title: Effects of Climate Change and Local Environmental Factors on Long Term Water Use Efficiency and Tree Growth in Different Forest Ecosystems
Creators: name: Liu, Tengjiao
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Climate change
subject: Tree growth
subject: Climate-tree relationship
subject: Tree species
subject: Acclimatization

description: Background Climate change has become one of the significant global challenges confronting all the people in the world. Climate change, particularly rising atmospheric carbon dioxide (CO2) concentration (ca) and temperature, and changes in water availability have also affected tree growth of forest ecosystems worldwide. Several studies about the effects of climate change on tree growth and physiological responses have been reported. However, these are mostly based only on experiments with isolated trees or seedlings grown under intensive and short-term exposure to one or two climatic factors. Since long-term and gradual impacts of climate change on tree growth and physiological responses could be different from the short-term effects, there is an urgent need to investigate how tree species respond to elevated atmospheric CO2 and temperature and water availability changes at larger scales and over more extended periods. This study is complicated because tree growth rates vary among genotypes and change as trees age and because climatic conditions, such as temperature, precipitation and humidity, and atmospheric CO2, have been changing over time. There are also indications that tree responses to climate change may change with time as the trees grow. Moreover, since tree growth (biomass) is a product of physiological processes, the physiological processes are affected by the structure of the organs or tissues where the processes occur. In turn, the structures of the organs are determined by the products of the previous physiological processes. To understand the mechanisms on how long-term climate change affects tree physiological processes and growth, we have to understand the relationships among the climate, tree growth and physiological responses, and how long tree growth trend will remain alongside the elevating CO2 concentration before it declines, considering that the forest ecosystems are one of the most important contributors to the global CO2 assimilation, which can effectively counteract the global warming. Hypothesis and objective My research has been focused on studying the long-term tree growth and water use efficiency in response to rising atmospheric CO2 concentration, in combining with other environmental factors, and their influences on climate change in the future. I hypothesised that tree growth is affected by biological effects, such as tree species and ages, and non-biological effects, namely locations, temperature, precipitation and humidity. Thus, all my experiments' main objectives were to confirm my hypothesis and quantify how those biological and non-biological factors would influence tree growth and water use efficiency in the context of both spatial and temporal scales. The goals and objectives of this research were: To determine the effect of long-term climatic conditions on tree growth and physiological processes. The objectives were to determine how climatic factors would influence tree growth and physiological responses; to determine the key climatic controls of the change in tree growth and physiological responses, and determine how each of the key climatic factors and their interactions affect tree growth and physiological responses. To determine the variation of the climate-tree relationship among tree species. The objective was to determine the phenotypic and genotypic variation of the effects of climate change on tree growth and physiological responses among tree species, To determine the acclimatization of tree species in response to climate change. The objectives were to determine the responses of tree species to climate change over time and determine tree species' responses to climate change before and after they are exposed to specific climatic conditions. Materials and methods To achieve the objectives, tree ring technologies were adopted. Trees record relevant information in their annual rings, represent important natural archives of climate changes, and provide archives of tree growth responses to the past climate variation. With tree ring width growth, information from tree-ring stable isotope compositions were used to better understand the dynamic relationships among the climate, tree growth, and physiological responses. My current research was commenced with seven tree species sampled from five different forests in China, covering both subtropical and boreal climatic conditions. The long-term tree-ring chronology was established by applying tree ring width measurement and cross-dating verified by COFECHA program; therefore, the basal area increments (BAI) were calculated sequentially. Meanwhile, the intrinsic water-use efficiency (iWUE) was calculated by measuring carbon isotope composition (δ13C) in tree ring samples. Tree ring δ13C relationships with BAI and atmospheric CO2 concentration were also quantified. Results and discussion From this study, we have gained further understanding of the relationships among long-term climate change, tree growth and physiology, as a basis for future projection of silvicultural manipulations under different climate change scenarios. In Chapter 2, both BAI values of the two tree species (Pseudolarix amabilis and Cryptomeria japonica, sampled from a subtropical monsoon forest located in eastern China), continuously increased with the rising of CO2 concentration until the atmospheric CO2 concentration tipping points were reached (the tipping points of Pseudolarix amabilis and Cryptomeria japonica were in year 1997 and 1996 when atmospheric CO2 concentration reached 365.1 ppm and 636.0 ppm respectively), after which tree growth started to decline with the rising CO2, while iWUE exerted a continuous increase trend with the increasing CO2 concentration. In Chapter 3, the results showed a decreasing trend in relative humidity over the past 70 years in a subtropical forest of south-east China with rising CO2 concentrations and temperature and the initial increasing tree growth for both Pinus massoniana and Cryptomeria japonica from the rising CO2, which peaked when CO2 concentration reached 330 ppm and 385 ppm in year 1974 and 2008 respectively, but decreased thereafter with increasing water limitation. Tree iWUE showed the same continuing increase trend as the two species in Chapter 2. In Chapter 4, three tree species (Cinnamomum micranthum, Pinus massoniana and Cunninghamia lanceolata) were sampled in two nearby subtropical forests of south-east China. The tree-growth also initially increased with the rising Ca, then decreased with the increasing Ca. The tipping points among the three species slightly varied but all happened between year 1995 and 1999. In addition, iWUE continuously increased with the rising Ca regardless the tipping points of BAI with the Ca. In Chapter 5, two species (Larix gmelinii Rupr and Betula platyphylla) were sampled in a boreal forest of north-east China, the results were similar to the previous chapters, while iWUE showed consistent increase during the entire growth period for both species, BAI reached the tipping points when Ca reached 366 ppm in year 1998 for Larix gmelinii Rupr, and 353.5 ppm in year 1989 for Betula platyphylla. In summary, the experimental results demonstrated that tree growth of BAI showed a continuous increase among all sampled tree species with the rising CO2 concentration until the CO2 concentration tipping points were passed. The trees’ responses were both species and site dependent. After reaching the critical points, tree growth started to decline even with the rising CO2 concentration, while iWUE exerted a continuously increasing trend with the increasing CO2 concentration, which biologically proofed that the decreased BAI was not dominated by tree age, but due to the rise of Ca and warming induced water limitation. The series of tree ring studies reported in this thesis has highlighted that there would be non-linear tree growth responses to the increasing Ca of the tree species in both subtropical and boreal forests, with the initial increases in tree growth detected as the atmospheric CO2 increased, but the tree growth peaked when the critical tipping points of Ca were reached and then declined thereafter. However, tree WUE continued to increase with the rising Ca, initially due to the increasing photosynthesis and tree growth, then later due to the warming induced water limitation. Unfortunately, the tipping points of Ca for tree species in both subtropical and boreal forests were reached between 1974 and 2008, and tree growth decreased with the rising Ca once the CO2 tipping points were passed, leading to a positive feedback to climate change.