Carbon and nitrogen stable isotope signals for an entire alpine flora, based on herbarium samples

Stable carbon and nitrogen isotopes provide time-integrated signals of plant carbon and nitrogen relations. We assessed an entire alpine flora in the Swiss Alps at ca. 2400 m elevation, using year 2007 herbarium samples of 245 species, 141 genera and 42 families to explore functional trait diversity. Despite overall similar macro-environmental conditions (moisture, soils, elevation), signal variation covered the full spectrum known for C3 plants. Variation among means for plant families for both δ13C and δ15N was smaller than variation among species within families. Species identity was of far greater importance than family affiliation. Similarly, tissue nitrogen and carbon concentrations varied in a rather species-specific manner, not permitting any a priori plant functional group definition based on such traits. The study also yielded tissue-type specificity of isotope signals. The elevation signal in δ13C (known to be less negative at high elevation) was much less pronounced than observed previously in con-generic comparisons. Thus, elevational δ13C trends are hard to distinguish from species effects in mixed populations over narrow ranges of elevation. δ15N data offer more space for ecological interpretation and show family specificity of signals in few cases. Cyperaceae, the most prominent family in this region, show no discrimination against 15N (like Fabaceae) and must have access to N sources different from most other families. This deserves experimental clarification, given the significance of Cyperaceae in cold environments. Overall, our study evidenced very high functional diversity among alpine plant species, as captured by these isotope signals.