Posted 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.

  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.

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.  

Posted by Cate Macinnis-Ng @LoraxCate

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

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.