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.