Altitudinal variation of leaf carbon isotope for Dendrosenecio keniensis and Lobelia gregoriana in Mount Kenya alpine zone

Abiotic factors vary along altitudinal gradients, and this may influence
plant morphology, physiology and function. This study aimed to test the
hypothesis that leaf δ13C,  a common proxy for water use
efficiency, was indirectly influenced by morphological
adjustments with changing climatic factors along an altitudinal gradient
on Mt. 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 δ13C 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 δ13C enrichment of 1.76 ‰km-1 and 1.62‰ km-1 with
altitude for D. keniensis and L. gregoriana respectively. Leaf δ13C 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
δ13C exhibited a depletion of -0.37‰ per o C increase of mean annual
temperature along the altitude gradient for D. keniensis and -0.34‰ per o
C increase for L. gregoriana. Our results also showed a negative
relationship between pCO2 and leaf δ13C and positive relationship between
pCO2 and ∆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 δ13C and ∆13C.