Carbon dynamics within kauri (Agathis australis) under drought conditions and infection by Phytophthora agathidicida

Abstract

Terrestrial forest ecosystems play a significant role in the global carbon cycle, functioning as carbon sinks and mitigating climate change impacts through carbon sequestration. Under climate change, there is a global risk of widespread forest mortality due to the increase in frequency and severity of drought events and pathogen attack. Plants respond to drought stress by reducing tissue growth and photosynthetic carbon uptake thus mobile carbon reserves within plants (in the form of non-structural carbohydrates, NSC) are crucial for the survival and tolerance of trees as soils dry and as carbon demand for metabolism continues under drought. Less is known about carbon dynamics in trees infected by pathogens. In Aotearoa New Zealand, knowledge of carbon responses of native plant species to drought stress and pathogen infection is limited and there is a pressing need to address this knowledge gap in a changing climate. We explored NSC dynamics and growth patterns of mature Agathis australis kauri trees under ambient and drought conditions created by a throughfall exclusion experiment at Huapai Scientific Reserve, Auckland. We investigated drought influences on seasonal patterns of total NSC concentrations, composition (soluble sugar, starch and total NSC) and allocation to three tissue types (leaves, stem and root). Drought effects on seasonal and annual growth of kauri were monitored with manual band dendrometers and automatic point dendrometers. NSC dynamics in tissues of kauri trees infected by the pathogen Phytophthora agathidicida were also assessed. We found that carbon dynamics in large mature trees are complex. Drought stress and infection had significant effects on NSC allocation to tissues. Stem NSC and root sugar concentrations decreased in drought-treated kauri and in infected kauri, NSC concentration in leaves increased and stem sugar content decreased. Higher canopy position increased total NSC concentrations within leaves of drought-treated and infected kauri. There were no significant differences detected in seasonal NSC concentrations and NSC compositions within drought-treated kauri tissues and drought conditions did not have significant impacts on seasonal and annual growth rates. The maintenance of consistent NSC concentrations and growth rates of drought-treated kauri suggested mature kauri trees are well adapted to and tolerant of drought conditions. The effect of P. agathidicida infection on NSCs indicates this pathogen interferes with the carbon dynamics of kauri. This study on NSC responses to drought within kauri trees contributes results towards understudied NSC dynamics of Southern Hemisphere species and their response to drought and disease.