1 Achnatherum calamagrostis (L.) Beauv. is a perennial, tussock forming gra
ss which is conspicuously successful as a pioneer in badlands in submediter
ranean areas. We analysed its population dynamics using a matrix population
model that is intermediate between traditional demographic models and full
y spatially explicit models.
2 Tussocks are considered as a set of spatial units, each classified accord
ing to the number and state (seedling, juvenile or adults) of the tillers i
t contains. The growth rate (lambda (i)) determined from the model estimate
s the spatial spread of A. calamagrostis as a rate of increase in the numbe
r of occupied quadrats.
3 We constructed six projection matrices from data for populations on two s
ubstrate types, in 2 years with differing climatic conditions and consideri
ng the whole life cycle or eliminating sexual reproduction or vegetative mu
4 All the matrices predicted growth rates greater than one. Sexual reproduc
tion was an important process for spatial spread in A. calamagrostis, unlik
e 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 conservat
ive tactic of slow spread. Tiller dynamics serve mainly for consolidation a
nd maintenance of the plant in already occupied space and sexual reproducti
on is the main way to ensure colonization of new safe-sites. Climatic condi
tions influenced growth rates which increased during non-drought years abov
e the near maintenance value seen in drought years.