Effects of microsite characteristics on Hieracium seedling establishment in tall‐ and short‐tussock grasslands, Marlborough, New Zealand

Abstract

The distribution and density of Hieracium pilosella, H. caespitosum, and H. lepi‐dulum seedlings were quantified at an early stage of invasion of a tall‐tussock (Chionochloa) and a more open short‐tussock (Festuca‐Poa) grassland. Seedling and available (unoccupied) microsites were compared in 11 stands. Recorded variables included establishment substrate, overhanging cover, and proximity and height of neighbouring plants. Total Hieracium cover was <7% in both grasslands. Total Hieracium seedling densities averaged 126 per 100 m2 in tall‐tussock and 38 per 100 m2 in short‐tussock grassland. The three species showed wide overlap, but also some consistent differences, in microsite characteristics. All three had established on all substrates, over 60% of seedlings were among vegetation or litter, and establishment was not dependent on the availability of bare ground. The least favourable substrates were tall‐tussock bases, large‐herb patches, deep litter, and rock fragments. Shorter/more open vegetation, shallow litter, and soil were either highly favourable or had no detectable effect on establishment. The effects of overhanging cover and canopy‐plant proximity and height were most consistently adverse for H. pilosella and most consistently favourable for H. lepidulum. Although biotic barriers to establishment were stronger in tall‐tussock grassland, it had higher seedling densities probably because it was located closer to established Hieracium populations and was subjected to a greater seed rain. Management of tussock grassland structure provides a key strategy for controlling Hieracium invasion.