Identifying population thresholds for flowering plant reproductive success: the marsh gentian (Gentiana pneumonanthe) as a flagship species of humid meadows and heathland
The threshold below which population declines impact the effectiveness of plant reproduction is essential for the identification of populations that can no longer spontaneously recover following habitat management or restoration, below the minimum viable population (MVP) size. We hypothesized that risk of reproductive limitation can be evaluated from combined analysis of pollen activity, ovule fertilization and germination in the context of population demographics and fragmentation. The marsh gentian (Gentiana pneumonanthe), a rare emblematic species of European heathland and fen, was investigated at the southern limit of its range in eighteen populations encompassing one to several hundred thousand individuals, spanning small fragments to extensive well-preserved areas. An index of habitat fragmentation was determined from GIS; field surveys determined the ratio of juvenile to reproductive age states; fluorescence microscopy of pistils determined, for each population, the proportion of flowers exhibiting active pollen tube growth. Analysis of seed lots determined the ovule fertilization rate and seed germination capacity. Some of the small populations occupying restricted habitat fragments showed high rates of pollination (100%) and ‘normal’ age state demographics. However, reproductive characters all exhibited exponential rise to maximum relationships with population size, indicating clear tipping points (for pollination, at a threshold of 7 reproductive adults, and for ovule fertilization rate and germination at 42 reproductive adults). Thus although small populations may set seed, exhibit a ‘normal’ age state structure, and may appear viable, reproductive effectiveness declines when population size falls below 42 generative individuals and < 7 is an indicator of strong limitation. Although many remnant populations of G. pneumonanthe are in the order of 50–150 individuals these should be not be considered as MVPs; they are on the brink of calamity.
KeywordsDemographics Dispersal Habitat fragmentation Ovule fertilization Plant conservation Pollination Pollen limitation
This study was supported by a grant from the Cariplo Foundation as part of the project “FraGenziane: Fragmented Gentiana pneumonanthe populations, habitats and associated fauna in local ecological network”, grant number 2014-1631: project leader, Parco Monte Barro. The regional parks of Parco Regionale delle Groane and Consorzio Parco Brughiera Briantea allowed access to their territories and actively facilitated the project, and we particularly thank Luca Frezzini and the ecological guards of the Groane Park (particularly Paolo Ventura and Antonella Pezzotta) and Daniele Piazza of Consorzio Parco Brughiera Briantea. We thank Dr. Maria Costanza Scarpini (Istituto L. Castiglioni) and students of the school for their enthusiasm whilst actively facilitating investigation of seed germination and plant production. We thank Guido Brusa for sharing his experience and for help with finding populations. Prof. Giuseppe Bogliani coordinated project activities for the University of Pavia, and Silvia Stefanelli and Arianna Musacchio provided entomological assistence in the field. We thank Letizia Manzoni for help with the analysis of autogamy. Prof. Maurizio Cornalba (University of Pavia) shared his expertise for the determination of Apoidea. We thank the regional governments of Lombardy and Piedmont for permission to collect samples of plant material. Comments from three anonymous reviewers, one of whom left strong hints as to their identity, greatly improved the manuscript and in doing so improved the chances of survival of G. pneumonanthe in northern Italy.
SP and RMC conceived and designed the study, SP, EC (Caporali), AS and AL performed the pollination analyses, SP, FP, LM, AL, AP and SC collected flowers and seed, characterized seed lots and performed in vitro germination tests for the study populations. ML and AG supervised population sampling, advised on statistics and revised the text. EC (Cardarelli) performed the analysis of pollinator visitation. SP performed statistical tests, produced graphics and wrote the manuscript, and all authors were involved in manuscript correction.
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