Coastal Temperate Rainforest – Chicken or Egg? Forest or Moss?

Written by Fred Bunnell

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

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.

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.

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.

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: