Variation across river channels in demographic dynamics of a riparian herb with threatened status: management and conservation implications

Abstract

Premise Gesneria pauciflora is a rare, threatened plant in riparian forests. Periodic disturbances, expected in this habitat, could influence demographic dynamics on plant populations, yet their impact may not be the same across the watershed. We hypothesized that differences in disturbances between the main channel and tributaries may lead to spatial dissimilarities in population growth rate (λ), structure, and fecundity. Methods In the Maricao River Watershed in Puerto Rico, 1277 plants were tagged and monitored for 1.5 years. Every 6 months, we measured plant size and recorded survival, fecundity, and appearance of seedlings. These variables were used in integral projection models to assess the population status of G. pauciflora. Results Plants in the main channel were smaller but more likely to flower and fruit than those in the tributaries. Overall mortality was greater in the main channel and greater during the rainy season. At both sites, λ ranged from 0.9114 to 0.9865, and survival/growth of larger plants had a greater effect on λ (>0.90) regardless of site. Conclusions Values for population growth rates suggest that G. pauciflora is declining across the watershed. Higher mortality rates in the main channel (more‐perturbed sites) might drive G. pauciflora to reproduce at smaller sizes, while tributaries (less‐perturbed sites) might be better for growth and lead to larger plant sizes. Extreme climatic events are expected to increase in the Caribbean and might decrease the population if the population is left unmanaged. Management strategies that reduce the time plants require to reach larger sizes might be necessary to increase λ, and reintroduction using cuttings might be a possible solution.