BIOGEOCHEMICAL IMPACT OFHIERACIUMINVASION IN NEW ZEALAND'S GRAZED TUSSOCK GRASSLANDS: SUSTAINABILITY IMPLICATIONS

Abstract

The establishment and spread of invasive plants could be enhanced by plant–soil feedbacks that alter the cycling of biologically important elements. In New Zealand, overgrazing of tussock grasslands in the South Island has led to land degradation and simultaneous invasion of exotic weeds (primarily Hieracium spp.) over large areas. While Hieracium continues to spread rapidly, little is known about variation in the impact of Hieracium across landscapes characterized by a range of environmental conditions. We examined the impact of Hieracium invasion on soil and ecosystem processes first at the scale of individual patches under one disturbance regime and uniform “environment” (i.e., one aspect and elevation), and then under different environmental conditions (aspects) and disturbance regimes (long-term grazing, no grazing since 1978). Around individual plants on heavily grazed north-facing slopes with significant bare ground, Hieracium invasion increased total soil C and N and lowered soil pH and mineral N relative to the adjacent herb-field vegetation. Soil C mineralization was higher, and net N mineralization was lower under Hieracium than under the adjacent herb-field vegetation. Litter quality differences did not explain differences in net N mineralization. However, results from 13C NMR spectroscopy suggest qualitative differences in organic-matter inputs under Hieracium compared to native vegetation. These results suggest that Hieracium may lower N availability by outcompeting native plants for mineral N, making it difficult for native species to reestablish and promoting the spread of Hieracium. Our results differed on cooler, wetter, less heavily grazed south-facing slopes, where soil C and N pools and cycling rates were lower under Hieracium than under the native herb-field vegetation (which differed across aspects). Whether the (longer term) impact of Hieracium on the south aspect resembles that on the north aspect may depend on management. Our results suggest that Hieracium invasion can alter ecosystem processes, but the impact of invasion may depend on the ecological context (aspect and disturbance) prior to invasion. For reprints of this Invited Feature, see footnote 1, p. 1259.