Reproductive strategies involving biomass allocation, reproductive phenology and seed production in two Asteraceae herbs growing in karst soil varying in depth and water availability.

Shallow soil with low water availability, poor nutrition, and limited living space is the key limiting factor of plant growth and reproduction in vulnerable karst regions. As pioneers adapted to these areas, little is known about the reproductive responses of annual herbs to these limited soil resources. In this study, we, therefore, investigated the reproductive strategies ("fight or flight") of two annual Asteraceae herbs (Xanthium sibiricum and Bidens pilosa) growing in the harsh karst soil environment, by comprehensively considering biomass allocation, reproductive phenology, and seed production. X. sibiricum and B. pilosa were grown under three soil depths and three water availability levels in a full two-way randomized block design, then biomass allocation, initial flowering time, fruiting stage, life span and seed number and mass were examined. In X. sibiricum, reproductive allocation decreased with decreasing water availability, soil depth, and water availability + soil depth. Meanwhile, the initial flowering time, fruiting stage and life span were all delayed with moderate decreases in resources. However, in B. pilosa, reproductive allocation increased with decreasing soil resources, while initial flowering time, fruiting stage and life span remained unchanged under moderate resource decline. Under severe resource decline, both species showed a reduction in fruiting stage and life span. These results support the hypothesis that X. sibiricum tends to adopt a "fight" reproductive strategy in response to declining resources via changes in biomass allocation, reproductive phenology and seed production. In contrast, B. pilosa tends to adopt a "flight" strategy in terms of biomass allocation, while maintaining a relatively stable reproductive phenology and seed size to cope with the harsh living conditions. However, neither species coped well under severe declines in water availability, nutrition and living space. These findings help our understanding of how herbaceous plants can adjust their reproductive strategies to support their regeneration in harsh karst soil environments. [ABSTRACT FROM AUTHOR]