Water and Carbon Relations in New Zealand Kauri Forest

Abstract

New Zealand kauri forest is the most species rich forest ecosystem in the country. It is dominated by Agathis australis (kauri), an iconic long-lived southern conifer. A. australis is an ecosystem engineer that influences the forest around it, but the tree species is also vulnerable to drought which poses a particular threat with drought frequency and duration being predicted to increase in northern New Zealand. The overall aim of this thesis was to gain a better understanding of the water and carbon relations of A. australis and co-occurring kauri forest species in order to quantify physiological responses to environmental drivers like vapour pressure deficit, solar radiation and soil moisture. I investigated species differences in plant water relations, seasonal and species-specific variations in water use and carbon, and water storage as a potential drought adaption for A. australis. First I explored leaf level water relations of A. australis and three co-occurring kauri forest species: the two podocarps Podocarpus totara and Phyllocladus trichomanoides, as well as the angiosperm Knightia excelsa. Leaf level processes, i.e. photosynthetic assimilation and stomatal conductance, influence whole tree and stand water and carbon fluxes. Even small changes at the leaf level can greatly affect the carbon and water cycles of the forest ecosystem. Despite marked differences in drought vulnerability of the investigated species, I found conservative stomatal regulation in all four species with similar responses to vapour pressure deficit and solar radiation. For A. australis, the degree of isohydry is vital to maintain hydraulic integrity, for the other species the conservative stomatal behaviour could indicate nitrogen limitation of photosynthetic activity. Due to a strong relationship between whole tree water use and diameter, I used the diameter to scale to stand water use. Even though stomatal conductance was low, the stand still transpired up to 3.4 mm per day. Solar radiation and vapour pressure deficit were the strongest drivers of transpiration, for individual trees and the stand alike. Soil moisture, on the other hand, had little influence on variations of tree and stand water use, likely due to access to deeper water sources. While the explanatory power of soil moisture was not significant, it did influence the strength of the relationship of water use and the environmental drivers. The slope of the linear relationship of water use and vapour pressure deficit declined markedly when soil moisture content was low, indicating greater stomatal regulation of transpiration when soils are dry. A. australis has very deep sapwood which could result in a large water storage capacity. I therefore explored the withdrawal of water from storage using sap flow sensors and dendrometers at multiple locations along the stem. Water from stem storage compartments did contribute up to 22 % of the tree's daily water use. The stored water mainly originated from inelastic xylem tissue and only to a small degree from elastic tissues. Withdrawal of water from storage could be a potential drought survival strategy of A. australis, as it buffers daily fluctuations in xylem tension and reduces the risk of hydraulic failure. Finally, I measured the seasonal variations in non-structural carbohydrate concentrations (NSC) in different organs of A. australis. Large NSC pools could be another strategy against drought-induced mortality as A. australis' isohydric behaviour could lead to carbon starvation during drought. I found comparably low NSC concentrations in all tissues, but the large sapwood volume of A. australis could still mean that there is a sizable storage pool in larger trees. Additionally, NSC concentrations were significantly correlated with tree diameter, which reflects an increased carbon supply due to greater leaf area but also a greater carbon demand. I found evidence for a variety of traits and strategies to cope with dry soil in A. australis. This includes conservative stomatal conductance, access to deep soil water stores and use of stored water in tree stems. These results indicate A. australis may be able to survive the predicted intensification of drought across its range. More research is needed to fully understand climate change impacts on this iconic species. A greater threat is kauri dieback disease (caused by the water mould Phytophthora agathidicida) and while I did not study the impacts of this pathogen directly, my research into the physiology of A. australis will assist with treatment and control approaches.