Kauri and drought – What’s their survival strategy?

Posted by Julia Kaplick @julekap

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.

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Roots

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.

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Kauri roots

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.

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Kauri cone in a bed of leaf litter

Leaf shedding

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.

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Base of a kauri stem

Water storage

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.

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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.  

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A Trip to Switzerland to learn some Wood Anatomy Skills

Posted by Julia Kaplick @julekap

In June this year I was lucky enough to escape the Auckland winter weather and learn some new skills at a Wood Anatomy Course in the Swiss Alps. It is a long running course organized by Dr Holger Gärtner, Prof Fritz Schweingruber from the Swiss Federal Institute of Forest, Snow and Landscape Research and Dr Alan Crivellaro from the University of Padua in Italy. The two main aspects of the course are the theoretical basics of the anatomical features of wood and the practical skills needed for sampling and preparing wood thin sections. This might not be obvious to everyone, but I was super excited to go and it was not because it took place in Klosters, where Prince Charles goes on skiing holidays.

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Left: Microscopy with a view of the Swiss Alps. Right: Gentian, the Swiss national flower. Right: Out in the field with Prof Fritz Schweingruber, one of the world’s leading experts in wood anatomy

There are many different scientific applications for wood anatomy, but I am most interested in the connection with tree water relations. Anatomical features like lumen area and cell wall thickness vary seasonally and are strongly influenced by climatic conditions. The wood anatomy also affects hydraulic characteristics of trees. Tree species with larger lumen areas can transport more water, but they are also more likely to suffer from embolism (the formation of air bubbles) during times of drought stress.

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Sample preparation – Top: With a microtome wood samples can be cut into thin section. Bottom: Staining of the sample and baking to create permanent slides

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Thin section of a kauri root – Staining of the wood thin sections makes anatomical structures more visible. Left: unstained. Right: same sample stained with Safranin and Astrablue.

The first day of the week-long course was all about the theoretical background. We spent the day looking at many thin sections under the microscope, starting with simple conifers, and later learned about the more complex structures of angiosperms and even had a glimpse at some crazy looking non-woody species. On the following days we went to some beautiful alpine valleys to try out different sampling techniques and learned how to prepare and stain professional thin sections from our own samples.

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Radial thin sections of rewarewa (left), tanekaha (middle) and nikau (right).

I could have easily spent the whole week cutting and staining my samples, but we also got to go on two little trips. The first one was a walk through a sustainably managed forest area, together with the responsible forester. The second trip was a visit to the Institute of Snow and Avalanche Research in Davos where we got to see the latest fashion accessories on the Swiss skiing field and also got to know a little more about how effective forest is as a protection against avalanches. Another highlight of the week was Helga, the lovely hotel cook who insisted on providing us with two hot meals a day, to keep our brains running. Yes, there was a lot of cheese and chocolate.

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Left: Fancy new avalanche protection. Middle: View of Klosters from above. Right: Happiness after a long day of learning

 

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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.  

Play time – Scientific toys

Posted by Julia Kaplick @julekap

My research focuses on trees, but when I look at the equipment I fill my car with when I go out to my field site, it looks more like I am an electrician. There are car batteries, cables, a bag full of tools, a laptop and lots more. Only a field notebook to record observations like Darwin or Humboldt have done it, is just not enough anymore. Nowadays most research involves specialised equipment to gather data or samples and advanced technology to physically or chemically analyse all sorts of sample materials. That can be daunting, but mostly it is just great to have so many toys to play with as a researcher.

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Kauri tree with sap flow sensor – nicely wrapped (for sun protection) like toys for christmas

I rely on sensors to gather most of my data. A meteorological station records environmental data for me, sap flow sensors (that I partly built myself) measure how much water my trees are using and I use a little thing called Trephor to collect wood samples (see how in this little video). The newest additions to our toy collection are called radius dendrometers. They will very soon record the expansion and contraction of my study trees’ stems on an extremely fine scale. That will give me an idea about daily patterns of water storage and growth. When the parcel with that equipment arrived I was excited like a little child at Christmas, but very soon the daunting part started. I needed to figure out how to get them working, how to install them and there is always that little bit of anxiety, because all that equipment is ridiculously expensive since it is so specialised and only few people in the world use it.

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Some of the toys I get to play with: Trephor micro corer to collect wood samples, our meteorological station, sap flow sensors (that is how it looks like under the wrapping paper) and data loggers to record measurements (with lots of fun coloured cables)

All these toys are great and playing with them is mostly fun, the harder part is the interpretation of the data they gather. Even though there have been many advances in how to get data the part where you have to make sense out of it to generate knowledge has not actually changed much since Darwin and Humboldt.

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#girlswithtoys, specifically #ecologytoys: Measuring water potential with a pressure bomb and (attempt to) shooting down leaves

To have a look at some cool toys scientists are using check out #girlswithtoys on twitter.

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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.  

A little ode to field work

Posted by Julia Kaplick @julekap

Julia takes aim!Spring is on our doorstep here in Auckland and nature is visibly getting busy. It is the start of the growing season for many plants and the most active time of the year for many animals. For many ecologists it also means that field work season is starting.

I am lucky enough to do my research in a more applied area of ecology. I get to go out into the forest and collect the data that forms the basis of my research myself. It is in fact the part that I enjoy the most and the main reason why I chose to work in ecology. During more than a year of field work in New Zealand, I have been soaking wet, freezing cold and muddy from head to toe, but at the end of the day I went home happy and with pages full of data. I have been hanging 15 metres high in the kauri forest canopy and freezing my feet off while taking predawn measurements in the mangroves.

The view from above.

The view from above.

Field work is challenging and fun. It teaches you to plan and organise, to improvise and to find creative solutions, as things do not always go as originally planned. Who knew that the party supply store around the corner would turn into one of the best sources for field equipment? What else could you possibly do with these metre long party straws than put wood samples into them and there cannot be any other proper use for Styrofoam cups than to use them as a radiation cover for temperature sensors. I also learned that a pressure bomb is a good thing and have significantly increased my electrical skills by building and connecting sensors. You also know that you are working with pretty cool instruments when your supervisor seems to be more worried about the machine dropping out of than canopy than about the PhD student who holds it. All this I have to admit came at a price. I probably lost several litres of blood to mosquitos and a scar on my middle finger will always remind me that soldering irons are extremely hot, not that I really needed that reminder.

Mud pie anyone?

Mud pie anyone?

Field work lets you see the world with different sometimes slightly nerdy eyes. It is exciting and rewarding to see theory come to life, even if it is sometimes unexpected. Someone recently told me that field work can become a little addictive and I can already see why.

What about the water? ‘Ecofriendly’ plantations will drink vast amounts of water

Margaret, Jacqueline and CatePosted by Cate Macinnis-Ng @LoraxCate

A government funding windfall to the Dryland Forest Initiative was reported in the NZ Herald today. The plan is to breed and grow drought adapted eucalypts in dry regions of the country with less than 1000 mm of rainfall per year.

Seems like an excellent plan. Eucalypts grow quickly, rapidly accumulating carbon in high quality wood. Even better, they are very comfortable in dry conditions. Sounds like a perfect ‘environmentally friendly’ product.

But what about the water cost? As I explain in this post, trees use vast amounts of water. A single tree can use over 2000 litres in a day and a stand of trees may use 90% or more of incoming rainfall. This means less water for other activities in the catchment including water required to maintain stream health. Eucalypts will exploit water resources as they become available with deep roots often accessing groundwater and physiological processes rapidly responding to rainfall events.

As droughts become more frequent, eucalypts are likely to remain healthy but they will use scarce water resources. Any plans for plantations or increases of woody vegetation in dry regions need to include a consideration of the impact on catchment water yield. A local water budget that includes groundwater is vital to ensure there is enough water for all purposes during dry periods. The often unseen impact is that groundwater become depleted over time. The trees will be fine because they are used to dry conditions but the rest of the system may not be alright.

Dr Cate Macinnis-Ng is a Lecturer in Ecology, School of Biological Sciences, University of Auckland.  She is a plant ecophysiologist and ecohydrologist working on plant-climate interactions.

Fascination of trees – Why are trees so thirsty?

photo_julia  Posted by Julia Kaplick @julekap

Water is crucial for most organisms on the planet. Humans are made up of more than 50% water and everyone knows how important it is to water the plants in the garden. Trees are especially fascinating in their water use. On a hot summer day an individual tree can take up close to 2000 litres of water and transport that huge amount several 10’s of metres from the roots to the canopy. But unlike us humans trees do not have a pumping organ to achieve that. The uptake is driven by negative pressure created at the surface of the leaves when water is transpired through the stomata to the atmosphere. More than 99% of water is transpired and only a very small amount is actually used during photosynthesis or to transport metabolites within the tree.

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Tall trees, canopy sampling and sap flow sensors at Huapai scientific reserve. Middle photo by Freddie Hjelm from The Living Tree Company

The great loss of water is necessary because at the same time carbon dioxide can diffuse into the leaf where it can be transformed into sugars and subsequently used for growth and maintenance. Water use patterns vary greatly between tree species. Some trees have a greater water use efficiency than others, which means they can gain more carbon while losing less water than other species. Some trees also have very conservative water use patterns, they control their stomata opening and lose less water, but also take up less carbon resulting in slower growth.

On a global scale trees and forested areas play a major role in the hydrological cycle due to transpiration but also because of evaporation of intercepted water. According to estimates, 41,000 cubic km of water are transpired globally every year. This is equivalent to 630 times the water volume of Lake Taupo and strongly influences rainfall patterns and the amount of water vapour (an important greenhouse gas) in the atmosphere. On a more local scale forested areas directly and indirectly influence many fresh water catchments and subsequently the supply and quality of drinking water.

Transpiration is still one of the biggest uncertainties in climate modelling, mainly due to a lack of data and the difficulty of measurement. We also know very little about the water use patterns of New Zealand trees. How much water do they use? How does that vary seasonally and annually? What effects does a rising atmospheric carbon dioxide concentration have on water use and transpiration? And how do changes in other climate variables affect the water use and related physiological processes of trees in New Zealand?

The recent global fascination of plants day highlighted the value and intrigue of plants and trees add an extra dimension to that. My research is exploring the physiology of four native tree species here in New Zealand. Stay tuned for some fascinating results!

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.  

What does drought mean for New Zealand’s native forests?

Posted by Cate Macinnis-Ng @LoraxCate

Margaret, Jacqueline and Cate

I don’t know much about Rolling Stone magazine. Like many, I presume, my impression is all electric guitars and rock and roll. So to stumble across one of the most engaging and accurate descriptions of the impacts of the current years-long drought on the forests of California was surprising in the least. The article is well-written and covers the complexities of plant water stress in an understandable way. It also addresses the global problem of drought and what that means for trees. The news is not good but since when did tree physiology become so rock and roll? Well, California is currently in the worst drought on record. Agriculture, food prices and water availability are suffering. The combination of low rainfall and extreme heat is killing forests and woodlands in a climate change double punch. And the situation is similar across the US and in other parts of the world where droughts are becoming more frequent and severe.

Here in New Zealand, we might expect to be buffered from such events. Our maritime climate is often described as mild. Temperatures are neither too cold nor too hot and annual rainfalls are generally more than adequate. Yet droughts are a common feature in the climate of New Zealand and images of browned agricultural areas have become common in the media in recent summers. The summer of 2012-13 was the most widespread and severe in 70 years. With future projections indicating parts of the country will spend more time in drought, there will be measurable impacts on agriculture, the economy and even health.

Cracking soil at the base of a kauri tree on March 2013

Cracking soil at the base of a kauri tree on March 2013

While droughts in New Zealand last months, not years, there is still a real potential for substantial impacts on forested systems. Tree mortality has been recorded in our native forests and there is evidence that trees living in wetter climates may have less of a buffer against dry periods. All forest biomes are equally susceptible to drought mortality no matter what their current rainfall is. If droughts are not severe enough to cause death, other processes and functions may be disrupted. For instance, carbon cycling can be altered and pathogen attack can be exacerbated. Some of the research I’ve been doing with students and colleagues is showing some native species have adaptations for drought survival (here and here) but we need more research to understand the thresholds of water stress, recovery after drought and impacts of subsequent droughts. I’m sure drought research will never reach rock and roll status in New Zealand but we’re certainly working towards improving our understanding of the threat of drought in native forests.

Dr Cate Macinnis-Ng is a Lecturer in Ecology, School of Biological Sciences, University of Auckland.  She is a plant ecophysiologist and ecohydrologist working on plant-climate interactions.