Intraspecific and interspecific variation in the xylem functional traits of Callitris species growing along an aridity gradient

Abstract

More severe and prolonged drought events as a result of climate change, have the potential to cause broad scale forest and woodland dieback worldwide. The Australian continent is primarily comprised of arid biomes. However, rapid climate change-induced desertification threatens these surprisingly diverse ecosystems. Callitris is Australia's most successful conifer genus, yet they remain they remain vulnerable to drought-induced decline. Given Callitris are the primary structural component of vegetation in many Arid-Australian ecosystems, their persistence is the most important factor preventing the collapse of these ecosystems. Resistance to drought-induced xylem cavitation has emerged as a key physiological trait determining the survival of tree species under water-limited conditions. Under the influence of aridity, Callitris have evolved the world's most cavitation resistant xylem, yet little is known about the xylem anatomy liable to convey this. The main objective of this thesis was to identify the anatomical xylem traits and attributes associated with cavitation resistance in Callitris. The main body of work in this thesis involved analysis of microscopic anatomical traits through the use and development of several microscopy techniques. An inter-specific study produced a complementary dataset of xylem anatomical traits for branches of 15 Callitris and closely related species, building on the physiological dataset by Larter et al. (2017). An intraspecific study among five C. glaucophylla populations required the physiological and anatomical traits measurements. An intraspecific increase in cavitation resistance with aridity was found among the five populations in both the primary branches and roots. To understand whole plant hydraulic function, variability in xylem anatomical traits in the tertiary branches, secondary branches and trunks, of C. glaucophylla, in relation to the primary branches and roots was also explored. A greenhouse experiment tested the plasticity of anatomical traits in C. glaucophylla seedlings grown under contrasting water treatments. Mainly, among seedlings grown under well-watered conditions, height growth and more hydraulically efficient roots are prioritised, while more mechanically reinforced tracheids and safer but less efficient pit traits are favoured among seedling grown under water deficit.

Date of Award	2020
Original language	English