Cross-pollination and pollen storage to assist conservation of Metrosideros bartlettii (Myrtaceae), a critically endangered tree from Aotearoa New Zealand

Context. Metrosideros bartlettii is one of the most threatened trees in New Zealand and with less than 14 individuals spread across three populations, the species is at high risk of extinction. Despite reproductive failure being identified as one of the factors contributing to population decline, little is known about its pollination biology. Aim. The current study aimed to gain knowledge of the reproductive biology of M. bartlettii by using trees in cultivation of which origin is known. Methods. We tested the effect of self-pollination, cross-pollination and hybridisation treatments on reproductive output. In addition, the viability of fresh pollen was determined for each tree and the impact of desiccation and storage temperature (5 degrees C, - 1 8 degrees C and - 1 96 degrees C) on pollen viability assessed. Key results. Metrosideros bartlettii was found to be highly self-incompatible with seed formed via autonomous self-pollination having low viability. Hybridisation with Metrosideros excelsa, another native species commonly found in cultivation, produced viable seeds, but seedlings failed to survive. Pollen viability differed significantly among trees, and pollen stored at - 18 degrees C and - 196 degrees C retained viability after 6 months. Conclusion. Metrosideros bartlettii is self-incompatible and cross-pollination is required to increase seed production and supplement wild populations. Implications. Trees in cultivation provide a valuable tool for the conservation of M. bartlettii. However, cross-pollination is essential to prevent hybridisation and ensure genetically robust seed. Long-term storage of pollen at the temperatures determined here will help to overcome challenges in cross-pollination of trees that are spatially and geographically isolated.