Ecohydrology in New Zealand. Water supply issues and the threat of drought for forests

Under a changing climate, parts of New Zealand will become warmer and drier. As yet, we know very little about the impact this will have on vegetation. In a recent post on the Waiology blog, I explain the importance of understanding responses of evapotranspiration (ET) to climatic conditions because ET is a major component of water budgets. It is particularly vital to quantify how much water is returning to the atmosphere as through this pathway in water supply catchments. In future climates, overseas studies indicate that water yield will decline, reducing water available for human consumption.

A further reason to explore the impact of future climates on forests is the potential for increased tree mortality. Several instances of drought-induced tree mortality in New Zealand were included in a global review in 2010. Such events are likely to become more common in all biomes because trees across the world operate close to their safety margins and even slight changes in climate can be catastrophic for trees. The combination of increased temperatures and lower rainfall leads to particularly challenging conditions for tress because both the soil and the atmosphere are drier, putting the plant’s water system under stress. This lethal formula has been implicated in California recently and was discussed as a global issue at the Tree Mortality Workshop in Jena, Germany last week. In addition to changing climate, attacks from insects, fungi and other pathogens will further weaken trees and impact on forest ecosystem services. With so many unique plants and ecosystems in New Zealand, there is an urgent need to improve our understanding of drought in the local context.

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