Nassella trichotoma – plant growth rates and effects of timing of grubbing on populations in North Canterbury grassland

To understand the plant growth and within-year timing of grubbing effects on the dynamics of a grassland population of Nassella trichotoma, three field experiments were conducted. In Experiment 1, at 25 farm sites spanning 6 years, established plants grew in basal diameter at a mean constant rate of 24 mm year 1, enabling rapid increase in seed output. In Experiment 2, autumn and spring grubbing compared on six 5-ha plots on four farms over six years gave reductions in N. trichotoma population size of 71 and 83 % respectively. In Experiment 3, the effects of timing of spring grubbing and pasture disturbance on recruitment were quantified and combined with published demographic process rates in a simple population model. Population stability occurs when 33.75% of plants are removed annually (the current mean grubbing rate) before mid-November with 3% pasture disturbance. Population stability also occurs with later grubbing at higher grubbing rates or lower disturbance levels.