Available and not total nitrogen in leaves explains key chemical differences between the eucalypt subgenera

Many ecologists regard nitrogen as a key element in the life-histories of herbivore populations. Consequently, those studying interactions between plants and animals often attempt to link feeding to the concentration of nitrogen (N) in plants. This approach disregards the possibility that in many plants, especially those rich in tannins, animals cannot digest much of the N. The ubiquity of tannins in plants led us to hypothesise that the concentrations of available nitrogen may be more informative for ecologists than are measures of total N. Eucalypts provide a good model for examining this hypothesis because subgeneric differences in foliar chemistry cause dietary niche separation in marsupials.We used an in vitro assay that integrates fibre, digestibility, tannins and N into a single measure of " available nitrogen" (AvailN) to compare the concentrations of total and available nitrogen in the leaves of 138 eucalypt species. There were distinct differences between the subgenera. Most notably, even though differences in total N were minor, species within Eucalyptus contain half the AvailN of those within Symphyomyrtus (0.27% vs. 0.59% dry matter). Among all species (N=138), there was a relationship between AvailN and total N (r²=0.23; P<0.001). The relationship, however, was much stronger (r²=0.84; P<0.001) and the concentrations of AvailN much higher after inactivating tannins with polyethylene glycol (PEG).By integrating several measures, we showed that tannins defend eucalypts against herbivory but this is pronounced in species within Eucalyptus. This likely explains the different feeding niches of arboreal folivores and likely affects many other processes in eucalypt forests and woodlands. Furthermore, it suggests that ecologists should be far more careful in their measure of N; AvailN rather than total N is important, especially when diets contain tannins.