Bedrock geology affects foliar nutrient status but has minor influence on leaf carbon isotope discrimination across altitudinal gradients.

Renato Gerdol,Rita Tonin,Paola Iacumin,Janusz Zwiazek

doi:10.1371/journal.pone.0202810

Renato Gerdol, Rita Tonin + Show 2 more

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https://doi.org/10.1371/journal.pone.0202810

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Abstract

Carbon isotope discrimination (Δ13C) in plant leaves generally decreases with increasing altitude in mountains. Lower foliar Δ13C at high elevation usually is associated with higher leaf mass per area (LMA) in thicker leaves. However, it is unclear if lower foliar Δ13C in high-altitude plants is caused by improved photosynthetic capacity as an effect of higher nutrient, especially nitrogen, content in thicker leaves. We investigated trends of foliar Δ13C in four species, each belonging to a different plant functional type (PFT), across two altitudinal gradients, each on a different bedrock type (carbonate and silicate bedrock, respectively) in a region of the southern Alps (Italy) where the foliar Δ13C was not affected by water limitation. Our objective was to assess whether the altitudinal patterns of foliar Δ13C in relation to leaf morphology and foliar nutrients were conditioned by indirect control of bedrock geology on soil nutrient availability. The foliar Δ13C of the four species was mainly affected by LMA and, secondarily, by stomatal density (SD) but the relative importance of these foliar traits varied among species. Area-based nutrient contents had overall minor importance in controlling C discrimination. Relationships among foliar Δ13C, foliar nutrient content and leaf growth rate strongly depended on soil nutrient availability varying differently across the two gradients. In the absence of water limitation, the foliar Δ13C was primarily controlled by irradiance which can shape anatomical leaf traits, especially LMA and/or SD, whose relative importance in determining C isotope discrimination differed among species and/or PFT. Decreasing foliar Δ13C across altitudinal gradients need not be determined by improved photosynthetic capacity deriving from higher nutrient content in thicker leaves.

Highlights

Supporting the general trend of increasing leaf mass per area (LMA) at high altitude, did not observe a parallel increase in carboxylation efficiency with the latter not varying [8], or even decreasing [10, 13] across altitudinal gradients. These findings suggest that, while high-LMA in thick leaves is always associated with high amount of support structures, the fraction of N allocated to functional cell compartments can vary depending on soil nutrient status [14]
The foliar Δ13C was negatively correlated with LMA and its proxy content per unit leaf area (Carea) in all four species independent of bedrock type
The foliar Δ13C presented negative correlations with foliar nutrient contents but such correlation was much stronger for N content per unit leaf area (Narea) than for per unit leaf area (Parea)

Summary

Introduction

Leaves of plants operating at lower pi/pa ratio discriminate less the heavy C isotope (13C) and present lower Δ 13C signature. Leaf mass per area (LMA) in plants from high altitudes generally is greater than in plants from low altitudes in association with thicker mesophyll cell walls, higher mesophyll cell density and higher C content per unit leaf area (Carea). This hampers CO2 transfer in the leaves, resulting in lower pi/pa through reduced mesophyll conductance (gm) [8, 9, 10]. The LMA increases with reduced water availability as an effect of smaller transpiring surface and more tightly packed tissues, which represents an effective adaptation to drought stress in water-limited ecosystems [11]

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