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

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.