NORTH-ISLAND SERAL TUSSOCK GRASSLANDS .2. AUTOGENIC SUCCESSION - CHANGE OF TUSSOCK GRASSLAND TO SHRUBLAND

The physiognomic stability of seral tussock grassland and tussock-shru bland was assessed by relating species cover and shrub density to envi ronmental and temporal factors using ordination and regression analyse s. Four small-leaved, sclerophyllous shrub species-Dracophyllum subula tum, D. longifolium, Leptospermum scoparium, and Kunzea ericoides-are rapidly invading Chionochloa rubra ssp. rubra tussock communities, cha nging tussock grassland to shrubland within 45-55 years. Elapsed time since the last fire disturbance, slope, and topographical position are reliable factors for predicting rates of vegetation change. Rates of shrub invasion are fastest on steep slopes and on topographically expo sed sites, and slowest on flat mesic sites dominated by close-canopied Chionochloa rubra ssp. rubra tussock grassland. The cover of wind-dis persed shrubs did not decrease with distance from seed source, but the cover of bird-dispersed shrubs did. The study highlighted the transie nt nature of some North Island tussock grasslands and the implications for management policy in terms of maintaining floristic, community, a nd landscape diversity, and problems with deliberate burning to mainta in early successional vegetation.