Foliar and soil N and δ15N as restoration metrics at Pūtaringamotu Riccarton Bush, Christchurch city
Delta15N values and associated N concentrations may be useful nutrient cycling metrics for assessing restoration progress, and complement biodiversity assessment. We tested this hypothesis at Pūtaringamotu Riccarton Bush (‘Pūtaringamotu’), a low-rainfall urban forest fragment under restoration since 1974. Foliar and soil measurements came from four indigenous forests (Pūtaringamotu; Saltwater, high rainfall, logged until 2002; Ōkārito, high rainfall, never logged; Scotchman’s Creek, high rainfall, burrowing seabirds present). Despite different disturbance histories, Pūtaringamotu, Ōkārito and Saltwater results all fitted global δ15N regressions between rainfall and foliar N concentration. Enhanced foliar N concentrations accompanying low rainfall at Pūtaringamotu were similar to seabird-affected Scotchman’s Creek. Although Pūtaringamotu foliar δ15N values showed differences between taxa, they were less pronounced than for Ōkārito; Saltwater showed no effect. Further investigation of nutrient cycling restoration metrics at Pūtaringamotu should therefore emphasise progressive development of differences in foliar δ15N between plant taxa, pooled values being insufficiently sensitive.
