Species Richness,Functional And Phylogenetic Diversity Of Plant Communities Along The Altitudinal Gradients Of Mount Kenya,East Africa

The spatial pattern of biodiversity has always been the focus of ecologists.The combined use of species richness,functional diversity and phylogenetic diversity can systematically reveal the diversity pattern of a region.Leaf stoichiometry and carbon isotope have also been used by ecologists as proxies of assessing the physiological adaptations such as water use efficiency of plants to different environmental conditions.Mount Kenya is the second highest peak in Africa and also a key ecological protected site of the world.This mountain is also a habitat to many endemic plant species such as Dendrosenecio keniensis and Lobelia gregoriana.Several studies on species diversity studies that have been conducted in this mountain have failed to combine species richness,functional and phylogenetic diversity,thus failing to provide reliable community structure information.Similarly,leaf stoichiometric and leaf carbon isotope studies for endemic species along their full altitudinal range in this mountain are lacking.This research addressed these knowledge gaps and was categorized into four key areas that provided a wholistic understanding of the community structure in Mount Kenya.(1)The impact of area and environmental heterogeneity on the species richness for the entire mountain was assessed.Where species richness data was retrieved from the collections of the lab,literatures and monographs from previous researchers.The climatic and environmental factors were considered,i.e.,mean annual temperature(MAT),mean annual precipitation(MAP),annual total solar radiation(ATSR),soil factors,soil organic carbon(SOC),soil total nitrogen(STN),soil extractable phosphorous(SEP),soil extractable potassium(SEK)and normalized difference vegetation index(NDVI).The coefficient of variance for the climatic and environmental factors was used to represent the environmental heterogeneity.The results showed that species richness had a skewed hump-shaped pattern,with the highest species richness being at mid-elevation.The heterogeneity of mean annual temperature,soil total nitrogen,soil extractable phosphorous,and annual total solar radiation significantly influenced species richness along the elevation gradient.Area and altitude had a significant correlation with all the environmental variables chosen.The hump shaped species richness pattern can be due to the ecophysiological constraints for example,low temperatures as elevation increases.The high species richness at the mid-elevation is because this zone has a large land area and also acts as transition zone between the extremes of the upper elevation range and lower elevation and species from either side can coexist since the environmental conditions are on the lower and higher limits for the existence of these plant species.(2)Ninety 10×10 m sampling plots were set up along the elevation gradient ranging from3500 to 4300 m.The altitudinal variation in morphological and leaf stoichiometric traits of endemic species Dendrosenecio keniensis and Lobelia gregoriana within their elevation range were assessed.Morphological traits were used,such as plant height,leaf area,leaf thickness,leaf dry weight,specific leaf area and the leaf stoichiometry traits nitrogen,carbon and phosphorous of the two endemic species D.keniensis and L.gregoriana.The relations of these morphological traits to soil organic carbon,soil total nitrogen,soil phosphorous,mean annual temperature,mean annual precipitation,annual total solar radiation,water vapor pressure and the topographic variables aspect,slope and hill shade along the elevation gradient were further explored.It was found that D.keniensis had wool-like pubescent leaves,while L.gregoriana had mucilage packed succulent and waxy cuticle leaves to avoid freezing.Both species exhibited reduced metabolic rates as shown by the low leaf phosphorous content.The results showed that changes in morphology and leaf stoichiometry were determined by a combination of climate,soil and topographic variables that change along elevation on Mount Kenya.The observed variations in the morphological and leaf stoichiometric traits of D.keniensis and L.gregoriana,which grow side by side along the elevation gradient,were interpreted as adaptation to the harsh environmental conditions.(3)The altitudinal variation of leaf carbon isotope for the two endemic species within their altitudinal range was studied.The same plots data of the previous work was used.The leaf?¹³C of the two species was measured using Delta V Advantage stable isotope ratio mass spectrometer.The results revealed that there was a leaf?¹³C enrichment of 1.76‰km^-1 and 1.61‰km^-1 with altitude for D.keniensis and L.gregoriana respectively.There was a leaf?¹³C enrichment of 0.01‰mm^-1 with mean annual precipitation along the altitude gradient for D.keniensis and 0.02‰mm^-1for L.gregoriana.D.keniensis and L.gregoriana have got high water use efficiency,an adaptation for surviving near freezing alpine temperatures and high diurnal range.Leaf?¹³C exhibited a depletion of-0.37‰per ^oC increase of mean annual temperature along the altitude gradient for D.keniensis and-0.34‰per ^oC increase for L.gregoriana.The results also showed a negative relationship between p CO₂and leaf?¹³C and positive relationship between p CO₂and?¹³C for both species.Low temperatures led to the increase in leaf thickness and specific leaf area for these two species factors that influenced leaf?¹³C and?¹³C.(4)The species richness,functional and phylogenetic diversity in Mount Kenya alpine zone was studied.The functional traits,i.e.,plant height,leaf thickness,leaf area,leaf dry weight,and specific leaf area were considered,for all the species occurring in the 10×10 m replicate sampling plots.The phylogenetic diversity,functional diversity,species richness,trait evolution and phylogenetic signal along the altitudinal gradient were assessed.A decline in species richness with altitudinal increase was found.Altitude affects species richness because it provides a wide range of environmental conditions that influence the plant species differently depending on their ability to adapt and functional plasticity.The results also showed functional convergence for all the traits used an indicator of periodicity and the sharing of ecological niches among the species.There was phylogenetic overdispersion for the plots occurring in the upper alpine zone(3900 to 4300 m)which can be attributed to competitive and adaptive exclusion which is also shown by a decline in functional divergence and functional evenness.There was phylogenetic clustering in the lower alpine zone(3500 to 3800 m)mainly due to presence of generalist species.An increase in functional dispersion and Rao quadratic entropy with altitude was an indicator of new species being recruited into plant communities were functionally similar to the existing ones.This study concludes that species richness patterns in Mount Kenya are correlated with environmental gradients since plant species have different degrees of physiological tolerance to environmental constraints.An assessment of environmental heterogeneity along the altitudinal gradient revealed that wide environmental range sizes favor establishment of generalist species along the altitudinal gradient while narrow or short-range size favors specialist species mainly due to microhabitat creation.The observed trend in leaf stoichiometry,enrichment of leaf?¹³C and reduction of?¹³C in D.keniensis and L.gregoriana alpine plants are also linked to adaptive strategies to the unfavorable alpine environmental conditions.The functional trait convergence and the overdispersed phylogenetic diversity along the altitudinal gradient is an adaptation to the harsh environmental conditions in the alpine zone.