Materials and methods

Study area. This study was carried out in the upper basin of “La Antigua” river in the central part of Veracruz State in Mexico. The weather is temperate and humid with a mean annual temperature of 18 °C and total annual precipitation that varies from 1,500 to 2,000 mm. The original vegetation was tropical montane cloud forest, in which the most common species of woody plants were Quercus lancifolia, Clethra macrophylla, Liquidambar styraciflua, Ilex discolor var. tolucana, Styrax glabrescens, Zanthoxylum sp. and Prunus rhamnoides (^{Williams-Linera 2012}). The sites selected for vegetation sampling were located within 19° 22’ 05” and 19° 32’ 31” latitude N and 96° 57’ 31” and 97° 06’ 08” longitude W (Figure 1) and ranged in elevation from 1,100 up to 1,800 m asl. Sampling sites corresponded to riparian forested belts that are part of anthropic landscapes in which cattle-raising pastures predominate. In this study, we defined forested riparian belts as the arboreal vegetation that grows on both sides of a river and that in our study area are usually 3 to 5 m wide in each riverbank.

Vegetation sampling. To determine community attributes and floristic composition of forested riparian belts, a total of 14 sampling sites were selected with a minimum separation of 1 km and a maximum of 18 km. Selected sites were 300 to 500 m long sections of the river (mean length of 400 m) having woody vegetation on both riverbanks. Belt transects modified from ^{Gentry (1982)} were placed along both riverbanks aligning its longest dimension parallel to the river. Six 50 × 2 m transects were placed at each selected site (three at each river side; Figure 1), for a total of 84 transects. Every plant rooted within each transect and having a total height ≥ 1.5 m was identified and measured (total height and diameter at breast height). Height was estimated with the help of a 6 m long post, graduated every 10 cm and for trees > 6 m an Abney inclinometer was used. Diameter at breast height (DBH) was measured with a diametric tape (in mm) at 1.3 m from the ground in trees and at the trunk base in shrubs. Additionally, the proportion of forest canopy cover was estimated at two sites within each transect using a spherical canopy densiometer. Herbaceous plants and other growth forms (e.g., palms, ferns) taller than 1.5 m were also recorded.

Taxonomic determination was based on the Flora de Veracruz (^{Sosa & Gómez-Pompa 1994}) and nomenclature on TROPICOS web site (Tropicos.org). Botanical specimens were deposited in the XAL-herbarium from the Instituto de Ecología, AC. Some collected specimens during field-work had no flower nor fruit, and for many of them botanist experts on the flora of Veracruz were able to identify them to genus or family level, being impossible to determine its species name.

Data analysis. Sampling completeness based on Hill numbers was assessed using the software iNEXT (iNterpolation and EXTrapolation; ^{Hsieh et al. 2016}), estimating the individual-based species accumulated curve of the 14 riparian belts sampled. Diversity profiles were drawn for each riparian belt (n = 6 transects/belt), estimating Hill numbers (q0, q1, q2) per belt, expressed in units of effective number of species (^{Chao et al. 2014}) for all species (q0 = observed richness), for typical species (q1 = Shannon diversity) and for very abundant species (q2 = Simpson diversity). The importance value index (IVI) for each species was estimated by combining its relative abundance, relative frequency and relative basal area recorded in all 84 transects. To compare species composition among the 14 riparian belts the Jaccard distance or dissimilarity (^{Jost et al. 2011}) was estimated between each pair of belts using incidence data. Jaccard distance varies from 0 (i.e., identical composition) to 1 (i.e., no shared species between sites).

Additionally, a multivariate ordination by canonical correspondence analysis (CCA) was used to summarize the spatial variation in floristic composition amongst the 14 belts and to explore the environmental factors that could explain the detected variation. The CCA and permutation tests (to assess the statistical significance of the CCA ordination axis) were run in the software R v. 3.4.3 (^{R Core Team 2017}) using the ‘vegan’ package (^{Oksanen et al. 2019}).

Remote sense images from Google Earth Pro v. 7.3.2 (year 2017; 2.5 m/pixel resolution) were used to estimate different landscape attributes or metrics of the sampled riparian belts and its surroundings. These images were processed in ArcGis v 10.4.1 as follows: a central point was defined for each tract of sampled riparian belt as the centroid of the 6 geo-referenced (with a Garmin-GPS) vegetation transects placed at each belt; then a circular area with 250 m radius centered on this centroid was defined; and finally, within each circular area three land cover categories were distinguished and their areas estimated in hectares: a) area covered by forest, b) area covered by agriculture (i.e., non-forest cover with open pastures or crop-fields), and c) area covered by urban or rural settlements including roads and streets. Each of these three areas was used as indicators of human disturbance in the vicinity of the sampled belts and were incorporated as environmental variables in the CCA ordination. Other environmental variables included in the CCA were the average value of arboreal canopy cover along the sampled belt estimated with the canopy densiometer; the elevation m asl of the centroid of each sampled belt; the distance to the nearest town and finally the distance to one of the 14 sampled belts located in the SW corner of the study area (shown in Figure 1 as the TL site). The latter was done to assess if proximity between sampling sites was related with similarity in composition or not. All environmental variables used in the CCA ordination are shown for each riparian belt in Supplementary material, Table S1. Only non-auto-correlated environmental variables were included in the CCA. The abundance matrix data in the CCA only included tree and shrub species identified at least to the genus level. The distinct species of Solanum that we recorded were grouped into a single category: Solanum spp. and the same was done for Piper spp. The latter was a consequence of the difficulty in identifying sterile individuals of these genera in the field.

Results

Total sampling effort amounted to 8,400 m² in the 84 transects, where a total of 2,062 plants were recorded and they belonged to 161 species, from 102 genera and 55 families (Table S2). The individual-based species accumulation curve pooling all transects showed that overall sampling effort reached 98 % of the estimated species richness (Figure S1). Families with the highest number of species were Asteraceae (18 spp.), Solanaceae (13), Rubiaceae (10), Fabaceae (9), Piperaceae and Melastomataceae (8 spp., each). The richest genera were Piper and Solanum (8 spp., each), Clethra (6), Quercus (5), Cestrum, Hoffmannia, Miconia and Oreopanax (each with 4 spp.). Of all plant species 66 were trees and 65 shrubs, the rest (19 % of total richness) had different growth forms, including herbs, palms and ferns. Regarding their dispersal syndrome, 112 species (69 %) were zoochorous (i.e., animal dispersed), 34 species (20 %) were anemochorous (wind-dispersed), and the remaining 15 species (11 %) had other dispersal syndromes (Figure 2).

The species with the highest IVI was Platanus mexicana with a value (0.95) much higher than any of the other species (IVI < 0.09), in great part due to the very large size of their trees (DBH > 1 m) and also because it was present in most transects and was very abundant (Table 1). Thus, we regard this tree species as over-dominant in the sampled riparian belts. The next most important species had IVI values that ranged between 0.086 and 0.040; in descending order, these species were Liquidambar styraciflua, Palicourea padifolia, Styrax glabrescens, Perrottetia longistylis, Alnus acuminata, Miconia minutiflora, Piper auritum, P. hispidum, Conostegia arborea, Clethra sp. and Meliosma alba (Figure 3).

Community attributes. More than 80 % of recorded plants ranged in size from 1.5 to 5 m tall (Figure 4A), while 70 % of all plants had a DBH smaller than 10 cm (Figure 4B). The tallest individual was a P. mexicana tree with 41 m recorded in the GR site and the one with the largest DBH (257 cm), was another tree recorded in the AB site (Table 2). The overall average of plant height was 6.3 ± 7.0 (s.d.) m and overall DBH average was 14.8 ± 29.1 cm. Vegetation physiognomy and plant sizes varied widely between as well as within riparian belts. Average plant height per sampled site varied from 4.4 ± 4.6 m in the AB site to 12.4 ± 11.8 m in TR. Average DBH per site varied from 7.9 ± 17.8 cm in AF up to 33.5 ± 43.8 cm in the TR site (Table 2).

Overall basal area adding the 14 riparian belts amounted to 190 m² in 0.84 ha of sampling area (i.e., 223.5 m²/ha). The TL site had the highest basal area with 27.9 m², while the lowest value was 2.6 m² recorded in the MG site. The percent proportion of tree canopy cover within the riparian belts, had a global average of 80 %, being the densest belt AF with 89 % and the least dense was PD with 72 % (Table 2). Plant abundance also varied widely, varying from only 79 plants in the LM and ET sites up to 327 plants in AF, with an overall average of 147 ± 68 plants per site (i.e., 2,450 plants/ha). Species richness per site varied from 22 species in MA and also in TR, up to 55 species in AF, with an average richness of 37 ± 9 species per site. Diversity profiles for each riparian belt, showed that the AF and TF sites were not only the richest in observed species (i.e., q0 = 55 and 49 species, respectively), but also in the number of typical species (Shannon diversity; q1 > 30 spp.) and also of very abundant species (Simpson diversity; q2 > 20 spp.). Whereas the TR belt was the site with the lowest number of typical (q1 = 12) and very abundant (q2 = 7) species of all sampled belts (Figure S2).

Variation in floristic composition. The Jaccard distance index showed that the highest dissimilarity was recorded between the MA and BA belts and also between VH and TR, being 0.92 in both comparisons, sharing only 4 and 5 species, respectively. While the least dissimilar sites were AF and TF with 0.61 in Jaccard distance, sharing 29 species. Overall dissimilarity between the sampled riparian belts was very high surpassing 0.7 in Jaccard distance between most paired comparisons (Table 3).

The CCA ordination of the 14 riparian belts in the species abundance space summarized 46 % of variation in floristic composition along the two most important ordination axis (i.e., eigenvalues for CCA-axis 1 = 0.36 and for axis 2 = 0.31). CCA scores for each sampled belt along axis 1 were significantly and positively related with site elevation (F = 1.95; P < 0.003), while those of axis 2 were significantly related (F = 1.65; P < 0.001) with forest cover area within 250 m around the sampled site and also with distance between sites. Riparian belts at highest elevation (MG, VH, TM; higher than 1,650 m asl.) were grouped towards the right part of the CCA plot (i.e., high positive values for axis 1), while those at lower elevations (MA, TR, ME, GR) were grouped towards the left of the graph (Figure 5). Most sites having less than 10 ha of forest cover within 250 m around them (RM, PD, TL and MG) had high negative values along CCA axis 2 (lower part of graph), while sites located in areas with higher forest cover in their surroundings and thus in less disturbed areas (MA, AF, GR, AB) had positive values along this axis (upper part of the graph).

The separation between sampling sites as denoted by the distance of each sampled belt to the TL belt (see Methods) also had a strong influence in the CCA results, since those sites far away from TL and located towards the NE corner of our map (Figure 1) were grouped relatively close together in the upper left corner of the CCA graph (Figure 5), while sites close to TL and located towards the SW corner of our map, were grouped towards the lower right corner of the CCA graph, albeit there was a relatively high floristic variation among them as revealed by the wide spread in their placement within the CCA graph. Other site variables related with the CCA ordination of riparian belts, were the distance to the nearest town or settlement and the amount of urban area within 250 m of the sampled site, albeit their relationship with the CCA scores (i.e., axis values) was much lower than that of the previous three variables (as shown by the length of their vectors in the CCA graph of Figure 5). The rest of the environmental variables measured for each sampled site (see Methods) was not related with the floristic variation summarized in the CCA graph or were auto correlated with at least one of the previous five variables mentioned.

Discussion

The recorded plant richness (161 species) in the 14 riparian belts sampled represents 2.4 % of total floristic richness for the national inventory of Mexican cloud forest (^{Villaseñor 2010}), and 5.0 % of total richness for the cloud forest of the Veracruz State (^{Villaseñor & Ortiz 2017}). For the central part of Veracruz in relatively well-preserved areas of cloud forest, ^{García-Franco et al. (2008)} found 67 tree species and 35 shrub species in 0.3 ha of total sampling area. While for the same area but within cloud forest remnant fragments of 1.2 up to 40 ha, ^{Toledo-Aceves et al. (2014)} found 45 tree species in 0.48 of sampling area and ^{Williams-Linera (2002)} found 71 tree and 24 shrub species in 0.7 ha of sampling area. In other states of the country with cloud forest, different studies have reported between 76 and 121 tree species and 59 to 151 shrub species (^{Mayorga et al. 1998}, ^{Alcántara-Ayala & Luna-Vega 2001}, ^{Cartujano et al. 2002}), reaching 300 or more species of woody plants in some regions (^{Ramírez-Marcial 2001}), albeit these studies covered larger areas and were carried out mostly in well preserved cloud forest. For the case of riparian habitats, other studies have found between 34 to 70 tree species and 33 to 49 shrub species in sampling areas ranging from 0.2 up to 1 ha, although these studies were carried out in deciduous oak forest of Morelos (^{Camacho-Rico et al. 2006}) and tropical rain forest in SE Mexico (^{Moreno-Jiménez et al. 2019}). Given that our total sampling effort amounted to less than 1 ha (0.84 ha) and that the 14 riparian belts sampled are narrow habitats completely subjected to intense edge-effects within human disturbed landscapes, the richness that they harbor (66 tree species and 65 shrub species) is remarkable. Even though riparian belts cover a relatively small area within our sampling sites (8 ha pooling the 14 sites) in comparison with open areas under agricultural activities (77 ha, mostly pastures) and the extent of secondary forest (184 ha) within 250 m of our sampling sites, these riparian belts are widespread within the studied landscape and as our results show they concentrate a relatively high density of native species of trees and shrubs.

Vegetation structure and composition of riparian belts. The structural features and floristic composition of the sampled riparian belts shows some similarity with cloud forest remnant fragments of central Veracruz. Total basal area in the 14 sampled riparian belts was 217.8 m²/ha (trees with DBH ≥ 10 cm), which is higher than the values reported by ^{Williams-Linera (2012)} in remnant cloud forest fragments of different sizes (58 to 100 m²/ha). However, the density of trees with DBH ≥ 10 cm in riparian belts (580 trees/ha) was lower than in cloud forest remnant fragments (900 trees/ha; ^{Williams-Linera 2012}). Tree canopy height in cloud forest remnants varies between 25 and 30 m, with some emergent trees reaching up to 40 m, having wide trunks with 1 m of DBH or more, which could be regarded as the giants of these forests (^{Williams-Linera 2002}). Most of the plants recorded in the 14 riparian belts were smaller than 5 m tall, however they were part of the understory, because trees having 20 to 30 m in height were widespread along the riparian belts and many of them had wide trunks (DBH ≥ 1 m). Even though most of the tallest and largest trees in the riparian belts belonged to P. mexicana, we also found very large trees of other species such as M. alba, Quercus lancifolia, Alnus acuminata and Clethra spp.

Plant families with highest number of species in riparian belts were Asteraceae, Solanaceae, Rubiaceae, Piperaceae, Fabaceae and Melastomataceae, which together contribute with 47 % of total richness reported so far for the cloud forest of Mexico (^{Gual-Díaz & Rendón-Correa 2014}). Other important families in Mexican cloud forest are Fagaceae, Clethraceae, Actinidaceae, Lauraceae, Gesneriaceae, Aquifoliaceae, Lamiaceae, Betulaceae, Clusiaceae and Styracaceae (^{Rzedowski 1996}, ^{Gual-Díaz & Rendón-Correa 2014}), all of which were also found in the riparian belts. The genera with most species recorded in our study were Piper, Solanum, Quercus, Clethra, Hoffmannia, Miconia, Oreopanax, Ardisia and Cestrum, many of which correspond to the richest genera in cloud forest of Mexico (^{Rzedowski 1996}, ^{Williams-Linera 2012}, ^{Gual-Díaz & Rendón-Correa 2014}). Trees and shrubs were the richest and most common growth forms recorded in riparian belts, however we also found some species of palms, ferns, herbs and epiphytes that were taller than 1.5 m within our sampled sites, in spite of the high density of cattle and frequent weeding with machete by farmers. However, it is important to remark that our sampling criteria (i.e., plant height > 1.5 m) was not adequate for sampling these latter growth forms, which are usually small plants or grow on top of trees and that could be an important and rich component of cloud forest (^{Rzedowski 1996}, ^{Flores-Palacios & García-Franco 2008}).

As many as 70 % of the plant species that we found in riparian strips have edible fleshy fruit corresponding to the zoochorous dispersal syndrome (i.e., plant species whose seeds are dispersed by frugivorous animals). Riparian belts are elongated and narrow arboreal elements that cross open areas converted into pastures or different types of crop-fields, which farmers left uncut to protect both riverbanks and thus are integrated into agricultural management, but as our results show they are also important reservoirs of plant species that might provide important edible fruit for different forest animals (^{Griscom et al. 2007}). Given that the studied landscape is dominated in extension by open agricultural areas, these narrow arboreal elements along rivers also provide crucial perching sites and movement corridors for different animals, ensuring and enhancing landscape connectivity (^{Pardini et al. 2005}). Within anthropic landscapes, arboreal riparian belts connect forest fragments from upper to lower areas and represent the most important venues for the displacement of forest animals across the landscape. Thus, riparian belts are not only important for the conservation of native species of woody plants but also for forest animals (^{Crome et al. 1994}). In particular, they are important for those animals that found edible fruit or other food sources (i.e., insects associated with tree foliage or their epiphytes) in them, as well as nesting sites, temporal refuge or perching sites in the middle of open areas with scant or null tree cover outside these narrow belts (^{Griscom et al. 2007}). Therefore, riparian belts are also crucial for the maintenance of the ecological interaction between zoochorous woody plants and frugivorous animals, without which forest regeneration is impossible.

The most important or over-dominant species in the sampled riparian belts was the tree P. mexicana, whose I.V.I. was notoriously much higher than that of all the other species. Other important tree species were L. styraciflua, S. glabrescens, P. longistylis, A. acuminata, M. alba and Clethra sp., while important shrub species include Palicourea padifolia, Miconia minutiflora, Piper auritum, P. hispidum and Cojoba arborea. The over dominance of P. mexicana is explained by its high frequency (i.e., recorded in most transects), its high abundance, and mainly due to its impressive basal area in riparian strips, related with the large size of their trunks whose DBH were usually larger than 80 cm. ^{Mayorga et al. (1998)} and ^{Williams-Linera (2012)} mention that P. mexicana and L. styraciflua are tree species strongly associated with riparian zones in cloud forest of Mexico. The latter coupled with the management practice of not cutting the trees along both sides of permanent rivers that cross the plots of farmers or cattle ranchers, explains the over-dominance of the first species and the high importance of the second within the sampled riparian belts. In riparian forest of the state of Puebla, ^{Aguilar-Luna et al. (2018)} have also found P. mexicana as the most important and dominant tree species, together with Alnus acuminata and Quercus rugosa.

Other plant species that are also known to be associated with riparian zones in cloud forest include Deppea grandiflora, Boehmeria caudata, S. glabrescens, and A. acuminata (^{Mayorga et al. 1998}, ^{Gual-Díaz & Rendón-Correa 2014}), all of which were found in this study. Trees of different species of Quercus are also abundant in riparian zones as well as in sites far away from rivers, being one of the most important genus of trees in cloud forest of Mexico and Central America (^{Johnson & Jones 1977}, ^{Granados-Sánchez et al. 2006}, ^{Nur et al. 2008}). Several species of Quercus are exclusive or quasi-exclusive of cloud forest, for example Q. sapotifolia, Q. corrugata and Q. pinnativenulosa, the last one being endemic to Mexico (^{Valencia-A & Gual-Díaz, 2014}). These three Quercus species together with other two more were found in the riparian belts sampled. Additionally, the genus Quercus is regarded as an important functional group, very useful for the restoration of native cloud forest in fragmented landscapes (^{Ramírez-Marcial 2001}, ^{Williams-Linera 2012}, ^{Gual-Díaz & Rendón-Correa 2014}). Lastly, is important to remark that the species of Quercus found in the riparian belts of this study are classified under some risk category or as threatened, together with other tree species such as Persea americana, M. alba, Carpinus caroliniana var. tropicalis, among others. The presence of endemic or threatened species also highlights the importance of riparian belts for the conservation of native species of the cloud forest.

On the other hand, in the riparian belts sampled it was also notorious the presence of numerous species that are favored by disturbance, and that are abundant in large canopy gaps or along forest edges with open areas. Among the latter, we recorded different species of the genera Piper, Solanum, Miconia, Conostegia, Rubus, Cnidoscolus, Telanthophora, Psychotria, Hampea, Trema, Ageratina, Sambucus, Bocconia, Alnus, Hedyosmum and Heliocarpus (^{Hamilton et al. 1994}, ^{Mayorga et al. 1998}, ^{Alcántara-Ayala & Luna-Vega 2001}, ^{Bruijnzeel et al. 2011}, ^{González-Espinosa et al. 2011}, ^{Muñiz-Castro et al. 2012}, ^{Williams-Linera 2012}, ^{Toledo-Aceves et al. 2014}). The early establishment of these pioneer heliophile species favors the arrival, establishment and further growth of intermediate or late successional species such as Saurauia, Myrsine, Liquidambar, Clethra, Quercus, Perrottetia, Cestrum, Turpinia, and Carpinus (^{Nadkarni & Wheelwright 2000}, ^{Muñiz-Castro et al. 2012}). As stated before, arboreal riparian belts crossing agricultural matrices in fragmented landscapes, are exposed to human activities and this intense disturbance is notable in vegetation structure and composition, however, these arboreal elements are formed by trees that were part of the original forest canopy and were left uncut to protect the riverbanks, but also represent sites that provide opportunities for the establishment and growth of late successional tree and shrub species, which explains the heterogeneous mixture of species typical of different successional stages within these belts. Even though riparian belts show clear signs of intense human disturbance they are not poor in forest species, genera and families as our data demonstrate, on the contrary they harbor a notable diversity of native plants of the cloud forest, including L. styraciflua, M. alba, C. tropicalis, Oreopanax xalapensis, as well as species from the genera Quercus, Clethra, Alnus, Prunus and Cinnamomum (^{Mayorga et al. 1998}, ^{Muñiz-Castro et al. 2012}, ^{Gual-Díaz & Rendón-Correa 2014}). Due to their wide distribution in current anthropic landscapes and their richness of woody plants, riparian belts crossing agricultural matrices if managed properly, could represent the most important and accessible source of propagules for the restoration of native cloud forest in agricultural fields.

Spatial variation among riparian belts. Floristic composition varied notoriously as shown by the relatively high values of Jaccard distance (i.e., dissimilarity index) amongst the 14 riparian belts, due to a high spatial heterogeneity and species turnover (i.e., beta diversity). Across the elevation gradient of central Veracruz where the cloud forest is found, a heterogeneous composition has been reported even in short distances along this gradient (^{Rzedowski 1996}, ^{Alcántara-Ayala & Luna-Vega 2001}, ^{Ruiz-Jiménez et al. 2012}, ^{Williams-Linera et al. 2013}), and this explains in part the high spatial heterogeneity that we detected. The multivariate ordination (CCA) showed that sites at the same elevation level had higher floristic similarity amongst them than with sites at different elevation. Also the CCA results showed that the amount of forest cover in the vicinity of the sampled site (i.e., within 250 m) was important, since those sites having less than 10 ha of forest cover around, had higher similarity in composition amongst them and were dissimilar to those sites having more than 10 ha of forest cover around. Additionally, another important factor explaining the spatial variation in floristic composition amongst the 14 riparian belts was their separation and location in a given sub-basin, since those sites closer together and forming part of the same sub-basin had a comparatively higher similarity among them than with riparian sites that were part of a different sub-basin and were more distant. It is important to remark that several other factors also affect the spatial variation in composition of cloud forest and of riparian belts, such as topographic and edaphic differences, natural and anthropic disturbance regimes. The latter varies widely from site to site in accordance with different practices of agricultural management followed by each farmer, including the frequency of cutting woody plants with machete to favor the growth of grasses, cow density and rotation regime, intensity of firewood extraction, among others (^{Williams-Linera 2012}), and all of these influenced the spatial variation in composition that we detected.

The management of riparian belts by each farmer has a strong effect on the spatial heterogeneity in vegetation structure and composition. In the sampled transects we detected a sharp variation in abundance of favored tree or shrub species that were planted by the farmer or that were spared from cutting or weeding. Some farmers are very selective in the species that they prefer as firewood and thus protect and favor these species within their riparian belts (pers. obs. OAHD). Others plant different fruit trees (particularly citric fruits and guava), or highly valuable crops such as coffee shrubs or macadamia nut trees, or lumber trees such as non-native pines. A widespread (almost universal) management practice in the region is to leave uncut only a single line of trees at each river bank, as a result the riparian belts are very narrow, usually less than 5 m from the maximum water level at each riverbank, in order to maximize pasture area and fit in more cows within their properties. The actual width of riparian belts is much less than the requirements of Mexican law (Ley de Aguas Nacionales en Mexico; ^{CONAGUA 1992}), which states that for rivers wider than 5 m, the federal zone at each riverbank should be at least 10 m from the maximum water level and in this zone the native original vegetation must be left untouched; for rivers less than 5 m wide, the width of natural vegetation at each side must be at least 5 m. This law is neither respected nor enforced in our study site or in any other region in Mexico. The mentioned law was written to protect the river and water quality, but it would also have a great positive effect on the biodiversity of forest plants and animals if respected, as our results show, because even when this belts are narrower than the width stated in the law they harbor a notable diversity of native species of woody plants. The role of riparian belts as reservoirs of native plant species and sources of propagules for forest regeneration would be greatly enhanced if the width stated in the law is enforced and also their importance as extra habitat and corridor for forest fauna would be enhanced. Thus, national programs and campaigns to benefit those farmers that respect the law and make wider the riparian belts crossing their properties should be promoted to increase the positive role of arboreal riparian belts in highly fragmented landscapes in order to increase the potential of biodiversity conservation of the cloud forest in transformed landscapes. In particular, the preservation of native flora and fauna of the cloud forest as well as their ecological interactions will be encouraged if riparian belts were of the width stated in the law, this in turn will ensure and enhance landscape connectivity and forest resilience within these anthropic landscapes.

In conclusion, vegetation structure and floristic composition of the arboreal riparian belts that we sampled showed a relatively high similarity with the vegetation of large fragments of cloud forest in Central Veracruz, however in riparian belts the tree Platanus mexicana is over-dominant and this species is absent or extremely rare in sites far away from rivers. Overall the spatial heterogeneity in composition was very high among the river belts sampled, mainly due to differences in the management regime of each belt by farmers, but also due to the high spatial heterogeneity and beta diversity of the original cloud forest, which is still very high in the current anthropic landscape, at least within the riparian belts that cross agricultural areas. This study shows that these arboreal elements that cross the agricultural matrix of the landscape do contain a remarkably high diversity of plant species (161 species 80 % of which are trees and shrubs), many of which are pioneer or secondary species of disturbed sites, but also they have many others that are late-successional or old-growth forest species. These riparian belts not only harbor a high diversity of plants but also offer extra habitat, temporal perching sites and edible resources for animals, which could be directly produced by the plants (i.e., fruits or leaves) or indirect resources associated with their foliage (i.e., insects and other invertebrates), that will not be there if the riparian belts were absent. In particular, frugivorous vertebrates (i.e., birds and bats) that feed on these riparian belts are crucial for seed dispersal and forest regeneration in the fragmented landscape. These riparian belts are arboreal elements of current landscapes already incorporated in the management of pastures and crop-fields but that should be added to conservation plans or programs of the cloud forest of the region, by explicitly recognizing their value in biodiversity preservation and landscape connectivity and thus by implementing a reward system or incentives for those farmers that maintain their riparian belts with a high plant diversity and as wide as the Mexican law states. We regard riparian belts crossing agricultural matrices as a critical arboreal element of the landscape that is crucial for the long-term conservation of the cloud forest, particularly in highly fragmented landscapes in which their presence and proper management would surely enhance forest resilience.

Supplemental data

Environmental variables for each sampled riparian belt used in the CCA analysis (Table S1). Species list of recorded plants with data on life form, dispersal mode, conservation status and I.V.I. (Table S2). Individual-based species accumulation curve and sample coverage for all 14 riparian belts sampled (Figure S1). Individual-based diversity profiles (Hill numbers; q0= observed richness; q1= Shannon diversity; q2= Simpson diversity) for each riparian belt sampled (Figure S2).