Leaf stable isotopes may be good candidates as traits in ecology and evolutionary biology studies because they integrate physiological processes and vary in relation to known environmental drivers. Leaf carbon isotope (δ13C) values reflect the long term balance between carbon dioxide (CO2) supply and demand, and integrate a fundamental tradeoff between water-use and photosynthetic efficiency. Leaf nitrogen isotope (δ15N) values integrate differences in plant metabolism and rates of ecosystem N cycling as well as mutualist associations that are involved in biotic N-uptake. Although isotope-environment relationships are well characterized, the role of evolutionary history in shaping δ13C and δ15N values is less understood. We compared evolutionary relationships, δ13C, δ15N and habitats in the heath family (Ericaceae). We constructed a maximum likelihood phylogeny for combined matK, nrITS and rbcL gene regions from 107 species that span the range of ecological and morphological diversity of the North American Ericaceae. We obtained δ13C and δ15N from leaf material collected from herbarium vouchers. Habitat information was obtained from voucher labels. We calculated phylogenetic signal of δ13C, δ15N and habitats using Pagel’s λ.
Results/Conclusions
We observed strong phylogenetic signal in leaf δ13C (Pagel’s λ = 0.7) and δ15N (Pagel’s λ = 0.9), suggesting that close relatives are adopting similar physiological strategies with respect to CO2/ H2O use efficiency and N-uptake. Variation in δ13C corresponded to habitat types: the most enriched species (-26.5 ± 0.3) were from dry and/or cold habitats (e.g., tundra) representing a strategy that limits water loss at the expense of C gain. The most depleted species (-28.9 ± 0.5) were from wet and/or warm habitats (e.g., swamps) representing a strategy that favors CO2 uptake at the expense of water loss. Variation in δ15N primarily corresponded to mycorrhizal association. Ericaceae species form mycorrhizal fungi associations endemic to the Ericaceae family: Monotropoid, Arbutoid and Ericoid mycorrhizae. Individuals with Monotropoid mycorrhizae displayed the most enriched δ15N (+11.7 ± 2.6), followed by Arbutoid (+2.6 ± 5.7) and Ericoid (-3.2 ± 0.7). Variation in δ15N also corresponded to habitat types: species from dry and/or hot habitats with faster rates of nutrient cycling (e.g., southern forests) were the most enriched in δ15N (+3.6 ± 0.8) while species from wet and/or cold habitats with slower rates of nutrient cycling (e.g., peatlands) were the most depleted (-3.2 ± 0.7).