Pine beetles and watersheds

Have you ever wondered what effect the beetle epidemic might have on the hydrology of our watersheds? Perhaps you've heard loggers attribute wet soils and poor winter logging conditions to the dead trees. Maybe you've heard of possible increases in spring runoff. These are valid concerns because dead trees allow more snow and rain to reach the ground and dead trees do not use soil moisture for transpiration. Meanwhile, accelerated salvage logging is also changing the hydrology of our watersheds. To complicate things, the weather is so variable, how can we distinguish one effect from another?

Hydrologists have a fairly good understanding of the effects of forest practices on the hydrologic cycle, fish habitat, and water quality. However, they are less certain about the effects of an extreme beetle infestation because they have had limited opportunities to study those effects until recently. Hydrologists with the Ministry of Forests and Range, Thompson Rivers University, UBC, and UNBC are now starting to answer these questions.

I am a hydrologist with the Ministry of Forests and Range in Williams Lake and, with funding from Natural Resources Canada, am studying the effects of dead pine stands on hydrologic processes that occur in the winter and spring. These are important because the highest annual streamflows are usually produced by the spring freshet, which can result in flood damage. Annual floods are also important to aquatic habitat because the physical structure of streams is largely determined by the size of these high flows. Natural resource managers would therefore like to have an idea of the potential effects of any large-scale disturbances which might cause peak streamflows to increase.

Most floods in the plateau occur when an above average snowpack melts quickly due to a warm period in late winter or early spring. Therefore, if dead pine trees allow more snow to accumulate on the ground, the potential size of spring floods increases. The provincial government maintains a network of snow survey stations which are used by Ministry of Environment scientists and engineers to help forecast floods. However, there are too few of these to detect changes in the snowpack due to the deterioration of pine stands or salvage logging.

I have established a new network of 30 snow survey sites to study these changes. They are in old pine stands, recent clearcuts, and pine plantations of different ages. With help from local contractors and assistants, I am measuring how much snow accumulates and how fast it melts in these different types of sites.

How do we measure snow in something as complex as a forest? Good question! Snow is highly variable in a forest so they measure it at 36 locations in each stand. Also, we don't simply measure snow depth, because the amount of water being stored depends on the snow's density. Instead, we weigh snow cores, which tells us the equivalent depth of water stored in the snow. Differences in this "snow water equivalent" between stands is one of the main things we are documenting.

Based on the first season of measurements in March 2006, it appears that both beetles and logging can increase the late winter snowpack for many years, but there are important differences. For example, the death of the overstory seems to increase snow accumulation on the ground by a moderate amount and this increase may persist for decades. This is based on similarities between snow in recently attacked pine stands and those that were killed in the 1980s. On the other hand, logging appears to increase the snowpack by roughly twice as much as beetles but cutblocks might "catch up" with killed pine stands within a couple of decades due to rapid tree growth. The distribution of cutblocks less than about 20 years old across a watershed may therefore be one of the more important factors over which we have control.

The high variability in the weather means that this type of study needs to continue for years before hydrologists can be confident with the results. What happens after 30 years or so is even more problematic, ecologically as well as hydrologically. For one thing, we don't have very many stands that were severely beetle attacked more than 30 years ago to study. Another problem is that we don't have very good inventory information on natural regeneration or understory trees in old pine stands. The abundance of understory conifers in old pine stands is quite variable over the landscape and this will affect how fast the attacked stands recover after the initial period of overstory deterioration.

So how will his snow research data be used? It will help hydrologic modellers forecast possible changes in flood frequency. A watershed model is a computer program that calculates snowmelt and stream flow from weather data and forest characteristics over a watershed. These models are not perfect but they are one of the best tools we have to estimate the potential effects of pine beetles and logging on flood magnitudes. The snow data being collected by me and my colleagues will be used by the modellers to improve the accuracy of the snowmelt component of the models. The first modelling study of the potential effects of beetles and salvage logging on future flooding in B.C. Interior has been done and will be published soon in a report by the Forest Practices Board.

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Pat Teti

Pine beetles and watersheds

About Us • Public Education • Species at Risk • Projects • Other Activities • Membership