Population growth of Nassella trichotoma in grasslands in New Zealand slower today than in the past

► Four long-term field experiments were conducted in Nassella trichotoma populations. ► On average, a population takes 210 years to increase to 90% of its carrying capacity. ► Key demographic rates are lower than in the past. ► Key demographic processes are plant-size-dependent. ► A size-structured model is needed to explain population growth in this species.