New drought response found in kauri trees

By Lynley Hargreaves 04/06/2014

Cate at HuapaiLike the Lorax, the University of Auckland’s Cate Macinnis-Ng speaks for the trees. A plant ecophysiologist, she translates how kauri trees respond to drought based on probes, loggers, and sensors. Dr Macinnis-Ng explains how her Marsden-funded research can help inform both the past and the future of these giants of Northland’s forests.

Why did you choose to work on kauri trees?

I’ve always been fascinated by how trees work and kauri are particularly interesting because they are so big and live for so long. Like many tree species, kauri store information about the climate in their tree rings. This climate record is particularly long for kauri because of their longevity and the sub fossil specimens buried in Northland extend the record thousands of years. The pattern of tree rings seems counter-intuitive to me because larger rings happen during dry periods But the physiological mechanisms behind these patterns have never been explored. It’s a complicated story and we may never get to bottom of that but we are looking into the physiology of kauri during drought.

What got you started on this research?

Global records of tree mortality during drought show an increasing frequency of tree death as droughts become more severe and we’ve had instances of tree deaths here in New Zealand even though rainfall is generally quite high. Trees in urban areas may be particularly unhappy, especially if they don’t have access to deep water stores. During the 2013 drought, there were plenty of examples of kauri street trees dying in Auckland and Hamilton. Previous studies have shown that kauri have weak water conducting systems that fail during periods of low water availability. Kauri also often remain on exposed ridges and some evidence suggested their root systems were shallow. These are all early indications of drought/climate change threat.

Are kauri at risk from climate change?

Despite these indications, we are finding that kauri in forested areas have several potential drought adaptations. During the drought, we found there was a significant increase in leaf fall. This reduced the leaf area of trees, a common water-saving mechanism used by trees. We also know that kauri close their leaf pores (stomata) early in the day to reduce water loss. Large trunks have the potential to act as water reservoirs during dry periods and we are in the process of studying this further. Finally, we found some trees had very little reduction in water use despite reductions in water in the top 60 cm of soil. This indicates the roots have access to deeper water stores. Indications are that kauri dieback is a greater threat than drought but often there are interactive effects between climate conditions and pathogen impacts so we need to explore those further.

How many trees do you measure?

We’re working at the University of Auckland Huapai Scientific Reserve in West Auckland in a stand of remnant kauri. Some are up to 1000 years old others are just saplings. Our measurement trees range in size from 175 to 20 cm in girth. We have multiple probes within trees and the sensors are logging all the time, recording the rate of water moving up the xylem. We are also looking at how fast trees growing and we do intensive field campaigns to explore leaf-scale processes. We are installing water sensors in other associated species like tanekaha and totara and we are exploring internal carbon allocation and usage across seasons to understand plant growth and activity patterns above and below grownd. Ultimately we want to understand how much water the trees are using and how this changes as rainfall changes.

That’s very detailed work! Can you see the wood for the trees?

We are coming to a point where we can start putting process models and physiological study together so we can simulate how climate change will influence water use and carbon uptake in native forests. This is really exciting because it hasn’t been done in New Zealand and we are looking forward to adding our unique vegetation to the global story of drought impacts on forests.

These interviews showcase researchers supported by the Marsden Fund which, since 1994, has been supporting fundamental, investigator-led research in New Zealand.

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