A multi-scale exploration of the drivers and implications of germination strategy in Australian alpine plants

Abstract

Alpine ecosystems are particularly sensitive to the effects of global warming because they are characterized by species adapted to low temperatures. Although adapted to low temperatures, alpine plants may still be capable of persisting under a changing climate and this will largely be dependent on the ability of their seeds to germinate, establish and reproduce in situ, or after dispersal to new sites. The effects of climate change on regeneration from seeds may be influenced by intrinsic factors such as seed longevity (how long seeds remain viable and able to germinate after dispersal), or germination success (the proportion of a seed population that will germinate following exposure to future climate scenarios), and the seasonal timing of germination. Germination strategy, among the earliest life history traits expressed, reflects germination timing and pattern across the progression of seasons after seed dispersal and I therefore expected germination strategy would be a key trait affecting the responses of species to climate change. To examine seed longevity, which reflects the potential for survival in a soil seed bank or in ex situ conservation, I explored the seed intrinsic lifespan (longevity) and its correlates across 56 species (Chapter 2). I showed that Australian alpine seeds are short-lived, similar to alpine seeds elsewhere and relatively shorter-lived compared to non-alpine Australian seeds. Although relatively short-lived, the seeds do survive long enough to form persistent soil seed banks. However, it is unclear whether the conditions required for germination will still be present in future climate scenarios. Thus, I conducted experimental studies that investigated how germination success in future climate scenarios will differ from germination in ambient climate scenarios, and whether germination strategy moderates the germination responses across 39 species (Chapter 3). This study highlights that when comparing across species, germination strategy moderates the effect of changing climate on germination success such that species with immediate germination strategy that germinate readily after dispersal are less sensitive to changing temperature and winter duration compared to species with dormant seed components (staggered and postponed strategies). Since germination strategy is a significant factor in determining the responses of species to climate change I also examined how within-species variation in germination strategy might affect the adaptability or plasticity of species in response to climate change. I examined the germination strategy variation within-species to specify the climate variables driving variation in germination strategy in wild populations (Chapter 4). I found that within the alpine herb Oreomyrrhis eriopoda, variation in germination strategies is mainly determined by temperature variability in the climate of origin. I also examined the consequences of germination strategy on not just germination but the whole plant life cycle, including early establishment, vegetative and reproductive traits and phenology, under ambient and future climate scenarios (Chapter 5). Under warmer soil temperatures, survival was reduced, and lifetime (time to senesce) and reproductive period were both condensed. Several vegetative and reproductive traits were affected by warming and the response indeed differed across germination strategies. This reveals that within-species variation in germination strategy may also enable species to express different patterns in life history traits across development, not just germination timing. Lastly, I also tested whether germination strategy is plastic across generations. Intraspecific germination strategies in O. eriopoda were not plastic across generations, even though labile in the species. The results highlight that across scales immediate species and immediate populations are less constrained by changing climate. What is interesting is the staggered germination strategy that maintained seasonal variations in seedling emergence, at current climate but also under future climate scenarios. Postponed strategy also tend to maintain germination under warmer germination temperatures, and hence, no shift in germination as reported in alpine seed elsewhere is observed across Australian alpine seeds. The germination strategy within species that is observed to be a conserved trait and did not change with maternal condition raises some questions. If germination strategy is not plastic across generations, then why does recent climate matter? Will germination strategy change when maternal conditions they are exposed to repeated over a few more generations? And how do the adult traits differ among the mothers? Together, the findings of this thesis contribute to improved understanding of alpine seed and seedling ecology, in particular how among- and within-species variation in germination strategy may influence species’ responses to climate change by affecting germination success and contributing to the persistence of species and/or populations. These findings have applied and pure ecological and evolutionary relevance to Australia and will make a substantive contribution to our global understanding of alpine plants.