Biogeochemical Impact of Hieracium Invasion in New Zealand's Grazed Tussock Grasslands: Sustainability Implications

Abstract

The establishment and spread of invasive plants could be enhanced by plantsoil 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 13 C 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.

A non‐native invasive grass increases soil carbon flux in a Hawaiian tropical dry forest

“…However, studies of invasive species' impacts on belowground C cycling have been few, have primarily dealt with C pools, and have varied widely in the magnitude and direction of inferred impacts (Scott et al, 2001;Ehrenfeld, 2003;Hook et al, 2004). Grass invasion into this forest greatly increased the flux of C into and out of soils, as well as the composition of rapidly cycling C pools.…”

Mechanisms underlying the impacts of exotic plant invasions

“…Further evidence could be provided by studies examining the differential impacts of invaders across sites (e.g. Stock et al 1995;Scott et al 2001). Such work is also important for understanding whether invaders exert their impacts via the same mechanisms in different systems.…”

“…Belnap & Phillips 2001;Ehrenfeld et al 2001;Evans et al 2001;Mack et al 2001;Scott et al 2001). The attention to nitrogen cycling, in particular, is largely the result of several dramatic examples, such as the work on Myrica invasion in Hawaii by Vitousek et al (1987) and Vitousek & Walker (1989) and studies of Acacia invasions in South African Cape fynbos (Musil 1993;Stock et al 1995).…”

Impacts of alien invasive plants on soil nutrients are correlated with initial site conditions in NW Europe

“…While most published studies report increased soil nutrient stock and/or availability under invasive plant species compared to uninvaded ecosystems (Musil 1993;Scott et al 2001;Duda et al 2003;Vanderhoeven et al 2005;Chapuis-Lardy et al 2006;Liao et al 2008), other studies show the opposite pattern (Christian and Wilson 1999;Leary et al 2006). In addition, the same species may have different impacts, depending on local conditions (Stock et al 1995;Meyerson et al 2000;Belnap and Philips 2001;Scott et al 2001). The general approach of most previous studies investigating the impacts of alien invasive plants was to consider only a single species in one or very few sites.…”