Disentangling the effects of population mixing and propagule amount in rare plant translocations

Genetic diversity and propagule amount have been identified as crucial
factors influencing the success of plant translocations. Population
mixing, serving as an indicator of higher genetic diversity, might enhance
overall plant fitness and resilience, while a higher propagule amount can
help buffer against environmental variability. However, mixing populations
could potentially reduce individual plant fitness in translocations,
possibly due to maladaptation, and a higher propagule amount might lead to
higher competition among individuals or attract more pests and pathogens.
To better understand the impact of population mixing and propagule amount
on the early fitness of translocated plants, we transplanted material from
either single or mixed source populations and varied the propagule amount
by introducing different quantities of plants into translocation plots for
four threatened herbaceous species in Switzerland. We recorded survival,
flowering status, and the number of inflorescences per individual for
reproductive fitness assessment, allowing us to track the fate of each
plant throughout the two years of monitoring. We then used aster models to
analyse survival and reproduction data collected over two years and
evaluate how these factors affected the mean fitness of individual
plants. Unexpectedly, we found that a higher propagule amount
negatively impacted plant fitness during the early stages of establishment
for two species, while population mixing had a slightly negative effect on
fitness for the other two species. This pattern may be due to increased
attraction of antagonists or maladaptation and low fitness of the source
populations. While the long-term effects of our treatment will become
clearer in future generations, our results suggest that population mixing
and a higher number of propagules may not always be the best strategies
for successful early establishment. We recommend creating multiple smaller
translocation plots instead of fewer larger ones and carefully considering
the vitality and suitability of source populations when planning plant
translocations.