DIVERSITY AND DISTRIBUTION OF VASCULAR CRYPTOGAMS IN RELATION TO ELEVATION GRADIENT IN OSOMBA RANGE OF THE CROSS

Vascular cryptogams also known as Pteridophytes have been found to colonize the lower altitudes of world’s tropical forest regions. Hence, a study to establish the relationship existing between elevation gradient and Pteridophytes heterogeneity and distribution within Osomba Range, Cross River State, Nigeria was carried out. In this study, we delineated four elevation classes and enumerated the vascular cryptogam communities found at each level within the range using standard scientific protocol. Shannon-Wiener's (3.434 and 2.833) and Simpson’s indices of diversity (0.968 and 0.941) as well as Taxa (31 and 17) assumed a dump bell pattern with peak values associated with the mid-elevation classes (178 -205m and 232 – 240m) respectively. On the contrary, dominance followed an inverted dump bell distribution with its optimum (0.1667 and 0.2500) skewed towards the extreme elevation classes (297 – 295 m and 347-405 m). Correlation analysis imprinted a significant (P<0.05) positive relationship between altitude and dominance (0.867*) but an inverse relationship between elevation and Simpson diversity (-0.867*). 178-205 m and 232-240 m elevation classes were more comparable with lesser species heterogeneity judging from Bray and Curtis (0.458), Simpson (0.647) and Jaccard (0.297) similarity indices respectively. Conclusively, forty-nine (49) species of vascular cryptogams were identified and their occurrence seemed patchy in distribution along the gradient. This is an indication that elevation is an important factor amongst the suite of ecological drivers which determine the diversity of this critical group of plants. Further investigation on individual species response to ecological gradient is recommended as this will enhance the conservation of these vascular cryptogams within and outside the study area.


INTRODUCTION
Ferns and fern allies otherwise referred to as vascular cryptogams are a unique group of plants which are lower plants and are valuable to man in various ways and find wide application as food, feed, indicators of environmental pollution, pesticide formulation alternatives, bioremediation agents, therapeutic remedies, ornaments, cultural craft components and others.Yet in spite of this, not enough research attention is given to these group of plants compared to spermatophytes within and around the study area.Even on a national level, the diversity of vascular cryptogams across the different ecological zones is not fully covered and the ecological drivers of their distribution is poorly documented.Though it has been established that species richness and distribution tend to align and vary seemingly along environmental gradients in different vegetation formations within the tropics, particularly in the tropical rain forest zones (Ubom et al., 2012;Anwana et al., 2019;Mbong et al., 2020), the specific patterns of species richness variation are still unknown for some locations and vegetation typologies (Tuonisto and Ruokolainen 2002).Notwithstanding, it is very typical to assume that abiotic factors within the environment are being implicated as ecological drivers of diversity and distribution of vegetation assemblages in different areas (Duque et al., 2001;Grandez et al., 2001;Romero-Saltos et al., 2001).Osomba range is a part of the Oban division of Cross River National Park (a protected area) within Akamkpa Local Government of Cross River State.Currently from a reconnaissance survey, Osomba range bears a part of rainforest forest vegetation physiognomy shaped by human disturbances (forest fragmentation, intense agricultural activities and bush burning) and natural influences.
The dominant tree species and their families have been reported for Cross River National Park and its neighboring buffer zones (Aigbe and Omokhua, 2015;Essien et al., 2020).However, these studies only focused on woody tree species leaving out the pteridophytes and lycophytes within the protected area.Also, Nwaka et al., (2022) in their research on pteridophytes diversity of the park only reported six (6) epiphytic ferns in the area and attributed this low diversity to anthropogenic factors.Thus, the scanty literature on pteridophytes and lycophytes within this all-important protected area shows the knowledge gaps on the extant families and species of the park.More so, is the lack of studies on the effect of altitudinal gradient on Pteridophytes and Lycophytes flora.Against this backdrop, our research objectives were to; (1) characterize extant pteridophytes presence within the Osomba mountain range, Cross River National Park (Oban division) and (2) evaluate the effect of elevation on pteridophyte diversity within the study area.Giving that plant environments are rarely homogeneous, especially across spatial and temporal scales, it is therefore logical to query how and to what extent elevation influences the distribution of vascular cryptogams within the study area.Answers to this question we opine are necessary to deepen understanding on ways environmental parameters shapes species composition and distribution, which could be used to enhance their management and conservation.

Study Area
Oban Division protected area situates within longitude 8°20′ E and 8°55′ E and latitudes 5°00′ N and 6°00′ N. The area extends up to 742.55km 2 with rugged and undulating slopes.
Its elevation rises gradually from low river valleys and extends beyond 1,000 meters in its mountainous parts (Jimoh et al., 2012).The area is characterized by a mosaic hilly patches ranging from 100 to over 1,000m.The area records mean monthly relative humidity ranging between 78% in the dry months up to 91% in the wet months and temperatures are generally high all year round with an average of 27 o C. The area records an annual rainfall ranging between 2,500mm-3,000mm (Schmitt, 1996;Aigbe and Omkhua, 2015).Osomba is one of the host communities to the Cross River National Park (hereafter referred to as CRNP), Oban division, located at the foot of the range with a peri-urban population consisting predominantly of farmers, hunters and gatherers.

Vegetation Sampling
Vegetation was sampled using systematic sampling of Ubom et al., (2012).Peculiar to each elevation class, species were sampled in 50 m transect consisting of five (5) 5 m x 5 m quadrats spaced at regular intervals of 5 m.In each quadrat, plants were enumerated and species were properly identified to the species level.Identification of the collected plants were done using Alston (1959), Stanfield andLowe (1979) and Plants of the World Online (POWO,2020).Voucher specimens of collected samples were deposited in the University of Uyo herbarium (UUYH) for storage.Species distribution was compiled using presence-absence scores for each elevation class and recorded in a field note book.Diversity, Dominance, and the Similarity indices were computed with the Paleontological software (PAST) version 17 (Hammer et al., 2001) according to the methods of Mbong et al. (2020).

Diversity indices
The Shannon -Weiner index (H) was calculated for each elevation level using the formula.

Statistical Analysis
Based on the elevation classes, Data on taxa, diversity and dominance values were presented using descriptive statistics.Also, Two-way analysis of variance will be carried out to draw inference on significant differences on species distribution across the distinct elevation groups according to the methods of Ubom (2006) and Okon et. al. (2019).Pearson correlation coefficients were computed to estimate Vascular cryptogams response or association with topographic heterogeneity using the methods of Mbong et al. (2022).The inferential statistics (ANOVA) and correlation coefficients were computed using Statistical package for Social Sciences (SPSS, Version 20.0;IBM Corporation, Armonk USA).Similarity indices for comparing the beta diversity between the plots were computed using Paleontological software (Hammer et al., 2001)
Species dominance was highest within the topmost elevation class 347-405 m (0.250) but least within the 178-205 m elevation class.Simpson diversity (0.9677 and 0.75), Shannon-Weiner (3.434 and1.386),Brillion (2.519 and 0.7945), Menhinick (5.568 and2.000) and Margelef (8.736 and 2.164) indices recorded higher values within the 178-205 m elevation class and least at the 347-405 m respectively.Tables 3, 4 and 5 reflect an overlap in terms of taxa similarities across elevation gradients.But common to these matrices, 297-295 m elevation recorded no taxa similarity features with 170-175 m and 347-405 m elevation classes respectively.Table 6 presents a matrix revealing relationships established based on taxa population and distribution indices in relation to elevation gradient.Clearly, the diversity (Shannon = 0.753*) of vascular cryptogams significantly (P<0.05)decreased with increasing elevation while dominance (0.867*) increased significantly with increased elevation within the range.

Discussion
Typically, the present study indicated the presence of 49 species of ferns and fern allies within the Osomba range.This value is high compared to those reported for other ecosystems in previous related studies within and outside Nigeria.For instance, while Bassey ( 2013 Banaticla and Buot, (2005) noted 36 in Luzon Island, Philippines.On the other hand, this value is low compared to 149 species reported by Tuonisto et al. (2002) in Yasuni National Park, Amazonia and 86 species reported by Sureshkumar et al. (2019) Kolli Hills of the Eastern Ghats, India.From the results, there is evidence of species selective response and sensitivity to elevation gradients in Osomba range.This is predicated upon the theoretical concept that explains the pattern of species richness and diversity on biotic and abiotic interactive factors, which include competition, historical or evolutional development, species phenological and adaptation dynamics, environmental gradient, and the nature and intensity of human activities (Zobel, 1997).Specifically, previous studies have confirmed that the abundance and distribution of plants including vascular cryptogams have been shown to oscillate with environmental factors including soil physicochemistry, Temperature, relative humidity, elevation, precipitation in different ecosystems (Zobel, 1997;Tuonisto and Ruokolainen, 2002;Banaticla and Buot, 2005;Mbong et al., 2020).This is clearly exemplified in this research since taxa, dominance and diversity indices differed significantly (P<0.05) between the six elevation classes Notably, a Shannon-Wiener Index (H) value of 3.434 reported for vascular cryptogams in this result compares favorably with H value of 3.795 which was recorded by Aibe and Omokhua, (2015) for tree species in the same ecosystem.This indicates the fact that ferns and fern allies contribute significantly to the ecosystem structure and functions of Osomba range of the CRNP.Obviously, the diversity indices also give an idea of the level of competition going on between species across the elevation classes.Also, as expected, the dominance values of elevation categories fluctuate clearly but inversely with the species richness and diversity indices hence elevation classes with high species diversity synonymously record low Simpson dominance value.Similar observations had been reported for other field studies (Ogbemudia et al., 2014;Anwana et al., 2020).Also, the similarity coefficients also give a basis for the measurement of beta diversity at different altitude levels of the range.Also, while some plots are grossly similar in terms of taxa composition, a coefficient of zero similarity coefficients for specific pair of classes indicates that the areas are not similar.
Again, the overwhelming and high similarity coefficients from the Bray and Curtis, Jaccard and Simpson similarity indices between paired groups is an indication of strong species overlap due to a continuum in vegetation space with the elevation classes seemingly existing as arbitrary but not true boundaries.This further emphasize the distribution of species such Bolbitis achrostichoides, Bolbitis auriculata, Tectaria fimbriata, Triplophyllum varians and Triplophyllum vogelli which are found on both low and extreme altitude transects.Remarkably, species richness on elevation gradient have been studied widely in different regions and ecosystems but from these, two correlational distribution models have been noted (Sureshkumar et al., 2019).Hence the distribution of plants along environmental gradient either follow a humped (dumb-bell) or monotonic distribution pattern.
The distribution of taxa and the species diversity on the criteria of elevation in Osomba range grossly aligns to the "humped distribution pattern".In line with this taxa, species diversity and richness indices intensify and assume higher values within the mid elevation classes (178 -240m) leaving moderate to low case intensities and observation beyond this range on both sides.This validates the findings of Rahbek (1995) and Grytnes and Beaman (2006).In favor of this pattern, Carpenter (2005) noted that up to about 49% of empirical studies relating plant diversity to elevation gradient as observed from different vegetation assemblages in different parts of the world followed a humped distribution form (Aldasoro et al., 2004;Kluge et al., 2008;Marini et al., 2010).The marked progressive decline beyond the 178-240 elevation classes is noted and well understood.This pattern synchronizes with the observation of Vazquez and Givnish (1998) who recorded progressive monotonic decline in species richness with persisting increase in elevation gradient.From this study, 178 -240m shows the elevation optimum for species diversity and richness beyond which there is progressive decrease in both taxa and diversity.Clearly, this decline gives room to increasing dominance by few elevations' tolerant species.The preferential exclusion of most species beyond the elevation optimum further confirms the postulations of Shellford's law of tolerance (Allabay, 2015) while species dominance within this transect lends credence to the Darwinian evolution theory.Clearly, these observations may be explained on the premise of environmental challenges and species tolerance limits (Spasojevic et al., 2014).Possibly beyond the elevation optimum there may exist unfavourable environmental conditions such as low relative humidity, rapid leaching of soil nutrients along steep slopes, reduced canopy cover, intense solar radiation and strong dry wind which serve as environmental challenges.Hence, the species with most efficient adaptive strategies thrives, reproduce and dominate within the seemingly difficult habitat conditions associated with the extreme elevation classes to the exclusion of other less adaptive species.
In a related study, Tuonisto and Poulsen (2000) noted that the amount of topographic heterogeneity within transects correlated strongly and significantly even at local scales with density of individuals, species richness indices even at the local scale.In this study, since transects where altitude biased, it is logical not to undermine the strong significant correlation coefficients between the elevation classes, Dominance (r=0.867* ) and diversity (r=-0.867* ) the elevation classes as seen from the correlation matrix.On the whole the matrix interprets that unrestricted progression in elevation gradient within the range unilaterally reduces vascular cryptogams richness and diversity along the range and increase the dominance trend within the extreme elevation classes.This is concurrent with the views of previous researchers (e.g., Vazquez and Givnish 1998).

CONCLUSION
With reference to the diversity and distribution of ferns and fern allies in Osomba mountain range, varied distinct patterns were found in association to topography.Clearly the area has a rich diversity for these species.Taxa seemed to increase steadily from the low lying terrain towards the mid elevation classes (170-240 m) where peak value for taxa was achieved.Beyond this ambience taxa showed a monotonous pattern, with sharp decline in diversity.The topographic heterogeneity at the intermediate levels secured an ecological optimum for Shannon-Weiner, Simpson, Bruilion, Margalef and Menhinick indices of diversity and richness.Also, Bray and Curtis, Simpson and Jaccard beta diversity indices indicated similarities which showed an overlap in taxa distribution at different elevation intensities with exceptions at 297-295m that had no similarities with 170-175m and 347-405m.This overlap is more obvious and prominent judging from the relative frequency scores of species including those of, Bolbitis achrostichoides, Bolbitis auriculata, Lomariopsis, gguuineensis, Oleandra distenta, Tectaria fimbriata, Triplophyllum varians Triplophyllum vogelli, Pteridium aquilinum and Pteris tripartita.From this research we conclude that topographic heterogeneity is a complex gradient which clearly influences the diversity and distribution of vascular cryptogams across the range.Further investigation on individual species response to ecological gradient is recommended as this will enhance the conservation of these ferns and other associated species within and outside the study area.
/N) Log1 (ni/N) where: ni = number of individuals of each species N = total number of individuals of all species The Simpson index of dominance (D) was calculated as follows: D = (n1/N) 2 Where n1 = number of individuals of each species N = total number of individuals of each species.Also, the Simpsons diversity was calculated as 1-D.

Table 1 :
Distribution of Vascular Cryptogams in relation to Elevation Gradient in the Study Area

Table 5 :
Simpson's Similarity Index of Elevation classes in the Study Area

Table 6 :
Correlating Elevation gradient and Pteridophyte community indices in the Study Area