Transcript Present-day Climate and Vegetation of the Willamette Valley Prepared by: Laura Stallard ES473 Environmental Geology Abstract Methods (cont.) This paper examines the present-day climate and vegetation.

Present-day Climate and Vegetation of the Willamette Valley
Prepared by:
Laura Stallard
ES473 Environmental Geology
Abstract
Methods (cont.)
This paper examines the present-day climate and vegetation patterns in
the Willamette Valley, and forms part of a larger theme session entitled “Earth
Science in Context: Land use and Watershed Function in the Willamette Basin”.
The Willamette Valley climate zone encompasses a 200-km-long lowland that
lies between the Coast and Cascade ranges. This region is characterized by
generally cool, wet winters and warm, dry summers. The Mediterranean climate
pattern results in a unique assemblage of vegetation, including the dominance
of coniferous forests. The focus of this poster is on elevation-controlled changes
in vegetation throughout parts of the Willamette Valley, as a function of local
climate variation.
The Willamette Valley itself is relatively warm and dry which is not as
favourable for conifers. Even in the time of the first settlers the valley was not
covered in forest and so was one of the first places settled.
•Because of this the valley has been experienced extensive humane
influence.
•Cities, farms, and other development have altered the valley.
•Now, even the natural areas of the valley have a trace of human impact,
which includes the affects of pre-settlement Native Americans and their use
of wildfires.
Introduction
The past and present regional climate and vegetation section of this theme
session is intended to bring a better understanding to the setting of the
watershed of the Willamette Valley and the physiographic framework. The
Willamette Valley has the most varied vegetation pattern in Oregon along with
the greater part of the state’s population.
The valley is made up of alluvial flats broken up by low rolling hills. Although
the largest contributing factor of vegetation difference is elevation change, the
valley is fairly flat, only rising from 50 meters in Salem to 129 meters in Eugene.
This leads to the sluggish meandering of the Willamette River. As is normal in
the Pacific Northwest, the amount of precipitation also falls as you move south
while the temperature rises.
Figure 3. A map of Oregon’s annual precipitation. (Oregon Climate Service, 2005).
Discussion and Conclusions
The climate and vegetation of today’s Willamette Valley has come
together to create a unique mosaic. The Pacific Northwest has a distinctive
climate that has shaped the valley and the human impacts of settlers and
pre-settler Indians have impacted the region as much as the climate has.
Changes in elevation and precipitation have affected the succession of the
forests in a way not common to other Mediterranean zones, leading to the
dominance of coniferous species. The forests have high stand productivity
and biomass accumulations that have benefitted the lumber companies
which in turn have their own affects on the valley. The valley’s diverse
agricultural area and long growing season also lends itself to a favourable
agricultural region
Mean Temperature:
Summer
Winter
High
Low 80's
~40
Low
Low 50's
Low 30's
Figure 1. Mean Temperatures in the Willamette Valley (Taylor and
Hannan, 1999).
Methods
In trying to understand the setting of the Willamette Valley we can look at the
factors that influence the mosaic of vegetation.
• The valley has a long growing season. It lasts 150-180 in the lower areas of
the internal valleys and 110-130 in and above the foothills.
Another characteristic of the PNW is the dominantly coniferous forests.
Evergreens are usually the pioneer or seral species and grow into a hardwood
dominated forest, but here in the wet, mild climate it is the opposite.
•It is thought to stem from the historical climate events combined with our
present climate that is very favourable to evergreen.
•The two climactic factors of today are the high precipitation and mild
winters.
Figure 2. Map of the Willamette Valley (Oregon State University,1999).
Results
All of the factors above have created the present day vegetation patterns
in the valley. Today there are five different categories of vegetation:
•Quercus woodlands.
•Coniferous forests: Large areas are clustered on the foothills.
•Grasslands: Before settlement and now grassland covers a wide area with
perennial, annual, and forbs grasses.
•Sclerophyllous shrub communities: More common in the southern valleys.
•Riparian forests: Dominated by hardwoods in places of poor drainage and
annual flooding.
Today 82% of Western Washington and Oregon is still classed as forest
land. The Willamette Valley also splits the Tsuga heterophylla zone which occurs
in the Coast Ranges and the Western and High Cascades. It is the most
extensive of the zones.
The combination makes the Willamette Valley and Oregon a favourable
place to settle. By understanding the region we gain a better grasp on the
land where we build and live.
References Cited
Franklin J.F.; Dyrness C.T., 1988, Natural vegetation of Oregon and Washington.
Reprinted with new bibliographic supplement, Oregon State University Press,
452p.
Oregon State University, 1999, <http://www.hort.purdue.edu/newcrop/
cropmap/oregon/graphics/will-vall.jpg>
Taylor, G., 2005, Special Report: Climate of Oregon, Special Climate Analysis
Service, Oregon State University, <http://www.ocs.orst.edu/page_links/
climate_data_zones/climate_oregon.html>
Taylor, G.H., and Hannan, C., 1999, The Climate of Oregon: from rain forest to
desert: Oregon State University Press, Corvallis, 45-56.