Spatial growth and population dynamics of a perennial tussock grass (Achnatherum calamagrostis) in a badland area

Abstract

Summary 1 Achnatherum calamagrostis (L.) Beauv. is a perennial, tussock forming grass which is conspicuously successful as a pioneer in badlands in submediterranean areas. We analysed its population dynamics using a matrix population model that is intermediate between traditional demographic models and fully spatially explicit models. 2 Tussocks are considered as a set of spatial units, each classified according to the number and state (seedling, juvenile or adults) of the tillers it contains. The growth rate (λi) determined from the model estimates the spatial spread of A. calamagrostis as a rate of increase in the number of occupied quadrats. 3 We constructed six projection matrices from data for populations on two substrate types, in 2 years with differing climatic conditions and considering the whole life cycle or eliminating sexual reproduction or vegetative multiplication. 4 All the matrices predicted growth rates greater than one. Sexual reproduction was an important process for spatial spread in A. calamagrostis, unlike many other perennial grasses. In contrast, new colonization by vegetative reproduction contributed little to spatial growth of populations, although some expansion may still be possible in the absence of sexual reproduction. 5 A. calamagrostis, like other tussock‐forming plants, exhibits a conservative tactic of slow spread. Tiller dynamics serve mainly for consolidation and maintenance of the plant in already occupied space and sexual reproduction is the main way to ensure colonization of new safe‐sites. Climatic conditions influenced growth rates which increased during non‐drought years above the near maintenance value seen in drought years.