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.