SpruceRoots Magazine - July 2001


by Erica Thompson

Some scientists consider fog as far more than a picturesque veil concealing coastal landscapes and oceanic climes. A closer look reveals the tiny droplets that make up fog are vital contributors to the forest ecosystems of the Pacific Northwest. Fog, it is said, may be the most underrated agent in supporting the largest living conifers on the planet.

For many, Haida Gwaii is synonymous with "misty isles" and lands covered with huge western red cedar, hemlock and spruce trees. The archipelago, and especially the windward inlets and sounds are frequently shrouded in fog, mist, haze or concealed by low lying cloud banks. The unique island climate is influenced by maritime conditions, prevailing winds, the mainland mountain barrier, the North Pacific high pressure during the summer, and the Aleutian low pressure system which brings dense clouds, strong winds and heavy precipitation during the winter. The predominant recipe for fog is warm moist air contacting cool coastal waters which lowers temperatures below the dew point. When fog sweeps into coastal forests, inland along valley corridors, up slopes or onto treed slopes, it marks the arrival of massive amounts of water in forest ecosystems. The trees strip water contained in the fog. As the droplets fall from thousands of individual needles the understory and soils absorb the water.

Water inputs supplied by fog have been known by many names over the centuries: fog drip, cloud drip, fog stripping, horizontal precipitation, negative interception and the eccentric occult precipitation, intimating stealth moisture arriving under the cover of darkness. But despite the many descriptions, fog water has rarely been taken into consideration in conventional resource management and while snow and rain are used to determine local precipitation input, fog is not. Researchers working in California coastal redwood forests and in Oregon's Bull Run watershed have offered clues to the meaning and functions of fog drip and their publications have opened windows on the relationship of fog and forest.

Plant ecologist Todd E. Dawson of Cornell University and University of California Berkeley, published a study on the utilization of fog by redwoods in northern California and southern Oregon. Dawson's The Use Of Fog Precipitation By Plants In Coastal Redwood Forests did much to dispel conventional wisdom deeming trees, especially old growth conifers, as harbourers of water, capturing and storing water then made unavailable to streams for example. Dawson's work indicates that fog and its interception by these giant trees contributes large volumes of water to forest habitat through the process of fog drip.

"Past literature has often claimed that fog may serve as a potential source of water for plants, yet no investigation has explicitly quantified if this is true and to what extent fog water is used by the vegetation," Dawson explains. To explore how redwoods use fog and the implications of fog drip on the surrounding forests, Dawson strove to determine which type of water ­ fog, rain, or ground water ­ plants use throughout the year.

Interestingly, all water is not equal. Hydrogen and oxygen, the two compounds making up water, are formed in different arrangements depending upon the source. Fog water and rainwater can be distinguished based on the different ratios of isotopes they contain. Dawson collected plant samples on fog days and analyzed water isotopes he had isolated from the xylem of plant species to determine which water source the trees and understory plants were using.

He found that during the summer months when fog was most frequent in northern California and southern Oregon, between 8-34% of the water used by the redwood, Sequoia sempervirens, was fog derived. Usually the species is dependent upon deeper soil or ground water provided by rainfall during winter rainfall events, says Dawson. Between 6-100% of the water used by the understory vegetation came from fog derived precipitation after it had dripped from the tree foliage into the soil. Hydrologic studies indicate that moisture input to the redwood forests from fog can constitute between 30-75% of the annual water budget, he claims. At certain times of the year, nearly half of the water input originated with fog!

As fog moves into a forested area, it travels through the canopy and the moisture is effectively stripped from the air by the tree's branches and array of needle-like foliage, which Dawson describes as a 'layered-like comb'. The water captured by the needles - an old tree collects droplets on perhaps 60 million needles, a surface area of one acre - then drips down branches and trunk to the plants growing at the base of the tree. During the summer months, sword ferns appeared to be completely dependent upon fog derived precipitation. Dawson's work suggests that fog drip can account for half of the water coming into a redwood forest in a year and is critical in maintaining the moisture that so many species depend upon in northwestern rain forests.

Dawson also discovered that not only are the plants of coastal redwood forests using high proportions of fog water but that the presence of the trees themselves significantly influences and moderates the magnitude of water input from fog. He noted that between 22­46% of the moisture input to the ecosystem was due to the presence of the redwood trees themselves (interception input) and when trees were absent interception input declined by 19-40%.

"From a management perspective," reports Dawson, "the fact that loss of redwood trees due to natural disasters (e.g., fire, windthrow, or floods) or from logging or other land use practices which convert the forest to open habitats will dramatically alter the hydrological and ecological balance of these forests." Loss of the canopy tree would mean not only the loss of biomass, nutrients within the biomass, and the soils, but also a fundamental conversion of a once moist, cool, forested ecosystem into a more drought prone, and warmer ecosystem, he says.

California conservation group, Friends of Old Trees, was successful twice after using this fog drip thesis to stop logging in old growth redwood forests. They argued the loss of water from fog drip had not been adequately addressed in logging plans for the area. (New York Times, 1998)

Identifying just how much water fog contributes to a forest ecosystem sheds light on the question of why more water runs off or out of watersheds than falls as precipitation. In a study conducted in Oregon's Bull Run watershed, Portland's primary water source, by the Portland Bureau of Water Works, where the annual rainfall is approximately forty inches while the average runoff is 135 inches. Fog drip contributed an estimated 35 percent of annual precipitation under the old growth canopy, the Bull Run study says.

Does the fog drip phenomena, in the northern California and Oregonian forests of the Pacific Northwest apply to Haida Gwaii? Canada's leading fog expert is Dr. Robert Schemenauer, Emeritus Research Scientist at Environment Canada, and he says the fog drip phenomena is most definitely at work in the Island's forests.

"Indeed they [BC's coastal forests] function just like more southerly forests and collect enormous quantities of water from the fog. Some tree species will be more efficient than others and windier locations will have more fog collection. The percentage of the total water going into the coastal ecosystems that is from fog is not being measured or even estimated," says Dr. Schemenauer. "My rough guess in the area that you are interested in [Haida Gwaii] is that it would be 10 to 30% of all the water going into the ecosystem. This is an enormous amount of water and the input is directly related to preserving the forest cover in the fog belt on the mountains."

Research shows that spruce canopies, along with redwood and Douglas fir, are efficient in stripping and inducing fog drip. In a recent article, Elizabeth Keppeler notes Joseph Kitteridge's 1948 study reporting 285 mm of fog drip collected during on summer season under an 85-year old spruce hemlock stand in coastal Oregon.
Using Trevey's model from A Total Forest Management Plan and Wildland Management Decision Support System as, fog drip volumes can be estimated for a watershed. Using information from areas receiving 40 inches of precipitation a year and with the Tlell receiving 48 inches there is an estimated input of ten inches from fog drip over the Tlell's, 33,000 hectares. Given that, the Tlell would receive an estimated 83,382,000 cubic metres of fog derived water.

Just how much water is this? It is enough to cover the surface area of Yakoun Lake to a depth of approximately ten metres or 33 feet. Or using the distance of highway between Masset and Queen Charlotte which is approximately 130 kilometers long and the pavement ten meters wide (1,300,000 square metres of pavement), the amount of fog drip derived water from a watershed the size of Tlell is estimated as the equivalent of distance of highway between Queen Charlotte to Masset to a height of 64 metres (210.4 feet) or the equivalent of a 21-story building of water!
More or less, fog and fog drip contribute an unquestionable volume of water to forests with frequent fog events. However, the presence of fog is not enough and the water inputs will be lost if there is no canopy to intercept it. To date research and measurement of fog contributions to Island watershed's has not been conducted.

Fog Harvesting
In other parts of the world, fog is being harvested as a source of fresh drinking water and irrigation for remote villages. Illustrating the life sustaining potential of fog and clouds is the experience of the remote and arid Chilean village of Chungungo where fog water now flows from the taps and the annual water budget has more than doubled since fog collectors were installed in the mountains beyond the village.

Dr. Shemenauer is a key player in creatively solving water supply problems in rural and remote villages through low tech fog harvesting.

In his essay, Collecting Mists from the Earth, Dr. Schemenauer says, "the water source is sustainable over periods of hundreds and likely thousands of years because the driving forces for the formation of the cloud decks are global in nature and will only change very slowlyAs clouds move over hills and mountains, the hilltops and ridgelines are enveloped in fogs. Just as the leaves and needles of trees can collect some of the water in these fogs, large artificial collectors can produce a flow of potable water."

Dr. Schemenauer explains that fog droplets, smaller than drizzle and raindrops, have low settling velocities and travel wherever the wind blows them. To capture these droplets, simple collectors are designed from a frame supporting a section of mesh in a vertical plane. The ultraviolet-resistant polypropylene mesh is stretched horizontally between the poles and positioned at a right angle to the prevailing winds and catches the fog droplets while winds pass with little resistance. As the fog collects on the mesh larger water droplets form and drip down into a collecting trough, then flowing through a system of pipes to a cistern.

"The large collectors are usually 12 long and 6 metres high. The mesh covers the upper 4 m of the collector. This gives a collecting surface of 48 m2 and typical water production rates from one collector of from 150 L day-1 (litres a day) to 750 L day-1 depending on the site." The fog collectors are simple, require no energy other than the wind and deliver their water by gravity flow, he says.

Beyond consistent fog conditions, the most important geographical and meteorological factors in successful fog collecting are global wind patterns, the presence of a mountain range and its orientation to the wind, altitude, the distance from the site to the coastline, the space available for fog collectors, relief of the surrounding area, topography and wind speed, crestline and upwind locations, the presence of upwind slopes, and the microtopography of the area.

Fog collecting projects, inspired by the average 11, 000 litres of water a day which now flows into Chungungo, Chile are happening in countries such as Nepal, Mexico, Oman, Croatia and Namibia. This year hundreds of delegates, from over sixty countries, will attend the 2nd annual International Conference on Fog and Fog Collection, chaired by Dr. Shemenauer, this July 2001 in St. John's, Newfoundland. Presenters will share their research on such topics as mountain fog/cloud experiments, fog physics and chemistry, fog contributions to sub-surface water supplies, socio-cultural issues related to fog and fog collection, assessment of the role of fog and dew in ecosystems. It appears the allure of fog, fog drip and fog collecting is catching on.


Keppeler, Elizabeth T; 1998. The Summer Flow and Water Yield Response to Timber Harvest. USDA Forest Service. 42p.

Dawson, Todd E; 1996. 'The Use Of Fog Precipitation By Plants In Coastal Redwood Forests. Website: http://www.cnr.berkeley.edu/~jleblanc/WWW/Redwood/rdwd-The-4.html

Queen Charlotte Islands ­ Haida Gwaii - Background Report: An Overview of Natural, Cultural, and Socio-Economic Features, Land Uses and Resources Management. Draft
Website: http://www.luco.gov.bc.ca/slupinbc/qcharlot/3.htm

Schemenauer, Robert; Collecting Mists From the Earth
Website: http://www.cru.uea.ac.uk/tiempo/floor0/recent/issue26/t26art3.htm

Carol Kaesuk Yoon, Clues to Redwoods' Mighty Growth Emerge in Fog: New York Times, November 24, 1998 (Section D, 1p.)
Website: http://www.jailhurwitz.com/media/pac_lumber_done/35_media.htm


SpruceRoots Magazine - July 2001

graphics - InHouse/SRs