Recovery, invasion, and decline over 37 years in a Marlborough short‐tussock grassland, New Zealand

Abstract

The composition of humid, unfertilised, short‐tussock (Festuca‐Rytidosperma‐Poa) grasslands was recorded on 42 permanent transects in 1959, 1970–72, and 1995–96 in the Wairau catchment, Marlborough. Changes in frequency of the most abundant plant species and bare ground were analysed. Reduced levels of grazing, browsing, and fire prompted significant recovery of native shrubs, tall tussocks (Chionochloa), and herbs. Invasive exotics (Hieracium, Anthoxanthum, Agrostis) also increased. Short native grasses (Poa, Elymus), a native herb (Acaena spp.), and an exotic herb (Rumex acetosella) declined. Hieracium pilosella, H. caespitosum, and H. lepidulum were rare or absent in 1959. By 1996 they had established on half the transects, but were still in the early stages of invasion (median frequencies ≤ 5%). There was no correlation between the increase in Hieracium and decline in other species, suggesting that other factors were driving the compositional changes. However, Hieracium species can establish in vegetated microsites and eventually dominate. Early in the invasion a lack of correlation may reflect a capacity for short‐tussock grasslands to “absorb” exotic species before interference becomes intense. Compared with five documented examples of more heavily infested tussock grasslands, the Wairau was the only one where the proportion of increasing species exceeded the proportion that decreased. This may indicate a threshold level of Hieracium, beyond which interference becomes involved in species declines. These grasslands will continue to change as native species continue to recover and potentially dominant Hieracium and exotic grasses continue to invade. In such grasslands, conservation management would benefit from integrating the inevitability of vegetation change with the need to optimise indigenous biodiversity. In appropriate areas, actively promoting long‐term succession to native woody vegetation and tall‐tussock grassland would result in communities that have high conservation values and higher resistance to exotic invaders such as Hieracium.