A new species index for forest vascular plants: development of functional indices based on mineralization rates of various forms of soil nitrogen

1 Soil nitrogen mineralization rates for ammonium, nitrate and total-N were determined by incubation experiments for 661 sample plots of deciduous forest from different regions in southern Sweden. 2 Species N indices for nitrate and ammonium were calculated as the weighted averages of mineralized N in those plots where the species were present. 3 Optimized linear models were used to combine mineralization rates for NH4+ + NO3- to produce a functional N index for plots (FNIP) and to combine the species indices for NH4+ + NO3- to produce a functional N index for species (FNIS). 4 The sample scores along the first axis of a detrended correspondence analysis (DCA) were highly correlated with soil acidity and mineralized ammonium, whereas total-mineralized N accounted for most of the variation along the second DCA axis. The species scores along the first axis were highly correlated with the ammonium index. 5 FNIP was somewhat better correlated with the sample scores along axis 1 than mineralized ammonium, but only slightly better correlated than total-mineralized N relative to axis 2. Similar results were obtained for FNIS. 6 Ammonium index and FNIS explained more of the floristic variation along the ordination axes than Ellenberg's N indicator values. Weighted averages of these indices also showed higher correlations with the sample scores along the axes than corresponding averages of Ellenberg's N values. However, when ammonium index and FNIS for the most frequent species were transformed into classes, they were generally similar to the Ellenberg scale. 7 The ammonium indices differed considerably between regions, caused by differences in N deposition, total-N mineralization and nitrification rate. In one of the regions with comparatively low nitrification, the ammonium index was found to be uncorrelated with the species scores along the first DCA axis. FNIS differed much less between regions and thus was more robust; it appears to be a good and generally applicable expression of the species' responses to N availability in deciduous forest soils.