Foliar N concentration and δ15N signature reflect the herb layer species diversity and composition in oak-dominated forests
Is the herb layer species diversity and composition in species-rich oak-dominated forests related to the (i) soil nitrogen (N) pool, (ii) foliar N concentration, and (iii) foliar 15N/14N isotope ratio (δ15N), as indicators of the N status in ecosystems? How do they correspond to the Ellenberg indicator value for N (EIV-N) of the herb layer vegetation?
Three areas in central Bohemia (Czech Republic) underlain by different geological substrates.
We assessed the herb layer species composition in nine oak-dominated forests growing on different substrates, in relation to both soil nutrient pools and oak leaf nutrient concentrations. Data on the herb layer vegetation composition were collected in 16 square plots (25 m2 each) distributed according to a fixed stratified random design at each site. Soil nutrient pools were calculated on the basis of nutrient concentration and weight of quantitative soil samples taken from six defined depth strata at each of two or three 0.5-m2 pits at each site. Foliar analyses were based on samples of 150 leaves from the upper part of the canopy of five oaks at each site.
The mean Shannon diversity index of herb layer vegetation was negatively related to the mean foliar Ntot. concentration, whereas the mean recovery rate (RR) of herb layer species was strongly positively related to foliar δ15N signatures and, less markedly, to the soil Ntot. pool. Similarly, both the foliar δ15N and the soil Ntot. pool were positively correlated with mean-weighted EIV-N in the herb layer vegetation.
The herb layer species diversity and composition was significantly related to N availability at our study sites. Foliar N concentration and the δ15N signature may reliably indicate: (i) the current N status of oak-dominated forests, in accordance with previous published findings; and (ii) the consequences of that N status for the herb layer species diversity and composition. Foliar δ15N may also be used to detect the susceptibility of an ecosystem to the spread of a few species with high colonization ability in the herb layer, at the expense of other forest species.