New Zealand’s future climate is likely to be warmer and dryer and the frequency and duration of drought events is predicted to increase. Drought-induced tree mortality is increasing world-wide, with several instances also reported in New Zealand. So far we know very little about the drought vulnerability of New Zealand forest trees, but due to our research on kauri we are beginning to understand more and more about the drought survival strategy of this forest giant.
The roots are integral for trees to extract water from the soil and a good root network is crucial for drought survival. During times of water stress many trees, including kauri, invest in root growth. This allows them to keep up their normal transpiration levels for a little longer. So far it is assumed that kauri roots are very shallow, but sap flow measurements during the 2013 drought suggest otherwise. The upper soil layer during that time was extremely dry, but the trees still used water which suggests that kauri roots must reach a lot deeper than we previously thought allowing access to deeper water stores.
Drought avoidance or toleration?
In general, every tree species falls somewhere on the spectrum between drought avoidance and drought toleration. Drought tolerating trees keep up transpiration as long as possible. Drought avoiding species on the other hand start closing their stomata to reduce water loss, when the soil moisture goes down. Both strategies have their downsides. Drought tolerators risk the formation of little air bubbles (xylem embolism) in their conducting tissue. This can lead to hydraulic failure if a drought lasts too long. Drought avoiders protect their hydraulic integrity but risk starvation, because the closure of the stomata also means a reduction of carbon intake. Kauri are clearly drought avoiders. Even under ideal growing conditions kauri are conservative water users, closing their stomata early in the day. They are known to be very susceptible to xylem embolism and protect their hydraulic integrity in that way.
During the 2013 drought the kauri in our study plot lost a substantial amount of leaves and twigs. The reduction of leaf area is an effective way to reduce the water-losing surface and consequently the reduction of transpiration and the need for water uptake.
All components of a tree (roots, stem, branches, leaves) can serve as water storage compartments. This is a drought survival strategy that succulents have perfected. Kauri make use of stored water on daily basis. Water is withdrawn from the stem and branches in the morning when the water starts to transpire from the leaves. During the afternoon and night these stores are refilled again. The massive stem volume paired with deep sapwood seem to make a great water store. During prolonged drought conditions kauri should be able to use the water reserves to their advantage. This is something we are investigation right now, stay tuned.
Julia Kaplick is a PhD student in the Centre of Biodiversity and Biosecurity, School of Biological Sciences, University of Auckland. She is researching the response of native trees to seasonal variation in climatic conditions using measurements of sap flow, water relations and carbon allocation. Julia is supervised by Cate Macinnis-Ng (University of Auckland) and Mike Clearwater (Waikato University). Julia is supported by funding from the Marsden Fund.