No two watersheds are the same

D-01: Long-Term Watershed Studies: What Have We Learned About Eco-Hydrological Functioning

 

Long-term research on watersheds shows that there's no simple formula to predict how watersheds will respond to land-use change. Photo by National Park Service.
Long-term research on watersheds shows that there’s no simple formula to predict how watersheds will respond to land-use change. Photo by National Park Service.

From studies in China to Ethiopia to Portugal, the United States, South Korea, and Japan, it’s clear that there’s no simple formula to predict how watersheds will respond to land-use change. This session spoke to the need for site-specific evaluations in making management decisions about forested watersheds.

Variations in vegetation, soil, climate, and topography are all important factors in watershed responses, but relationships that hold true in one system may be quite different in another.

Much of the session focused on plantation forests established to restore deforested land and prevent erosion. Yanhui Wang spoke about large-scale afforestation of larch in Northwest China and how variation in stand density impacts ecosystem services. He and his colleagues found that a stand density of 1000 to 1200 trees/ha is optimal to maximize total ecosystem service value.

A different afforestation study of a pine/eucalypt plantation in Portugal by Daniel Hawtree examined the progressive changes in streamflow that occurred during a vegetative shift from 1936 to 2000 from a shrubland, to a pine-dominated system, to a primarily eucalypt monoculture.

While negative streamflow trends were expected to occur with afforestation, the only significant trend he found between streamflow and afforestation was a positive trend during the pine afforestation period. Kevin Bishop presented another study on streamflow responses to long-term forest cover change. Despite drastically different types of land cover, there was no consistent change in streamflow.

Research on large-scale disturbance had more consistent results. Xiahua Wei presented a study in British Columbia of large-scale forest disturbance by logging, fire, and bark beetle outbreaks that found that the magnitude of high flows significantly increased following disturbance. Similarly, Jason Hubbart reported increased water yield from harvested catchments in northern Idaho.

The above studies showed that vegetation effects streamflow and water yield, but the characteristics of specific watersheds also impact the vegetation.

John Yarie conducts long-term precipitation elimination experiments in forests in upland and floodplain areas in interior Alaska. He found that white spruce growing in the floodplain showed significantly reduced growth when summer rainfall was eliminated, but no change when winter snowfall was eliminated. This contrasted with the upland sites where aspens, birch, and balsam poplar showed no significant change in response to the summer precipitation elimination experiment, but significantly lower growth when snowmelt was eliminated.

Yarie concluded that it was important to discern which water sources were tied to tree growth, especially when making predictions of how forests will respond to climate change.

In light of land-use change and heightened concern over sustaining our forest and water resources, understanding how to manage watersheds in a way that maximizes ecosystem services such as water yield, biodiversity, carbon sequestration, and timber yield is a high priority topic.

While there seems to be no general prescription for how to best manage watersheds, the speakers in this session stressed the continued importance of long-term ecohydrological research and the long-term data they yield.

Written by: Allison Chan

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