive understanding of these ecosystems and serve as a tool for future research.

While the existing literature provides insights into the lichen diversity in the Atlántico Department, knowledge of their local patterns of diversity and spatial turnover remains limited. In this study, we present an analysis of the (i) regional (gamma) and (ii) local diversity (alpha) and (iii) spatial turnover patterns (beta diversity) of Corticolous lichens associated with trunks in the understories of four TDF localities in the Atlántico Department. These localities exhibit varying environmental conditions and conservation statuses, providing a unique opportunity to investigate the influence of environmental factors on lichen diversity patterns.

2. Materials and Methods

2.1. Sampling Sites

Four sites were selected (Figure 1) based on their environmental characteristics, degrees of human disturbance and vegetation types; this selection allowed for an approximate understanding of the variability of the lichen diversity in the Caribbean region of Colombia.

2.1.1. Morro—Tubará

The sampling area was located on a private farm in the town of El Morro (203 masl), within the municipality of Tubará. The forest fragment at El Morro is characterized by vegetation at the early successional stages, with plant species adapted to light saline conditions, e.g., Bursera, Tabebuia, Tecoma and Ochoroma. The matrix surrounding the sampling area is mainly composed of temporary pastures with herbaceous formations, shrubs and some native trees such as Croton niveus Jack., Euphorbia tithymaloides L. and Cousetia ferruginea Kunth [23,24]. This area is exposed to winds coming directly from the sea, potentially increasing the environmental salinity.

2.1.2. Distrito Regional de Manejo Integrado (DMI) Luriza—Usiacurí

This fragment of TDF, with an area of 837.17 ha at 170 masl, has been legally protected since 2011 and includes relatively well-preserved seasonal streams. The best-preserved forest areas are located in the lower parts of the slopes of the Luriza stream. The other remaining forest areas are characterized by an intermediate successional state, with human intervention that varies from moderate to low [25]. Biodiversity inventories conducted in Usiacurí reveal the high diversity of birds (138 spp.), reptiles (44 spp.), mammals (43 spp.) and amphibians (19 spp.). Typical plant species belong to the Apocynaceae, Arecaceae, Bignoniaceae, Euphorbiaceae, Fabaceae and Polygonaceae [26,27].

2.1.3. Reserva Natural de la Sociedad Civil Los Charcones (Los Charcones)—Piojó

This protected area (42,98 ha; up to 450 masl) includes gallery forests, savannas and fragments of secondary TDFs with little recent human intervention. In the area, more than 300 plant species have been documented, including species of the genera Bauhinia, Bulnesia, Bursera, Caesalpinia, Capparis, Hura, Pithecellobium, Plumeria, Spondias and Tabebuia. There are historical records of emblematic animal species in the area, such as jaguars (Panthera onca), and current records of species such as howler monkeys (Alouatta seniculus), as well as green, blue, yellow and red macaws (Ara militaris (Linnaeus, 1766), Ara arauna (Linnaeus, 1758) and Ara macao (Linnaeus, 1758)) [28,29].

2.1.4. Distritos Regionales de Manejo Integrado Banco Totumo Bijibana—Repelón

The fragment of TDF associated with this protected area covers 1524 ha at 100–226 masl. It is a secondary forest, with some well-preserved relicts. In this area, approximately 196 plant species from 55 families have been recorded, some of which are important tree species, such as Bulnesia arborea Engl., Prosopis juliflora (SW) DC., Ceiba pentandra (L.) Gaertn., Anacardium occidentale L. and Pachira quinata (Jacq.) W.S.Alverson. In addition, animal diversity is important in terms of birds (171 spp.), reptiles (30 spp.), mammals (25 spp.) and amphibians (21 spp.). Some large mammals, such as reindeers (Mazama sp.) and howler monkeys (Alouatta seniculus (Linnaeus, 1766)), have been observed [30,31].

2.2. Specimen Sampling and Collection

In each one of the forest fragments, trees and lichens were selected as follows: (i) one tree at every 10 m along a trail; (ii) all trees inside a perimeter of 10 m from each previously selected tree; (iii) trees with a diameter at breast height (DBH) greater than 50 cm; and (iv) lichens located between 50 and 110 cm high on the trunk (trunk section, TS 50–110 cm). This was conducted as described by Gradstein et al. [32], with additions including the selection of only trees with a DBH greater than 50 cm and considering trees within a 10 m perimeter around each previously selected tree, as well as lichens located at a height of between 50 and 110 cm on the trunk. In total, we collected lichens from 112 trees: 50 in Usiacurí, 31 in Repelón, 18 in Piojo and 13 in Tubará. Sample size differences among localities correspond to differences in forest structure and composition that result in variability in the tree DBH. Henceforth, we use lichen community or community to refer to all taxa coexisting in the bark of trees with a DBH larger than 50 cm and between 50 and 110 from the ground. Specimen collection was authorized by the Autoridad Nacional de Licencias Ambientales (ANLA) through permit number 589 (10 April 2019), given to researchers at the Universidad del Norte. Lichen specimens were stored in paper bags (at 25 °C for 72 h) and identified at the Universidad del Norte (UNO, Barranquilla) and the Botanischer Garten und Botanisches Museum, Freie Universität Berlin (BGBM), using standard methods via light microscopy (LEICA DM500 (UN) with camera LEICA ICC50W; LEICA M205 C (UN) with camera LEICA MC190 HD; and LEICA Zoom 2000 (BGBM) are also used, all from Leica Microsystems in Wetzlar, Germany. Additionally, ZEISS Axioskop (BGBM) microscope, manufactured by Zeiss AG in Oberkochen, Germany) and thin layer chromatography (TLC, elution solvent mixtures A and C) [33,34]. Species identification was conducted using a comprehensive set of taxonomic resources, including monographs, taxonomic keys and online databases. Additionally, all identifications, including the consideration of potential new species, were supervised, reviewed and verified by Dr. Robert Lücking (BGBM). For a detailed list of all references used in the identification process, please refer to Supplementary S1.

2.3. Data Treatment and Statistical Analysis

To estimate the lichen community diversity at the sampling localities, we first calculated species accumulation curves to check for sampling completeness. Next, we applied the well-known asymptotic estimators of the q0 (richness), q1 (Shannon’s exponential index) and q2 (Simpson´s inverse index) Hill diversity numbers. Asymptotic estimators account for sampling differences and allow a direct comparison among sites irrespective of the sampling size. In this case, since no measure of absolute abundance was available, the frequency of each species in the entire sample was used as a measure of its abundance [35,36,37]. We computed the confidence intervals (CI) of the richness estimators, as well as the observed species richness and diversity indices, by non-parametric bootstrapping, resampling the observed frequency matrix 1000 times. Significance in the diversity metrics was assessed by the amount of overlap among the confidence intervals. We estimated the beta diversity using the Bray–Curtis index and the resulting pairwise dissimilarity was used in non-metric multidimensional scaling (NMDS) to visually compare the lichen community composition across the sites [38]. Statistical differences among sites were estimated using an analysis of similarity (ANOSIM) and significance was assessed based on 1000 permutations of the distance matrix. Finally, dissimilarity was partitioned into nestedness and turnover components to explore geographical patterns in species distribution. All analyses were performed in the R statistical software version 4.4.1 and the iNEXT and betapart packages version 3.0.1 are available on CRAN [39,40].

3. Results

3.1. Gamma Diversity

Across the four sampling sites, we collected, in total, 706 lichen specimens. From the sampled specimens, 83.0% (508) were identified to the species level and 12.6% (89) at the genus level (due to sterile samples) and 4.2% (30) were unidentified. The lichen community in this study (see Section 2.2 for lichen community definition) was composed of 23 families, 48 genera and 135 species (Figure 2) and included 19 possibly undescribed species (Supplementary S2).

The most diverse families were Graphidaceae (8 genera and 30 spp.) and Arthoniaceae (5 genera and 30 spp.), whilst the most diverse genera were Graphis (23 spp., including three spp. possibly undescribed), Opegrapha (22 spp., including five spp. possibly undescribed), Pyrenula (14 spp., including one sp. possibly undescribed), Porina (eight spp., including one sp. possibly undescribed) and Pseudopyrenula (seven spp.). Moreover, the most frequent species were Arthonia rubella (Fée) Nyl. (41 specimens), Coniocarpon cinnabarinum DC. (29 specimens), Graphis supracola A.W.Archer (25 specimens), Bacidina medialis (Tuck ex Nyl.) Kistenich, Timdal, Bendiksby & S.Ekman (23 specimens), Mazosia carnea (Eckfeldt) Aptroot & M.Cáceres (22 specimens) and Pyrenula ochraceoflava (Nyl.) R.C.Harris (22 specimens). The families with only one sampled specimen were Coenogoniaceae (Coenogonium sp.), Eremithallaceae (Eremithallus sp.), Gyalectaceae (Gyalecta nana Tuck.) and Teloschistaceae (Lacrima epiphora (Taylor) Bungartz, Søchting & Arup) (Supplementary S3).

The accumulation curve was close to reaching an asymptote (Figure 3A), with observed sample coverage of 86% (95% CI: 84–90). Based on the sampling, the asymptotic richness estimator suggests that the total lichen species richness in our community at the Atlántico Department is 246 species (95% CI: 206–287) (Figure 4A). The observed q1 and q2 indices (Figure 4B,C) showed an uneven community with a few high-frequency species but no dominant species.

3.2. Alpha Diversity

The distribution of the total lichen samples in our community was as follows: 332 in Usiacurí, 192 in Repelón, 104 in Piojó and 48 in Tubará. According to the uncorrected data, the most diverse site was Usiacurí (94 spp.; 8 gen.; 1 fam.), followed by Repelón (84 spp.; 8 gen.), Piojó (37 spp.; 9 gen.; 1 fam.) and, finally, Tubará (23 spp.; 3 gen.). The sample coverage in decreasing order was Usiacurí > Repelón > Piojó > Tubará (Figure 3B–E). The estimated richness varied between 100 spp. and 200 spp. Based on the asymptotic estimators for the species richness and their confidence intervals (q0) we found significant differences in the number of estimated species between Piojó and Repelón and marginally between Piojó and Usiacurí (Figure 4A). According to the Hill diversity numbers of the first order (q1), there were no significant differences between Repelón and Usiacurí or between Piojó and Tubará, but both Repelón and Usiacurí showed higher diversity compared to Piojó and Tubará (Figure 4B). The sites with the highest variability in their lichen communities were Repelón and Usiacurí. With respect to q2, Repelón and Usiacurí had significantly higher diversity than Tubará (Figure 4C). Despite Tubará having the lowest coverage, it was the only site where the observed diversity estimates were within the confidence intervals of the asymptotic estimates.

3.3. Beta Diversity

An analysis of the shared and unique species per site (Supplementary S4) based on the beta diversity revealed that Usiacurí had the highest number of unique species (38 ssp.), followed by Repelón (32 spp.), Piojó (nine spp.) and Tubará (five spp.), while nine species (6.7% of the total number of species and 31.5% of samples) were shared among the four sites. The frequency (number of specimens) of these species was as follows: Arthonia rubella (41) and Coniocarpon cinnabarinum (29)—Arthoniaceae; Graphis dendrogramma Nyl., 1877 (11), G. supracola (25) and Leucodecton occultum (Eschw.) Frisch (13)—Graphidaceae; Mazosia carnea (23) and M. viridescens (Fée) Aptroot & M.Cáceres29)—Roccellaceae; Opegraha dekeselii Ertz (8)—Opegraphaceae; and Pyrenula ochraceoflava (29)—Pyrenulaceae. The highest species turnover between Usiacurí and Repelón covered the highest number of species, with 23 species, and these two sites shared 11 species with Piojó.

The ANOSIM showed significant differences among the lichen communities (ANOSIM statistic = 0.07, p-value = 0.002); in particular, Usiacurí and Repelón had the highest similarity, while Tubará had the most dissimilar community compared to the rest of the sites (Figure 5A). The nestedness and spatial turnover components of the beta diversity showed that the differences between Tubará and Usiacurí and between Tubará and Repelón were mostly due to nestedness and less to spatial turnover (Figure 5B). In contrast, the differences between Tubará and Piojó and between Repelón and Usiacurí were mainly due to spatial turnover (Figure 5B).

4. Discussion

Our study contributes to the lichen knowledge of the Atlántico Department, increasing the number of records by approximately 87%. Here, we report, for the first time, an analysis of the diversity of tree lichen communities in both Atlántico and the Caribbean region of Colombia. We found a total of 135 taxa of lichens distributed across 23 families. We also found that the most diverse sites were places away from the ocean, with a larger proportion of forest. Sites like Repelón and Usiacurí were significantly more diverse than Tubará, which is much closer to the ocean and with a smaller proportion of natural cover. Finally, we found significant turnover between Tubará and the other three sites. Interestingly, the differences in species composition between Tubará, Repelón and Usiacurí were mainly due to species loss and less to spatial turnover.

Previous studies have documented the lichen diversity in the Caribbean region of Colombia. For example, Rincón et al. [41] stated that the lichen diversity for Juan de Acosta, another municipality in the Atlántico Department, was only 19 species. Lücking et al. [12] documented 73 lichens in Piojó and Usiacurí, including 57 new records for Atlántico and four species new to science. More recently, Coca [42] identified 42 spp. at four localities in Atlántico (Luruaco, Piojó, Repelón and Suan), of which 17 species corresponded to new records for the department. Finally, our study identified a total of 99 species reported for the first time in the department, which increases the known species richness from 103 spp. to 211 spp.

The genera Allographa, Celothelium, Dichoporis, Diorygma, Eremithallus, Gyalecta, Lacrima, Lecanographa, Nigrovothelium, Sarcographa, Stirtonia, Vigneronia and Zwackhia were found for the first time in Atlántico. Moreover, for the genera Graphis, Opegrapha and Pyrenula, an increase in the number of species was observed, as well as 19 previously undescribed species belonging to the genera Astrothelium (one sp.), Bactrospora (one sp.), Coenogonium (one sp.), Cryptothecia (one sp.) and Hyperphyscia (one sp.). It is noteworthy that the most diverse genera, e.g., Graphis, Opegrapha, Porina, Pyrenula and Pseudopyrenula, exhibited some similarities related to their ascomata, although some of them had perithecial ascomata and others lirelliform ascomata, and, in both cases, these fruiting bodies were partially submerged in the thallus, with continuous walls and black excipula. These characteristics are likely to play a pivotal adaptation role in the resilience of these genera to the high water stress conditions in the TDF and the related implications [21].

The comparison of the species richness in our study to that reported by Lücking et al. [12] and Coca [42] showed some differences. For Usiacurí, Lücking et al. [12] found 42 spp. of lichen, including two undescribed species; meanwhile, in this study, we found 110, of which 86 were new records for the area and six of them may be undescribed species. In Repelón, 93 spp. were collected; 87 spp. were new records and five were undescribed species; in contrast, Coca [42] reported only 23 spp. Finally, 63 species had been recorded for Piojó, but, with this study, this is increased to 109 spp., including a possible species that is undescribed. For Tubará, we found no previous studies in the area with which to compare our inventory.

The species diversity was significantly higher in Repelón and Usiacurí compared to Tubará and Piojó when measured using the Shannon and Simpson indices. This difference is in part explained by the differences in species richness. However, the asymptotic estimators for q1 at Tubará and Piojó are approximately half of the q0 asymptotic estimator. This suggests that there are few species that account for most of the samples collected and they were collected in many trees. In contrast, for Repelón and Usiacurí, the asymptotic estimators of q1 are similar to those of q0. In this case, the samples were more evenly distributed across species, with all species having roughly the same abundance. This could be explained by differences in the selective pressures among the sites. When the selective pressures are stronger, it is expected that few species will dominate the communities, while, in places where the selective pressures are relaxed, the abundance is more evenly distributed across species, as in Repelón and Usiacurí [43]. The differences in the selective pressures in this case may be attributed to differences in salinity, humidity and human intervention [44].

Interestingly, the patterns of beta diversity provide some insights into the mechanisms driving the differences in the community composition among the sites. First, we found that the turnover between Repelón and Usiacurí was the lowest and was mainly attributed to taxonomic turnover rather than nestedness. This suggests that these sites are similar in environmental conditions, the selective pressures are similar and the turnover is simply a result of stochastic species replacement. Second, the differences between Repelón and Usiacurí and Tubará are large, but, in this case, the turnover is mostly attributed to nestedness. This may suggest the intensification of the selective pressures, potentially due to the higher salinity in Tubará compared to the other two localities. Third, we also found high turnover between Tubará and Piojó, but, in this case, the turnover was almost entirely attributed to species turnover. This suggests that these two sites are radically different in environmental conditions, including human alterations. Previous studies have also shown high heterogeneity in lichen and flora in tropical dry forests with varying levels of human intervention [45,46].

A closer analysis of the turnover patterns using shared and unique species reinforces the idea that the sites in the Atlántico Department may be different in their environmental conditions and human intervention, which leads to different sets of species. While Usiacurí and Repelón share the highest number of species (53 spp.), Usiacurí shares only 24 species with Piojó and eight spp. with Tubará. Furthermore, Repelón shares 30 species with Piojó and 17 with Tubará, while Piojó shares nine species with Tubará. For instance, Tubará features a shrub-dominated forest and is closer to the Caribbean Sea than the other sites, resulting in soil conditions with high salinity, strong winds and some smooth-barked phorophytes. Of the species documented in Tubará, one constitutes a new record for Colombia (Pyrenula mastophoriza Nyl. Zahlbr), two are new records for Atlántico (Opegrapha aperiens Vain. and Pyrenula minor Fée), and one is a possible new species for science in the genus Opegrapha. Notably, some of these lichen species have been previously reported in tropical coastal areas on the islands of Fiji and Martinique [47,48].

5. Conclusions

This study provides the first record of tree lichen community diversity indices for the Atlántico Department and the Colombian Caribbean region. The findings reveal the high diversity of lichens in the studied TDFs, with a significant number of taxa possibly undescribed to science. The observed differences in species composition among the sites could be influenced by the habitat heterogeneity and anthropogenic pressures on lichen communities. The most diverse sites were Repelón and Usiacurí, in contrast to Tubará, which showed less diversity. The differences in species composition between Tubará and the other sites are primarily due to species loss, likely influenced by the varying habitat conditions and human impacts. Nonetheless, we emphasize that there is still much to learn about the lichen diversity in these forests. Further research is needed with more exhaustive sampling that includes other substrates and parts of the trees, such as branches and leaves, to have a more complete picture of the total TDF lichen diversity in the Colombian Caribbean region. Despite this, the results emphasize the need for conservation strategies to protect these ecologically valuable lichen communities within the TDFs of Atlántico.