PS 43-166 - Whole-tree responses to drought for better predictions of drought vulnerability: New Zealand kauri (Agathis australis) as a case study

Abstract: Global syntheses of tree mortality often use plant functional traits to predict drought vulnerability. These studies can help us identify sensitive species or geographical hotspots and can be particularly powerful for understanding forest mortality under future climatic conditions. However, assessments made using a small suite of traits to determine drought mortality risk may be overly simplistic, especially where other traits may be able to protect a tree from drought stress. For instance, vulnerability to xylem embolism (P₅₀ value), is a commonly used predictive trait but a tree with deep roots may be able to prevent declining water potentials in stems by accessing deep water stores to maintain plant water status.

Southern conifers are an under-studied group with an ancient lineage. They are found in Australia, South America, New Caledonia and New Zealand and evolved around 200 million years ago. Many conifers in this group are long-lived and therefore need to be able to survive fluctuating environmental conditions throughout their lifetime. New Zealand kauri (Agathis australis) is an iconic species with ecological and cultural significance. It is confined to the north of New Zealand’s North Island in an area that is suffering more frequent, severe and hotter droughts. Tree ring records indicate kauri are highly responsive to climatic conditions and published P<sub>50 values for stems and roots of kauri suggest the species should be highly vulnerable to drought mortality. However, by studying a full suite of plant functional processes under changing seasonal and soil moisture conditions, we have found kauri have a range of strategies for maintaining water status under drought. We present data from sap flow sensors, point dendrometers, leaf gas exchange, isotopic analysis of water use efficiency, non-structural carbohydrates and leaf water potentials under ambient and simulated drought for mature trees (around 300 years old). Our whole tree analysis indicates the trees have a range of adaptations to protect a vulnerable water conducting system, including conservative stomatal conductance, deep roots, reduction of sap flow and increased use of stem water stores under drought and the capacity to acclimate to drying soils indicated by buffering of tree water deficit during dry summer months. Our results have implications for global models of drought mortality but provide a positive picture for this species under a drying climate.