Hawkweed invasion does not prevent indigenous non-forest vegetation recovery following grazing removal

Abstract

Factors controlling vegetation restoration of depleted short-tussock grasslands are poorly understood. We investigated effects of mouse-ear hawkweed ('hawkweed', Pilosella officinarum) cover and environmental stress associated with landform and soil type on the rate and pattern of indigenous vegetation recovery from grazing in the highly-modified 1000-ha Lake Tekapo Scientific Reserve in the north of the Upper Waitaki ('Mackenzie') Basin. The reserve has been destocked of sheep and under effective rabbit control since 1992. At that time, mouse-ear hawkweed dominated vegetation on three of its five major landforms (up to 42% cover), and 44-89% of soil was exposed. In 2011 we resampled 12 original vegetation monitoring plots that were established in 1993. Indigenous vegetation recovered in the 18 years following removal from grazing despite high levels of initial modification and exotic cover dominance. Exposed soil and rock decreased, and indigenous plant cover and litter increased across all landforms, while vascular and nonvascular indigenous plant cover increased at different ends of a landform-driven productivity gradient. Hawkweed invasion did not retard recovery; to the contrary, the extent and rate of recovery was higher on more productive landforms with higher initial hawkweed cover. The pattern of change across the reserve was consistent with grazing having exerted a powerful constraint on the growth and biomass of both indigenous and exotic palatable species prior to reservation. Soil moisture stress appeared to delay the timing of hawkweed invasion, and to constrain productivity and hence the rate of indigenous vegetation recovery following release from grazing. We propose a testable model of interacting influences of grazing and environment on indigenous vegetation and the niche of mouse-ear hawkweed, in which recovery outcomes depend on environmental productivity. We suggest that removal of feral grazing as well as stock, size of recovery area, limited monitoring-disturbance, and timing of grazing-release relative to hawkweed invasion may explain why our results and conclusions contrast with those from other studies of release of fescue-tussock grasslands from pastoral grazing.

Plant invasions in New Zealand: global lessons in prevention, eradication and control

“…Yet in all non-native plant species management programmes, the first action should be to identify the drivers of invasion since the target nonnative species may be as much a symptom as a cause of ecosystem change. Thus forest fragmentation (Williams and Buxton 1995), increased disturbance (Jesson et al 2000), disrupted river flows (Brummer et al 2016), eutrophication of soils and water (de Winton et al 2012), altered fire frequencies (Williams 1992), and changes in the abundance of grazers or browsers (Bellingham and Coomes 2003;Walker et al 2016) can all act as drivers of plant invasion (Hulme 2006). Unless these drivers are managed, then despite successful removal of the target species, ecosystems will remain prone to re-invasion, either by the same or by other plant species.…”

Predicting ecological change in tussock grasslands of Aotearoa New Zealand

“…Removing livestock grazing can facilitate local recruitment of Chionochloa spp. and other native plants (Lee et al 1993;Walker et al 2016), but can lead to dominance by exotic grasses in more modified short-tussock grasslands in some situations (Lord 1990). Exclosure studies across grassland types suggest there is no substantial impact on plant community structure when grazing is removed over relatively short time frames of 5-16 years (Primack 1978;McIntosh & Allen 1998;Grove et al 2002;Rose & Frampton 2007; and no significant changes in soil chemical properties (Lord 1990;McIntosh & Allen 1998).…”