why so few hardwoods in the PNW?

Kirk Johnson newkirk at olywa.net
Wed Aug 19 14:09:54 EST 1998

Jim Christensen <jchris at ptinet.com> wrote:

> Joseph Zorzin wrote:
> > So why are conifers dominate in the PNW, a mild wet climate?
> According to J. F. Franklin & C.T. Dyrness, "Natural Vegetation of
> Oregon and Washington", it is the current and past climate.  The PNW
> has two climatic factor that caused this.  One is the high
> precipitation of which most occurs during the winter and is followed
> by a dry summers and the other is the very mild winters.  Other
> factors were the absence of appropriate genotypes during the
> Pleistocene  and the absence of strong winds that disturb and weaken
> the forests -- no hurricanes, typhoons or tornados.

Another good paper that should go a long way to answering Joseph's
question is: "Evergreen Coniferous Forests of the Pacific Northwest" by
R.H. Waring and J.F. Franklin  Science, Vol. 204, 29 June 1979. 

Here's what the abstract says:

"The massive, evergreen coniferous forests in the Pacific Northwest are
unique among temperate forest regions of the world. The region's forests
escaped decimation during Pleistocene glaciation;they are now dominated by
a few broadly distributed and well-adapted conifers that grow to large
size and great age. Large trees with evergreen needle - or scale-like
leaves have distinct advantages under the current climatic regime.
Photosynthesis and nutrient uptake and storage are possible during the
relatively warm, wet fall and winter months. High evaporative demand
during the warm, dry summer reduces photosynthesis. Deciduous hardwoods
are repeatedly at a disadvantage in competing with conifers in the
regional climate. Their photosynthesis is predominantly limited to the
growing season when evaporative demand is high and water is often
limiting. Most nutrients needed are also less available at this time. The
large size attained by conifers provides a buffer against environmental
stress (especially for nutrients and moisture). The long duration between
destructive fires and storms permits conifers to outgrow hardwoods with
more limited stature and life spans."

A professor of mine with a strong background in tree and plant physiology
still distributes this paper to students every year. If you feel ambitious
about learning about PNW forests, there is also the book:

Schoonmaker, Peter K., et al., eds.  "The Rainforests of Home: Profile of
a North American Bioregion"   Washington D.C.  Island Press  1997   ISBN:
1-55963-479-0 (cloth), 1-55963-480-4 (pbk.)

Kirk Johnson

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