Background: Climate change has become a catastrophic event that has altered Earth's ecosystems. Climate change has accelerated climate extremes globally, thereby affecting water availability and its impacts on carbon (C) and nitrogen (N) cycles within terrestrial ecosystems. These climate extremes such as droughts, floods and bushfires play an important role in N cycling in terrestrial ecosystems and have the potential to alter the N availability in ecosystems. Nitrogen isotopic composition (δ15N) is an important indicator that has been used to further understand the relationships between climate extremes (such as floods, droughts and bushfires) and N cycling in terrestrial ecosystems. However, majority of the research is based on data from experiments with soil and plant tissues and comparing them with historical climate data and tree ring width data. There has been little research done using tree ring δ15N to study the intensity and frequency of climate extremes (such as floods, droughts and bushfires) across various climate zones and tree species. The majority of the literature on planetary responses to climatic extremes has focused on droughts, heat waves and temperature focused disasters. However, there is a gap in understanding how major flooding events and bushfires would affect forest ecosystems, particularly in the context of the shifts in C and N cycles in different forest ecosystems. under climate change. Observed genetic shifts, DNA sequence, mutation rate or evolutionary rate are notably lacking in the analysis of neutral theory and molecular evolution for future performance, of distribution and productivity of long-lived plants that show strong responses to climate change and climate extremes. Aims and objectives: Chapters 4,5,6,7and 8: the research aimed to examine whether tree ring δ15N technology can be used to fingerprint floods, droughts and bushfires with different tree species in different subtropical and boreal forest ecosystems of Australia and China. Another aim was to quantify the responses of N availability to both climate change and climate extremes in these diversified ecosystems. The thesis would be the first to evaluate how N availability and cycling affected by climate extremes. The patterns of N responses to this climate change is the same as the C response pattern of the tipping point curve due to climate change since the C and N cycles are closed coupled in these forest ecosystems. Chapter 9: In this preliminary study, we aimed to examine the potential empirical evidence of mutation rates and patterns, as well as a calculation of the evolutionary rate using DNA extracted from tree rings to test the prediction of neutral theory. Methods (Chapter 3-9): Tree rings were extracted from locations that represent the major subtropical and boreal forest ecosystem of Australia and China with native conifer species dominated. Three trees of Betula platyphylla and three trees of Larix gemlinii of boreal forests in Gen-he County, Inner Mongolia of northern China were used. Five trees of Pseudolarix amabilis and four trees of Cryptomeria japonica in a subtropical forest of Tiantong, Zhejiang, China were selected. Three trees of Cinnamomum micranthum, seven trees of Cunninghamia lanceolata and four trees of Pinus massoniana in Jian’Ou County, Fujian, China, were used. Four trees of Pinus massoniana and five tree of C. Japonica, Luobo Yan, Fujian, China were selected. Three native hoop pines trees (Araucaria cunninghamii) in a subtropical rainforest in Binna Burra, Lamington National Park, Queensland, Australia. Three native hoop pine trees (Araucaria cunninghamii) and three native Bunya pine trees (Araucaria bidwillii ) were collected from the subtropical dry forest of Yaramman, Queensland, Australia. Four hoop pine trees and two Bunya pine trees were also selected from the subtropical wet forest of Imbil, Queensland, Australia. Tree rings were measured with a mean annual basal area increment (BAI), while tree ring stable C isotope composition (δ13C) and N isotopic composition (δ15N) as well as total C and N concentrations were measured on mass spectrometer at either annual-intervals, three-year intervals and 5year intervals depending on the locations and species used. DNA extraction was taken from each sample tree and analysed through DNA kit and genomic sequencing and genomic library through ancient DNA methodology. Multiple regression analysis was used to quantify the relationships among tree ring δ15N, BAI, atmospheric CO2 concentration, temperature, and precipitation of the study sites. Tree ring total C and N, δ13C and δ15N were determined at Griffith University. Tree ring samples were cut every year or every three or every five years, to obtain the isotopic results and to effectively analyse the intensity and frequency of climate extremes as well as the relationship of N availability. Correlations between mutation rate, mutation comparisons, phylogenetic trees and PCA results were undertaken to understand the genome of the each individual sample tree and their mutation rate and patterns. [...]