Effect of floral traits mediated by plant-soil feedback on the relationship between plant density and fecundity: Case study of Tamarix chinensis in the Yellow River Delta, China

Abstract

Density-dependent fecundity is important for the successful recruitment and reproduction of plant populations, especially for plants inhabiting saltmarshes with highly varied environmental gradients. The highest fecundity of plants at intermediate densities is usually correlated with high pollinator visitation rates mediated by floral displays of medium density. However, little data explains the mechanism that floral traits and pollinator visitation optimize at intermediate densities. Here, we investigated how the density of Tamarix chinensis (in the Yellow River Delta) influence soil conditions, floral traits, pollinator visitation rates, and plant fecundity in environments with different soil salinity and moisture. At all sites, soil moisture and floral traits (number and length of racemes, amounts of floral volatiles) increased parabolically with increasing density, whereas soil salinity declined. Soil conditions and floral traits of T. chinensis were optimal at intermediate plant densities, which, in turn, maximized pollinator visitation and seed set. Reduced fecundity at plant densities that were higher than optimal was related with higher soil salinity, shorter raceme length, and lower amounts of floral volatiles in T. chinensis. Regular freshwater restoration showed a positive impact on pollinator visitation and fecundity of T. chinensis. Consequently, positive plant-soil feedback enhanced floral traits and pollinator visitation, which are related with the optimal fecundity of T. chinensis at intermediate densities. Thus, the density of T. chinensis should be managed in restoration projects of the T. chinensis population through artificial transplantation. The transplanting density of T. chinensis should be strictly controlled in harsh environment, while this requirement could be relaxed in mild conditions.