Temperature Dependence of Tree Reproductive Processes: Flowering Phenology, Pollen Viability, and Seed Germination

Abstract

Many plant species are already shifting their geographic ranges in response to climate change. Species range shifts in response to temperature are conditioned by many factors. Reproductive phases, e.g., flowering phenology, pollen germination, pollen tube growth, fruit set, and seed germination, have proven to be the most temperature-vulnerable stages. Therefore, the impact of climate change on reproduction could be a major limitation on future tree distribution. Some researchers indicate that all species distribution models have to integrate or improve species dispersal and migration processes to be able to provide accurate projections of future distributions. However, species distribution models rarely consider temperature-induced reproductive disorder and failure, e.g., mismatches with pollinators or seasonality and inability of pollen to germinate, because this species-specific information is hard to obtain for trees. My study used Yoshino cherry (Prunus x yedoensis) blossom data in Japan to demonstrate that phenological changes in flowering caused by higher-than-normal temperatures may induce reproductive disorders and further impact their distribution. We will be able to apply the same method to investigate the warming effects on male and female cones with increasing flowering phenological observations of conifer species in the future. In addition, I utilized the pollen viability test on conifer species, Pinus contorta Dougl. ex. Loud., Picea engelmannii Parry ex Engelm., and Pinus ponderosa Dougl. ex Laws., as a method to study temperature effects on reproductive success and seed germination tests on the same three conifer species plus Pseudotsuga menziesii (Mirb.) Franco to understand the possibility of seedling regeneration rate under climate change. My goals are to (1) evaluate the effects of winter warming versus spring warming on flowering phenology and how warming in these different periods exerts variable effects on flowering dates; (2) explore how temperature dependence of pollen germination and tube growth vary among conifer species and populations of the same species collected from various elevations; (3) examine how temperature dependence of seed germination varies among different conifer species and populations of the same species collected from various elevations and whether the variation of germination traits relates to any climate factors at seed-collecting sites. I found that rates of change in Yoshino cherry bloom dates differ significantly between before and after 1980, and excessive warming could delay blooms, which has manifested at low latitudes. Optimal temperature and temperature ranges for pollen germination and tube growth vary among species and elevations within the same species and reflect the acclimation or adaptation of species to spring mean temperatures. Seeds from different species and different elevations within the same species had different germination traits correlated with their ecological niche, and germination traits do not show the environmental gradients. The results provide the exact optimal temperatures for pollen germination, tube growth, and seed germination that people always suspect but never know the values. These findings also provide the information needed for improving current conifer species distribution models based on reproductive biology and contribute to more effective climate adaptation strategies, e.g., seed transfer, assisted migration, locating corridors and refugia, etc., for policymakers and stakeholders to help mitigate climate change impacts, and increase our understanding of forest resilience.