Aerial seed bank and dispersal traits in Anthemis chrysantha (Asteraceae), a critically endangered species

Mature plant density and fruit production were monitored in the main population of four successive cohorts of the endangered winter annual Anthemis chrysantha (Asteraceae) in southeastern Spain. Experiments were conducted with artificial rainfall and a wind tunnel to determine the temporal and spatial dispersal pattern of the species and the relationship with rain and wind.The population fluctuations of A. chrysantha observed during 2006–2009 could be explained by the rainfall amount and seasonal distribution, which in turn influenced the production of achenes. This study demonstrated the formation of a seasonal aerial seed bank that remains on the dead plant from the fruiting period (early summer) to late spring. We experimentally demonstrated that rain was necessary to release the achenes from the capitula (ombrohydrochory), with wind having no effect on dispersal. The pattern of achene release was related to rainfall events. The main timing of dispersal was during autumn, decreasing the aerial bank by around 80% by December. However, in March, when the next cohort was flowering, some achenes still remained on old capitula (up to 1700 achenes per m2 in some years). White non-dormant achenes were first released, and dark dormant achenes were released later. The non-dormant achenes on the soil surface could germinate quickly (2 d) after receiving a minimum of 10 mm of rainfall at autumn temperatures. Most achenes dispersed across very short distances (atelechory): 75% of them landed beneath the plant canopy and only 7% landed >30 cm from the canopy edge. Results suggest that a seasonal aerial seed bank is an effective trait to ensure the maintenance of this species in its unpredictable habitat.