Critical junctures in eco-hydrologic control of forest water balance

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Forest harvesting and natural disturbance by wildfire and insects converts older forests into younger forests and this can have a large effect on regulation of water production from forests in headwaters regions. As young forests grow, increasing water consumption by growth of canopy leaf area increases the amount of water lost by transpiration during photosynthesis and reduces the amount of water entering the forest by intercepting rain or snowfall. Therefore leaf area regulates how much water the forest vegetation consumes through interception and transpiration, which in turn, regulates how streamflow is produced by forests. Thus changes in forest structure can produce very large changes in water production from headwater regions as forests establish, mature, and develop into old growth phases. Other changes in forest vegetation such as slowly changing species composition with forest succession can also cause large changes in the amount of water the forest vegetation consumes because some species use more water than others. Developing a better understanding how much water is used by forests of differing age and by different forest tree species is important to be able to predict how Canadian water supplies might change with both natural changes in forests over time and those related to human use of the forest.