19 – Agriculture, Pasture, and Mixed Environs
W. Daniel Edge, Rex C. Crawford, & David H. Johnson
Geographic Distribution. Agricultural habitat is widely
distributed at low to mid-elevations (<6,000 ft [1,830 m]) throughout both
states. This habitat is most abundant in broad river valleys throughout both
states and on gentle rolling terrain east of the Cascades.
Physical
Setting. This habitat is maintained across a range of climatic conditions
typical of both states. Climate constrains agricultural production at upper
elevations where there are <90 frost-free days. Agricultural habitat in arid
regions east of the Cascades with <10 inches (25 cm) of rainfall require
supplemental irrigation or fallow fields for 1-2 years to accumulate sufficient
soil moisture. Soils types are variable, but usually have a well developed A
horizon. This habitat is found from 0 to 6,000 ft (0 to 1,830 m) elevation.
Landscape Setting. Agricultural habitat occurs within a matrix of
other habitat types at low to mid-elevations, including Eastside grasslands,
Shrub-steppe, Westside Lowlands Conifer-Deciduous Forest and other low- to
mid-elevation forest and woodland habitats. This habitat often dominates the
landscape in flat or gently rolling terrain, on well-developed soils, broad
river valleys, and areas with access to abundant irrigation water. Unlike other
habitat types, agricultural habitat is often characterized by regular landscape
patterns (squares, rectangles, and circles) and straight borders because of
ownership boundaries and multiple crops within a region. Edges can be abrupt
along the habitat borders within agricultural habitat and with other adjacent
habitats. Structure. This habitat is structurally diverse because
it includes several cover types ranging from low-stature annual grasses and row
crops (<3.3 ft [1 m]) to mature orchards (>66 ft [20 m]). However, within
any cover type, structural diversity is typically low because usually only one
to a few species of similar height are cultivated. Depending on management
intensity or cultivation method, agricultural habitat may vary substantially in
structure annually; cultivated cropland and modified grasslands are typified by
periods of bare soil and harvest whereas pastures are mowed, hayed, or grazed
one or more times during the growing season. Structural diversity of
agricultural habitat is increased at local scales by the presence of
noncultivated or less intensively managed vegetation such as fencerows,
roadsides, field borders, and shelterbelts.
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Composition.
Agricultural habitat varies substantially in composition among the cover types
it includes. Cultivated cropland includes >50 species of annual and perennial
plants in Oregon and Washington, and hundreds of varieties ranging from
vegetables such as carrots, onions, and peas to annual grains such as wheat,
oats, barley, and rye. Row crops of vegetables and herbs are characterized by
bare soil, plants, and plant debris along bottomland areas of streams and rivers
and areas having sufficient water for irrigation. Annual grains, such as barley,
oats, and wheat are typically produced in almost continuous stands of vegetation
on upland and rolling hill terrain without irrigation.
The
orchard/vineyard/nursery cover type is composed of fruit and nut (apples,
peaches, pears, and hazelnuts) trees, vineyards (grapes, Kiwi), berries
(strawberries, blueberries, blackberries, and raspberries), Christmas trees, and
nursery operations (ornamental container and greenhouses). This cover type is
generally located on upland sites with access to abundant irrigation.
Cultivation for most orchards, vineyards and Christmas tree farms includes an
undergrowth of short-stature perennial grasses between the rows of trees, vines,
or bushes. Christmas trees are typically produced without irrigation on upland
sites with poorer soils.
Improved pastures are used to produce perennial
herbaceous plants for grass seed and hay. Alfalfa and several species of fescue
(Festuca spp.) and bluegrass (Poa spp.), orchardgrass (Dactylis
glomerata), and timothy (Phleum pratensis) are commonly seeded in
improved pastures. Grass seed fields are single-species stands, whereas pastures
maintained for haying are typically composed of two to several species. The
improved pasture cover type is one of the most common agricultural uses in both
states and produced with and without irrigation.
 Unimproved pastures are
predominately grassland sites, often abandoned fields that have little or no
active management such as irrigation, fertilization, or herbicide applications.
These sites may or may not be grazed by livestock. Unimproved pastures include
rangelands planted to exotic grasses that are found on private land, state
wildlife areas, federal wildlife refuges and U.S. Department of Agriculture
Conservation Reserve Program (CRP) sites. Grasses commonly planted on CRP sites
are crested wheatgrass (Agropyron cristatum), tall fescue (F.
arundinacea), perennial bromes (Bromus spp.) and wheatgrasses
(Elytrigia spp.). Intensively grazed rangelands, which have been seeded
to intermediate wheatgrass (Elytrigia intermedia), crested wheatgrass, or
are dominated by increaser exotics such as Kentucky wheatgrass (Poa
pratensis) or tall oatgrass (Arrhenatherum elatius) are unimproved
pastures. Other unimproved
pastures have been cleared and intensively farmed in the past, but are
allowed to convert to other vegetation. These sites may be composed of uncut
hay, litter from previous seasons, standing dead grass and herbaceous material,
invasive exotic plants (tansy ragwort [Senecio jacobea], thistle
[Cirsium spp.], Himalaya blackberry [Rubus discolor], and
Scot’s broom [Cytisus scoparius]) with patches of native black
hawthorn (Crataegus douglasii), snowberry (Symphoricarpos spp.),
spirea (Spirea spp.), poison oak (Toxicodendron diversilobum), and
encroachment of various tree species, depending on seed source and environment.
Modified grasslands are generally overgrazed habitats that formerly were
native eastside grasslands or shrub- steppe but are now dominated by annual
plants with only remnant individual plants of the native vegetation. Cheatgrass
(Bromus tectorum), other annual bromes, medusahead (Taeniatherum
caput-medusae), bulbous bluegrass (Poa bulbosa), and knapweeds
(Centaurea spp.) are common increasers that form modified grasslands.
Fire, following heavy grazing or repeated early season fires can create modified
grassland monocultures of cheatgrass. Agricultural habitat also contains
scattered dwellings and outbuildings such as barns and silos, rural cemeteries,
ditchbanks, windbreaks, and small inclusions of remnant native vegetation. These
sites typically have a discontinuous tree layer or one to a few trees over a
ground cover similar to improved or unimproved pastures.
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Other
Classifications and Key References. Quigley and Arbelbide181
referred to this as agricultural and exotic forbs-annual grasses
cover types. Csuti et al.58 referred to this habitat
as agricultural. The Oregon Gap II Project126 and
Oregon Vegetation Landscape-Level Cover Type127 that
would represent this type is agriculture. U.S. Department of Agriculture
Conservation Reserve Program lands are included in this habitat.
Natural Disturbance Regime. Natural fires are almost totally suppressed
in this habitat, except for unimproved pastures and modified grasslands, where
fire-return intervals can resemble those of native grassland habitats. Fires are
generally less frequent today than in the past, primarily because of fire
suppression, construction of roads, and conversion of grass and forests to
cropland.159 Bottomland areas along streams and rivers
are subject to periodic floods, which may remove or deposit large amounts of
soil. Succession and Stand Dynamics. Management practices disrupt
natural succession and stand dynamics in most of the agricultural habitats.
Abandoned eastside agricultural habitats may convert to other habitats, mostly
grassland and shrub habitats from the surrounding native habitats. Some
agricultural habitats that occur on highly erodible soils, especially east of
the Cascades, have been enrolled in the U.S. Department of Agriculture
Conservation Reserve Program. In the absence of fire or mowing, westside
unimproved pastures have increasing amounts of hawthorn, snowberry, rose
(Rosa spp.), Himalaya blackberry, spirea, Scot’s broom, and poison
oak. Douglas-fir or other trees can be primary invaders in some environments.
Effects of Management and Anthropogenic Impacts. The dominant
characteristic of agricultural habitat is a regular pattern of management and
vegetation disturbance. With the exception of the unimproved pasture cover type,
most areas classified as agricultural habitat receive regular inputs of
fertilizer and pesticides and have some form of vegetation harvest and
manipulation. Management practices in cultivated cropland include different
tillage systems, resulting in vegetation residues during the non- growing season
that range from bare soil to 100% litter. Cultivation of some crops, especially
in the arid eastern portions of both states, may require the land to remain
fallow for 1-2 growing seasons in order to store sufficient soil moisture to
grow another crop. Harvest in cultivated cropland, Christmas tree plantations,
and nurseries, and mowing or haying in improved pasture cover types
substantially change the structure of vegetation. Harvest in orchards and
vineyards is typically less intrusive, but these crops as well as Christmas
trees and some ornamental nurseries are regularly pruned. Improved pastures are
often grazed after haying or during the nongrowing season. Livestock grazing is
the dominant use of unimproved pastures. All of these practices prevent
agricultural areas from reverting to native vegetation. Excessive grazing in
unimproved pastures may increase the prevalence of weedy or exotic species.
Status and Trends. Agricultural habitat has steadily increased
in amount and size in both states since Eurasian settlement of the
region. Conversion to agricultural habitat threatens several native
habitat types.166 The greatest conversion
of native habitats to agricultural production occurred between 1950
and 1985, primarily as a function of U.S. agricultural policy.96
Since the 1985 Farm Bill and the economic downturn of the early
to mid 1980s, the amount of land in agricultural habitat has stabilized
and begun to decline.164 The 1985 and
subsequent Farm Bills contained conservation provisions encouraging
farmers to convert agricultural land to native habitats.96,
153 Clean farming practices and single-product farms
have become prevalent since the 1960s, resulting in larger farms
and widespread removal of fencerows, field borders, roadsides, and
shelterbelts.96, 153, 164 In Oregon, land-use planning
laws prevent or slow urban encroachment and subdivisions into areas
zoned as agriculture. Washington’s growth management is currently
controlled by counties and agricultural land conversion to urban
development is much less regulated.
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