Fire increases invasive spread of Molinia caerulea mainly through changes in demographic parameters

Abstract

We investigated the effects of fire on population growth rate and invasive spread of the perennial tussock grass Molinia caerulea. During the last decades, this species has invaded heathland communities in Western Europe, replacing typical heathland species such as Calluna vulgaris and Erica tetralix. M. caerulea is considered a major threat to heathland conservation. In 1996, a large and unintended fire destroyed almost one-third of the Kalmthoutse Heide, a large heathland area in northern Belgium. To study the impact of this fire on the population dynamics and invasive spread of M. caerulea, permanent monitoring plots were established both in burned and unburned heathland. The fate of each M. caerulea individual in these plots was monitored over four years (1997–2000). Patterns of seed dispersal were inferred from a seed germination experiment using soil cores sampled one month after seed rain at different distances from seed-producing plants. Based on these measures, we calculated projected rates of spread for M. caerulea in burned and unburned heathland. Elasticity and sensitivity analyses were used to determine vital rates that contributed most to population growth rate, and invasion speed. Invasion speed was, on average, three times larger in burned compared to unburned plots. Dispersal distances on the other hand, were not significantly different between burned and unburned plots indicating that differences in invasive spread were mainly due to differences in demography. Elasticities for fecundity and growth of seedlings and juveniles were higher for burned than for unburned plots, whereas elasticities for survival were higher in unburned plots. Finally, a life table response experiment (LTRE) analysis revealed that the effect of fire was mainly contributed by increases in sexual reproduction (seed production and germination) and growth of seedlings and juveniles. Our results clearly showed increased invasive spread of M. caerulea after fire, and call for active management guidelines to prevent further encroachment of the species and to reduce the probability of large, accidental fires in the future. Mowing of resprouted plants before flowering is the obvious management tactic to halt massive invasive spread of the species after fire.