Written by Fred Bunnell

Data from the forest biodiversity plots indicate that the streamside forest at Brooksdale is borderline coastal temperate rainforest (CTR); the forest higher on the slope much less so. So, what’s a CTR anyhow? It begins with climate – mild and mellow and has been for about 6,000 years. Coastal Washington gets fewer freezing nights than northern Louisiana; but in July daily maxima are lower than in Minnesota or Maine.

View All

Did You Know?

Creation is fascinating. The Did You Know? series examines this wild and wonderful world we live in. Check out the ponderings of biologist Dr. Fred Bunnell for a fun way to learn more about creation near you!
View All

It’s also wet because the moisture-laden jet stream runs into mountains. Prince Rupert is the wettest city in Canada, Henderson Lake on Vancouver Island is the wettest place on the continent. Despite all that rainfall, CTR can still experience summer drought. There is a long frost-free season, very few hurricane force winds and no tornadoes. The forest at Brooksdale is much closer to CTR in species composition than in precipitation. Climate influences all life. In the CTR the dominant life form is coniferous trees.

Graph comparing less precipitation in Surrey, more precipitation in Prince Rupert, and the most precipitation in Henderson Lake.

About the time of the dinosaurs most of the world was dominated by conifers. The long necks of Brachiosaurus, Diplodocus and Mamenchisaurus were for feeding in conifer canopies. Gradually the angiosperms or flowering plants took over, evolving into about 350,000 species. There’s only about 700 species of gymnosperms left, most of them conifers. Flowering plants excluded conifers from most areas other than on sites too soggy, sandy, dry or cold for broadleaved trees. The CTR is the one place on the planet where conifers are truly dominant. The tallest eastern conifer (white pine) gets up to about 200′ or 60 m, but at least 13 species of CTR conifers reach 300′ and the tallest were 400′ or 122m.

Tallest trees of select conifer species growing in the Pacific Northwest

First Second Third
Douglas-fir 99.8 m / 327 ft (OR) 98.3 m / 323 ft (OR) 96.8 m / 318 ft (OR)
Western hemlock 75.6 m / 248 ft (BC) 59.4 m / 195 ft (WA) 59.4 m / 195 ft (OR)
Western redcedar 59.4 m / 195 ft (BC) 55.5 m /182 ft (BC) 53.0 m / 174 ft (WA)
Sitka spruce 80.8 m / 265 ft (BC) 78.3 m / 257 ft (WA) 62.5 m / 205 ft (BC)
Yellow-cedar 61.0 m / 200 ft (BC) 46.9 m / 154 ft (BC) 45.1 m / 148 ft (BC)
Mossy Forest

Heights in the table exclude dead tops. It won’t be current when you read it; trees blow down, are cut down and re-measured. It does not include the tallest trees. The tallest Douglas-firs in British Columbia, for example, were about 400’ (122 m) – the Cary fir just north of Vancouver (127 m, 417’) and a tree that went to the Hastings Mill in 1896 at 120 m (393’). At least one Douglas-fir over 300’ still stands in the Coquitlam watershed, 310’ (94.3 m). The only conifer taller than those in the PNW is the coastal redwood of California. The very wet and warm coast of British Columbia hosts the tallest individuals for 4 of the 5 species in the table. It would be five had the tallest Douglas-fir been left standing. Although western hemlock grows well in the PNW as defined here, the three tallest western hemlocks measured anywhere are further south in California.

Why is it that conifers don’t simply dominate our coast, they flourish? Simple really. In most of the world’s temperate forests precipitation occurs year-round or mainly in the warm months. Trees receive their moisture when warm temperatures and high light levels allow trees to photosynthesize rapidly. Under those conditions, broadleaved trees outcompete conifers.

In the Pacific Northwest, summer temperatures and high light levels, which create the highest respiratory and evaporative demands, occur just when moisture is least available. In the tropics and savannahs, broadleaved trees cope with low moisture by losing leaves in dry season and regrowing them in the wet season. But if our local broadleaved trees tried that, they’d be in a heap of trouble. Our broadleaved species cannot photosynthesize at subfreezing temperatures, so they lose their leaves during winter. Arbutus or madrone makes it by restricting itself to areas where subfreezing temperatures are relatively rare, but now appears to be in trouble with moisture stress. Our local broadleaved species have difficulty maintaining leaves both in the summers (dry soils) and the winters (subfreezing). Conifers predominate. Where soils are moist during the summer, as on floodplains or riparian areas, we do find broadleaved species: red alder, cottonwood, bigleaf and vine maples. Generally, however, the CTR climate favours conifers because of the peculiar combination of ample rain, summer drought, and occasional subfreezing temperatures. The mellow climate and rather gentle natural disturbance regime means they can live an exceptionally long time. That in turn means they get uncommonly big.

Conversely, mosses and liverworts are small. Despite that, they accomplish a lot in a forest. They are tiny plants, but plants without plumbing. What look like miniature leaves have no veins to move water through them. Water and nutrients must be absorbed through the leaf surface and moved from cell to cell across cell membranes. What look like tiny roots can anchor the plant, but have no conductive tissue (though water and dissolved nutrients can move along them by capillary action).

Without plumbing to move water and nutrients they are limited to a few cm in height. Faced with this problem, bryophytes made it an opportunity by exploiting the nooks and crannies among larger plants and the advantages of the boundary layer, where air meets ground, runs into vegetation or other obstacles and slows down.

Kostan Inlet, Haida Gwaii
Kostan Inlet, Haida Gwaii
Coastal Leafy Moss
Plagiomnium (Mnium) insigne
Coastal leafy moss
Mossy Forest
Forest biodiversity plot at Brooksdale.
(Note moss-covered logs.)
Mossy Forest
Hoh River Valley, Olympic Peninsula, Washington
(Note moss-covered stems.)

The boundary layer, or floating greenhouse, does not extent far from any surface. Bryophytes grow taller on the forest floor, though never more than a few centimeters. Striving upwards after light is not part of their game plan. Their chlorophyll is finely tuned to the particular wave lengths of light that penetrate forest canopy to the forest floor. Tree stems create their own boundaries with air and host all manner of bryophytes when the air is moist. In the boundary layer, they crowd together grabbing and holding water like a wee sponge. Examining bryophytes under a microscope is invariably a romp through a miniature zoo housing all manner of creatures. The more you look, the longer the list.

Moss CloseUp
Moss-covered rotting log in Brooksdale forest.

Rotting logs are particularly good at retaining water and host a rich, moist boundary layer. Mosses and liverworts thrive. Logs cover relatively little of the forest floor in the forest biodiversity plots at Brooksdale (photo above). But 1 x 1 m bryophyte plots sampling the forest floor in the biodiversity plots found that 83% of the ground-layer bryophyte cover was on wood, usually logs, within the plots.

If you spend any considerable time in older west coast rainforest you are struck by three things: the trees are surpassing tall, the forest is surpassing green and everything is almost constantly wet. All are correct. The tallest individuals of most west coast conifer species are found in British Columbia. Green moss and liverworts do cover almost everything. Henderson Lake on Vancouver Island received over 9 m (3 stories) of rain in 1997. That poses another question – what came first: the big trees or the moss.

Mossy Forest
Windy Bay, Haida Gwaii

There is a chicken and egg conundrum about all this. Almost certainly the rain came first. But what came second – the towering stature of the trees or the carpets of moss? It’s not clear. If the rain stops for a bit and a beam of sunshine breaks through, chances are you will see a shimmery halo of moisture around moss clumps on stem bases or fallen logs. The halo reveals the large of amounts of moisture the bryophytes have soaked up. Because it is frequently wet they have crawled up the tree trunks, taking miniature gardens and zoos with them. The trees intercept moisture. As it travels down the trunk, bryophytes grab on to moisture and the nutrients in the moisture, slowing their return to earth and depositing a rich nutrient mix right at the tree roots. The moisture is no longer rainfall, but full of nitrogen and other goodies leached from needles, nitrogen-fixing lichens and algae plus a lot of poop from mites and other wee creatures. A surprisingly large percentage of the forest floor is mite poop. The wee streams of fertilizer running down the trunks are grabbed by the mosses at the base and released slowly and directly into the rooting zone. Dead mosses keep it from being leached quickly from the soil.

The trees naturally grow taller – often taller than anywhere else in their range. The mosses climb higher in the air and intercept more water, which gathers more nutrients as it flows down the stems. The mosses deliver the nutrients to the stem base so the trees grow still taller. The trees would not be nearly so tall without all that moss intercepting and storing water and nutrients to release slowly. The moss would not be nearly so abundant if the trees didn’t hoist some of them up to intercept rain and water vapour. The stature of the trees also helps shade the forest floor so the moss and liverworts do well. Moss would have helped create soil for the first trees, but now they are a quite happy mix, benefiting each other.

The bryophyte series contributors are:

Fred Bunnell
Fred Bunnell
Lead on text; some photos
Corey Bunnell
More photos
Anthea Farr
Still more photos