Genetic variation and resilience to climate change in Mediterranean-type trees

Climate models predict that temperature means will continuously increase globally, and that heatwaves and drought periods will become more frequent and intense, particularly in Mediterranean-type climates. The Southwest Western Australia (SWWA) Biodiversity Hotspot has extensive forest environments that have been subject to heatwaves and drought-induced forest mortality in recent years, impacting forest carbon sequestration and local ecological structure. Although, species may persist through enhanced physiological tolerance, phenotypic plasticity and/or genetic adaptation. Genetic variation is critical for ecological adaptive capacity - the potential and ability to adjust to, and persist through, external factors - and consequently, the evolutionary potential of the species. Evolution to a specific environment through natural selection results in patterns of local adaptation (when a local population experiences higher fitness compared to non-local counterparts). Local adaptation can be identified by either genome wide surveys that link genetic variants to climate variables or measuring plant traits indicative of plant performance and survival through reciprocal transplants in common environments. Exploring genetic adaptation patterns associated with physiological tolerance to climate can guide forest management approaches to enhance forests’ resilience to climate change, such as assisted gene migration. The genetic survey (Chapter 2) sampled natural jarrah populations and obtained 13,534 independent single nucleotide polymorphic (SNP) markers across the genome. Three genotype-association analyses were used to identify putatively adapted SNPs associated with independent climate variables. While overall levels of population differentiation were low (FST=0.04), environmental association analyses found a total of 2,336 unique SNPs associated with temperature and precipitation variables, with 1,440 SNPs annotated to genic regions. Considerable allelic turnover was identified for SNPs associated with temperature seasonality and mean precipitation of the warmest quarter, suggesting that both temperature and precipitation are important factors in adaptation. SNPs with similar gene functions, had analogous allelic turnover along climate gradients, while SNPs among temperature and precipitation variables had uncorrelated patterns of adaptation. These contrasting patterns provide evidence that there may be standing genomic variation adapted to current climate gradients, providing the basis for adaptive management strategies to bolster forest resilience in the future. The second experimental chapter (Chapter 3) explored seed germination response to temperature in jarrah and marri populations from wide-ranging climate origins, to estimate the thermal optima and constraints. Seeds from across the entire geographic distribution were collected from independent populations of each species. Patterns of germination observed differences between species on a thermal gradient plate (5-40°C) and provided a temperature range for explicit germination tests. Germination tests were carried out at five constant temperatures between 9 and 33°C. We discuss how the germination niche (1) differs between species, (2) varies among populations, and (3) relates to climatic origin. Temperature response of germination differed among species, specifically the optimum temperature for germination (jarrah – 23.4°C; marri - 31°C). Temperature response of germination also differed among populations within species and was related to the climate-origin only for marri. Lastly, for the third experimental chapter (Chapter 4), a reciprocal transplant common garden experiment was used to investigate variation in marri`s functional traits using hyperspectral data. Hyperspectral remote sensing has the potential to assess plant functional status rapidly and non-destructively across climatic gradients to support conservation and management strategies, such as assisted migration, for forests under climate change. This study explored the variability of functional traits in marri to estimate patterns of local adaptation. Trees from natural populations spanning marri’s geographic distribution were grown in two common garden plantation sites with different climate settings. High-resolution field-based spectral measurements were collected from leaves of adult plants at both sites in two seasons (summer and autumn). Partial least squares regression analyses of full reflectance spectra highlighted differences among populations, sites, and seasons in spectral regions associated with photosynthetic pigments and water content, among other spectral traits, related to leaf condition and stress responses. Variation in these traits was further explored with analyses of spectral indices tailored to pigment and water absorptions. Analyses of spectral indices variation identified significant differences between populations, suggesting there is heritable variation in climatic tolerances, but stronger effects of season and site