Which tree to plant where? Do we know enough to plant 1 billion trees?

Posted by Cate Ryan, @cate_ryan

Aotearoa was once cloaked in lush temperate rainforests and beech forests. They ensured the land was well looked after by providing ecosystem functions such as habitat, soil protection, carbon storage, fuel, localised shading and cooling effects, and filtration of water.

Today only 43% of Aotearoa is in native vegetation compared to c.80% before human settlement (Norton and Pannell, 2018). Ecosystem functions have degraded, especially water quality and soil protection (erosion) and we must also increase carbon sinks to play our part in tackling climate change.

The current NZ government seeks to significantly increase the number of trees in NZ with the 1 billion trees policy. Species that will be planted include radiata pine, redwood, eucalyptus, Douglas fir, totara and mānuka and these will be planted on private, public and Māori owned land (MPI , 2018). However, which species we plant has a big impact on ecosystem function. Plants trade-off water and carbon during the gas exchange process so that the quicker a tree grows, sinking carbon in the process, the more water it uses, and vice versa. Introduced species like eucalypts grow quickly so are great from a carbon perspective but they use a lot of water. Natives grow more slowly so they don’t accumulate carbon as quickly but they often use less water. We don’t yet have the numbers to properly quantify this in the field so we can’t currently make good decisions on prioritising carbon over water. Moreover, we need to plan ahead for climate change – which is expected to increase dryness and water scarcity.

My PhD research will determine the water use of a range of native compared to plantation exotic tree species in Northland and Canterbury farms, from the leaf to the landscape level – to help inform decisions about what trees to plant where. This is in collaboration with a wider Biological Heritage National Science Challenge programme around ‘Enhancing the ecological function of native biodiversity in agroecosystems’.

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Cate Ryan is a PhD student in Biological Sciences at the University of Auckland, supervised by Dr. Cate Macinnis-Ng

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