Title: Population growth of Nassella trichotoma in grasslands in New Zealand slower today than in the past
Abstract: Nassella trichotoma established in modified tussock-grasslands in New Zealand from about 1860. Management programmes since 1946 have reduced populations to levels no longer impacting pastoral production. Optimising future management requires knowledge of the trajectory of population growth and its regulating demographic processes. To that end, four long-term field experiments were conducted. Net reproductive rate varied from 1.021 to 1.237 year−1 and growth in plant basal diameter from 8.1 to 16.6 mm year−1. The probability of flowering increased with basal diameter and was essentially unity above 50 mm diameter. Populations grubbed annually declined abruptly but recruitment was unaffected and extinction did not occur. Of seeds sown into disturbed and intact pastures, 0–51% produced seedlings and more arose on sunny slopes and from disturbed than intact pasture. Death rates were high; 7 years after sowing, surviving plants represented only 0–9% of the seed sown. Seeds buried 25 mm deep in the pasture litter on two occasions declined in viability at rates of 74 and 89% in the first year and first three months respectively and 26 and 0% year−1 thereafter. Seed production plant−1 (square root scale) increased linearly with plant basal diameter; for example, plants of 11 and 100 mm diameter are predicted to produce 0 and 11,092 spikelets (each with one seed) respectively. We estimate that a N. trichotoma population today will, in the absence of management, take 210 years to increase to 90% of its carrying capacity supporting the hypothesis that population growth in this species is slower than occurred historically. We show that the rates of some demographic processes may be much lower than in the past and suggest this is due to more competitive vegetation resulting from improved management. The size-dependence of many processes supports the need for a size-structured model to explain population growth in this weed. [Shona L. Lamoureaux, Graeme W. Bourdôt and David J. Saville (2011). Population growth of Nassella trichotoma in grasslands in New Zealand slower today than in the past. Acta Oecologica, online July 29, doi: 10.1016/j.actao.2011.06.008]