Do climatic and habitat conditions affect the reproductive success of an invasive tree species? An assessment of the phenology of Acacia longifolia in Portugal

Abstract

Plant phenological events are some of the most sensitive indicators of how plant species respond to favourable or stressful conditions. The evaluation of the flowering phenology of invasive plant species is particularly relevant, mainly due to its crucial importance in determining plant reproductive success and the outcome of invasion. We studied the phenology of Acacia longifolia, an aggressive, invasive plant species in the Mediterranean basin. We measured its vegetative growth and reproductive traits, specifically flowering phenophases and fruit production, under different climatic conditions (mesic and xeric Mediterranean climates), and in two different habitats (pine forest and open area). All the measured phenological phases began earlier at the xeric site than at the mesic site; this was particularly evident when comparing reproductive phenophases. Flowering dates were significantly associated with air temperature, with early peak flowering dates linked to increases in air temperature. The number of fruiting flowers per inflorescence in A. longifolia trees was higher at the mesic site, mainly in the pine forest plot, and the number of aborted fruits was notably lower than in the xeric plots. The presence of a pine forest at the mesic site strongly influenced the flowering phenology of A. longifolia and resulted in the highest reproductive success and the lowest branch growth rate. Our results demonstrate that a combination of climate and forest structure can cause pronounced differences in phenology and reproductive success of A. longifolia. These data can help to understand the variations in invasive rates of A. longifolia across the Mediterranean basin.