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.