Aims The aim of this paper is to assess morphological and leaf stoichiometric responses of Dendrosenecio keniensis and Lobelia gregoriana to extreme environmental conditions along an elevation gradient in tropical mountains. Methods In this study, we assessed the variation of the morphological traits, including 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. We further explored the relations of these morphological traits to soil organic carbon, soil total nitrogen, soil phosphorous, annual mean temperature, annual mean precipitation, annual total solar radiation, water vapor pressure and the topographic variables aspect, slope and hill shade along the elevation gradient. Ninety 10 m × 10 m sampling plots were set up along the elevation gradient ranging from 3500 to 4300 m. We used 1 km × 1 km grid cells to rasterize our study area in ArcGIS 10.5 for easy access to data pertaining to the climate of each elevation band. We performed linear regression of the morphological and leaf stoichiometric traits with elevation as explanatory variable. We conducted correlation analysis on the morphological and leaf stoichiometric traits with climatic, soil and topographic variables along the elevation gradient. Important Findings Dendrosenecio 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. Our results also 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.

Abiotic factors vary along altitudinal gradients, and this may influence plant morphology, physiology and function. This study aimed to test the hypothesis that leaf δ 13 C-a common proxy for water use efficiency-was indirectly influenced by morphological adjustments with changing climatic factors along an altitudinal gradient on Mount Kenya. We sampled leaves of Dendrosenecio keniensis and Lobelia gregoriana using seventy-two 10 × 10 m plots situated every 100 m starting from 3600 to 4300 m. We determined leaf δ 13 C using stable isotope mass spectrometry. We also quantified the following morphological factors; leaf area, leaf mass per area, specific leaf area and leaf thickness. Climate data included mean annual temperature and precipitation, diurnal temperature range and water vapor pressure. Our results revealed that there was a leaf δ 13 C enrichment of 1.76 ‰ km −1 and 1.62 ‰ km −1 with altitude for D. keniensis and L. gregoriana, respectively. Leaf δ 13 C was enrichment by 0.01 ‰ mm −1 with mean annual precipitation along the altitude gradient for D. keniensis and 0.02 ‰ mm −1 for L. gregoriana. D. keniensis and L. gregoriana have high-water use efficiency, an adaptation for surviving near freezing alpine temperatures and high-diurnal range. Leaf δ 13 C exhibited a depletion of −0.37 ‰ per °C increase of mean annual temperature along the altitude gradient for D. keniensis and −0.34 ‰ per °C increase for L. gregoriana.Our results also showed a negative relationship between pCO 2 and leaf δ 13 C and positive relationship between pCO 2 and ∆ 13 C for both species. Low temperatures led to the increase in leaf thickness and specific leaf area for these two species, factors that influenced leaf δ 13 C and ∆ 13 C.