Layered in ancient mud below Sooke Lake’s surface and deposited as carbon in the forests around it are clues to climate change that could affect the future of Victoria’s drinking-water supply.

Now, a team of scientists is studying how those changes fit in the history of the Sooke Lake Watershed and what they might mean for its future.

article continues below

“The ecological characteristics of those lands are responsible for the high quality of our drinking water, so it’s important for us to understand those lands and how human disturbances and natural disturbances affect them,” said Joel Ussery, resource planning manager for the watershed protection division of the Capital Regional District’s integrated water services.

Existing research points to increased summer temperatures and more pronounced summer drought, warmer water bodies and streams, as well as warmer winters and more severe winter storms.

The CRD is supporting two studies by research scientists from the Canadian Forest Service, as part of its efforts to minimize the impact of climate change on Greater Victoria’s water supply.

One study looks at forest fires and climate history since the ice age, while the other focuses on the past 100 years. Together, the data collected can be entered into models projecting how the ecosystem might change.

Kendrick Brown has been studying the “stratigraphic sequences” of that mud since June.

Layered in that mud is evidence of the landscape’s transition from a blanket of ice at the end of the last Ice Age 15,000 years ago to a moon-like terrain of unconsolidated sediment, and the rainforest it is today.

“What I wanted to do was take us through that transformation,” Brown said.

In practice, Brown collects and analyzes core samples up to 15 metres below the sediment’s surface, reaching mud up to 12,000 years old. Within each layer are charcoal and pollen samples, as well as other indicators of historic forest fires, vegetation changes and more.

The sediment acts like an archive, Brown said. Each layer is like the page of a book.

In one example, samples from between 11,700 and 7,000 years ago identify a warm and dry climate susceptible to droughts — conditions that might occur with climate change.

“We’re honing in on a past warm-dry period and saying — what did vegetation look like? What did fire disturbance look like? — as a potential analogue for the future. We’re also trying to understand how the fire regime changed in response to climate, so we can understand those processes better as we prepare for the future changes in climate.”

Brown and his team are also measuring the magnetic-susceptibility characteristics of the sediment, which can determine how the land surface responds to forest fires. Is there significant movement of sediment after a fire?

“That has impacts on water quality,” Brown said.

Tony Trofymow’s research focuses on changes in the past 100 years, a topic he has studied for 25 years as part of the Coastal Forest Chronosequence Project.

The soil ecologist is studying the role of forests as both carbon sinks and carbon producers — and how that has changed over time. He and his team compile historical maps and aerial photos to identify disturbances such as fires, as well as changes in forest types over time.

“Our objective is to see what the effects of historic land use and natural disturbances have on the landscape carbon budget,” he said.

A “carbon budget” is like a bank account, Trofymow said, with uptakes and emissions similar to deposits and withdrawals. “Carbon stocks” are like the bank balance and include carbon in live biomass of trees, as well as dead organic matter including woody debris, forest floors and mineral soil. Trofymow calculates those stocks over time.

In the Sooke Lake Watershed, the biggest carbon emissions occurred in the 1930s and 1940s, he said, stemming from major forest fires as well as large-scale logging. Logging continued in the 1950s on a smaller scale and emissions declined. Since the 1990s, the watershed forests have been a carbon sink, with carbon stocks accumulating.

Overall, total carbon stocks have declined from 710 tonnes of carbon per hectare in 1910 to 540 in 2012. But forests are taking in more carbon each year than they did in the past, reflecting a shift from mature and old-growth forests to younger ones, he said.

“Since all the disturbances occurred, we have younger forests, which are more rapidly growing. So their annual uptake of carbon is now higher,” Trofymow said.

Although he can’t isolate the influence of climate change so far, the information he collects can be used along with prediction models of disturbances in future climates.

Once more information is gathered about the past, the CRD will be able to look to the future.

That means studying potential climate-change scenarios as they apply to the reservoir and surrounding forests.

“That’s our next big step,” Ussery said. “What, if anything, can we do as stewards of the land to minimize any negative effects that may occur?”

Among the potential implications already identified in the CRD’s strategic plan are more algal blooms and changes in the species composition in the water. Warmer water and less transparency could favour species that cause taste and odour or produce toxins. And the forests in the water-supply area may die from drought or become susceptible to new insect species and disease.

Responding could mean higher doses of chemical treatment in the water supply and better forest-fuel management, such as cleaning up woody debris or planting drought-resistant tree species.

In the meantime, Ussery said the CRD is lucky to have Canadian Forest Services researchers doing their work in the watershed.

“Their research is a huge benefit to us, because not only does it help us understand the ecological processes going on in any forestry area, but it’s very specific to our water-supply area,” he said.

“We have a real responsibility to try and understand how these ecological characteristics and processes affect water quality and how any change that affects these processes may affect water quality. And what, if anything, we can do to deal with these negative effects.”

asmart@timescolonist.com