Which biome is considered an exception to the positive relationship between ET and NPP?

In the context of ecosystems, evapotranspiration (ET) refers to the sum of evaporation and plant transpiration from the Earth's surface to the atmosphere. Net primary productivity (NPP) is a measure of the amount of carbon uptake after subtracting the carbon lost through respiration from gross primary productivity. Generally, there is a positive correlation between ET and NPP, as higher moisture levels tend to support greater plant growth.

However, tundra ecosystems present an exception to this trend. In tundra regions, which are characterized by cold temperatures and short growing seasons, the availability of water can actually be limited despite high levels of evapotranspiration occurring during the growing season. The permafrost layer restricts root growth and water availability for plants, resulting in lower NPP than might be expected from the amount of water available through ET. Additionally, the vegetation in tundra is typically low and limited to herbaceous plants, which have a lower overall productivity compared to the forests and grasslands that might exhibit higher NPP with increased ET.

This unique combination of climatic and soil factors means that in tundra ecosystems, even as evapotranspiration rates might increase, the NPP does not increase correspondingly, highlighting the complexities of ecological relationships in different bi