PLANT-ANIMAL INTERACTIONS - METHODS A simple, integrative assay to quantify nutritional quality of browses for herbivores Jane L. DeGabriel Æ Ian R. Wallis Æ Ben D. Moore Æ William J. Foley Received: 20 April 2007 / Accepted: 19 December 2007 / Published online: 21 February 2008 Ó Springer-Verlag 2008 Abstract Many regard the concentrations of nitrogen (N), tannins and plant cell wall constituents (fibre) as key indicators of food quality and habitat suitability for browsing herbivores; yet there is no method for measuring their combined effects. We have developed a simple in vitro assay for measuring the effects of tannins and fibre on N availability in browse. We determined the effects of tannins by measuring the polyethylene glycol (PEG)- binding capacity (PEG-BC) of Eucalyptus leaf samples, followed by a two-stage in vitro digestion with pepsin and cellulase to determine the digestibility of dry matter and N. There was a significant relationship between concentra- tions of digestible N and the PEG-BC of the leaves. Furthermore, adding PEG significantly improved the digestibility of N. Our results concur with in vivo observations from several mammalian species. This sug- gests that our method is effective for measuring the nutritional quality of browse and the benefits of adding PEG, providing some index of the detrimental effects of tannins. We further simplified the assay by removing the PEG step, allowing us to quickly analyse samples in bulk. Nevertheless, this simplified method is still not practical for analysing the many samples necessary to compare the nutritional values of different tracts of forest. We used near-infrared reflectance spectroscopy to produce calibra- tion equations and predicted total and digestible N in 322 trees at eleven sites. Both within and between sites, we found a wide variation in concentrations of digestible N but a much lower variation in total N, with either no rela- tionship or poor relationships between the two measures. This confirms the variability in the nutritional quality of eucalypt forests, which may explain the distribution and abundance of mammalian herbivores. Thus, our assay provides a useful tool for understanding how food resour- ces influence herbivore populations at different scales. Keywords Tannins  Nitrogen availability  PEG  Eucalyptus  Folivory Introduction The nutritional quality of food for browsing herbivores may constrain both habitat use and reproductive fitness (e.g. Batzli 1986; Moore and Foley 2005). However, measuring food quality depends on the ability to easily assess the proportion of food that can be digested and metabolised by animals (Foley et al. 1999). Plant second- ary metabolites are ubiquitous in browse and can reduce the digestibility of energy and protein (Iason 2005). Communicated by Jo ¨rg Ganzhorn. Electronic supplementary material The online version of this article (doi:10.1007/s00442-008-0960-y) contains supplementary material, which is available to authorized users. J. L. DeGabriel  I. R. Wallis  W. J. Foley School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia J. L. DeGabriel  B. D. Moore School of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia B. D. Moore The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK J. L. DeGabriel (&) School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK e-mail: Jane.Degabriel@anu.edu.au 123 Oecologia (2008) 156:107–116 DOI 10.1007/s00442-008-0960-y