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.

Methods
Four rare and threatened plant species were reintroduced in their historic distribution range in the Canton of Bern in Switzerland. There is one reintroduction per species. At each reintroduction site, individuals of a given species were planted in plots: A full factorial design with six rectangular plots representing all combinations of single and mixed populations and two propagule-amount levels was used. In mixed-population plots, individuals from both source populations per species were mixed. For half of the single-population plots, we used individuals exclusively from the first source population, and for the remaining half, individuals from the second source population. Mixed-population plots are expected to have a greater diversity of genotypes and phenotypes compared with single-population plots. The propagule amount was manipulated by planting four times as many plants in high propagule-amount plots compared to low propagule-amount plots. To maintain a constant propagule density, plants were always planted at the same distance from each other. On the reintroduction day, each reintroduced individual's position was recorded within each plot and within the reintroduction site, and the length of the longest leaf (i.e., distance between root collar and tip of the longest leaf) was measured for initial size assessment. Individuals were then monitored in the northern hemisphere's spring and autumn of 2020 and 2021. Individual survival, flowering status, and the number of inflorescences per individual for reproductive fitness assessment were recorded.

For further information on the methods used to conduct the plant reintroductions, whose data are available in the two datasets provided here, as well as the assessment of the impact of population mixing and propagule number on the individual fitness of the reintroduced plants, please refer to the Materials and Methods section of the article:

Bürli, S., Fischer, M., & Ensslin, A. (2025). Disentangling the effects of population mixing and propagule amount in rare plant translocations. Biological Conservation, 306, 111132. DOI: 10.1016/j.biocon.2025.111132