Plants that favor similar living conditions group into separate
but related communities. You have seen how the mixture of plants
on a north slope differs from those on a south slope. They are
two plant communities. The individual plants in each community
are always working together and competing with each other.
A complex community consisting of several life forms is a desirable
community to have on rangelands. Grasses are good conservers
of water and soil, and grasses are also good forage producers.
The sagebrush-grass community is typical of ranges of the Pacific
Northwest. This community usually consists of a mixture of bunchgrass
(such as bluebunch wheatgrass, Idaho fescue and Sandberg bluegrass)
interspersed with sagebrush. Other plant communities of importance
in the Northwest include trees. For example, ponderosa pine
is often found with an understory of bitterbrush and bluebunch
wheatgrass or Idaho fescue.
These are only a few examples of plant communities. It is natural
to find many kinds of plant communities, since plants tend to
sort themselves into groups according to their needs. Because
this is true, many plants have ‘indicator value’. That is, their
presence indicates many things about the habitat in which they
are growing.
The plant community is never stable, but is always changing
for better or for worse. Range condition can be changed by management.
In a typical sagebrush-grass community certain changes may take
place as a result of continued early and heavy use. For example:
The taller, early growing but late maturing grasses such as
bluebunch wheatgrass or Idaho fescue are grazed closely, preventing
seed formation and storage of food in the roots. Many plants
die as a result of continued, heavy use. The remaining plants
are low in vigor and unable to compete successfully with less
desirable plants (such as sagebrush and weedy grasses) which
are being utilized to a lesser degree.
If a plant is grazed and then allowed to make top growth again,
it will not be seriously hurt. But if the shoots are kept grazed
close to the surface of the ground, the plant suffers. Where
the shoots are kept down, the roots are shortened. This makes
the range plant less able to compete for moisture and nutrients
with thrifty, ungrazed, less preferred plants and undesirable
plants. Undesirable plants that decrease habitat health and
provide lesser quality forage for livestock and wildlife thus
replace desirable, usually native, plants.
If all plants are grazed to the same height, the short ones
will have the advantage over the tall ones. Compare a tall (12-inch)
and a short (3-inch) grass growing side by side. If both were
grazed down to the same height, say 2 inches, what fraction
of the tops of each would be lost to the plant? Grazing short
grass to the height of 2 inches would remove only about one-third
of the tops. In the case of tall grass, the 2-inch grazing height
would result in more than three-fourths of the top growth being
removed. It is easy to see that the tall grass would be crowded
out because its food-making ‘machinery’ would be reduced too
much to work well. They are usually replaced by less preferred
or shorter plants. Poisonous and injurious plants may increase
on rangeland where there is continuous heavy use.
How does healthy range happen? It takes many thousands of years
for stable, healthy range plant communities to develop. But
finally, the plants on the range are balanced with the soil
and climate. There are communities of plants that make the best
possible use of the available soil nutrients, soil moisture
and the energy from the sun. The healthiest range is approaching
the potential natural community.
One of the objectives of range management is to maintain the
most efficient combination of desirable plants on the land,
so that these growth factors – nutrients, moisture and sunlight
– may be converted into the largest possible amount of ecological
services.
State and Transition Plant
Communities
Managing rangelands effectively requires an understanding
of plant communities and community succession/transition. Generally,
if left undisturbed long enough, a region, defined by its climate,
abundance of water, type of soil and location, supports a certain
type, or types, of plant community. If a site has been disturbed
the stable-state community present is changed and replaced by
an early seral community consisting of hardy, less competitive
pioneer plant species, or by another stable-state community.
If the stable-state community has been replaced by a seral,
or transition, community and the site is left undisturbed then
the early seral community gradually changes as pioneer species
are replaced by more competitive, less hardy, late-successional
species. These later seral stages eventually give away to a
stable-state community. The rangelands of the Pacific Northwest
generally support a stable-state community dominated by perennial
bunch grasses, some forbs and sparse brush such as sage or juniper.
In some regions such as the Pacific Northwest regular disturbance
plays a critical role in maintenance of the stable-state community.
Occasional fires in the Pacific Northwest rangelands, prior
to Euro-American settlement, insured that perennial bunchgrasses
remained dominant. In these regions, when fires are not allowed
to burn occasionally, brush species begin to competitively exclude
bunchgrasses and gain dominance in the plant community.
Plant Community Succession Theories
Early in the 20th century Clements introduced the idea of
succession in plant communities from early seral stages to the
given region’s climax stage. Clements believed that succession
took place in a linear, ‘start and stop’ manner; a seral plant
community occupied a region until enough factors built up to
push it into the next stage. The transition would be rapid with
little of the prior community remaining. About a decade after
Clements presented his theory it was challenged by Gleason,
who believed that succession happened in a more gradual manner,
with later seral species slowly invading earlier seral communities
and eventually out-competing the earlier species. Both Clements
and Gleason believed that a region, left undisturbed long enough,
would support one specific plant community called the climax
community. Over time, theories on plant succession and communities
have abounded.
Five vegetation classification theories that have influenced
the debate on community succession are: Monoclimax Theory, Polyclimax
Theory, Polyclimatic Climax Theory, Climax Pattern Theory and
Site Climax Theory. The first three theories (monoclimax, polyclimax
and polyclimatic climax) require mature soils and geomorphological
equilibrium, which are not the case in many regions, to identify
climax communities. They also take a Clements-ian view on plant
succession as occurring in discontinuous communities. The Climax
Pattern and Site Climax theories do not require geomorphological
equilibrium or soil maturity in determining climax communities.
For this reason, these two theories are may be more useful to
range managers in assessing rangeland health. The main difference
between these last two theories is that Climax Pattern views
vegetation communities from a Gleason-ian continuum while Site
Climax views communities on a discontinuous basis. Finally,
Site Climax Theory is the most useful of all five theories discussed
in management, in that it can be employed in any area and most
closely represents actual plant community dynamics. Site Climax
allows for vegetation communities that are in dynamic equilibrium
with their sites to be considered climax even if they are on
disturbed or eroded soils. (Meeker,
1984)
Some theorists have challenged the very idea that plant communities
move in a linear fashion towards an end, or climax, community.
If plant communities do not evolve in a linear fashion then
management plans that are based on the more traditional views
of plant succession may be ineffectual. In range management
managers are often seeking to improve rangelands from poor condition
/ low seral woodlands to good condition / high seral range dominated
by perennials. Oftentimes managers try to achieve this end through
grazing management alone. Several studies have shown that grazing
alterations alone produce little or no improvement in range
conditions. The ‘cup in ball’ analogy and state-and-transition
model are employed to show that moving rangeland from one of
many stable states to another more desirable state requires
specific effort and management beyond simple grazing controls.
Though heavy grazing often has been responsible for pushing
rangelands into poorer condition simply removing grazing usually
is not sufficient to push them back into better condition. More
traditional plant succession theories are considered insufficient
tools for management in that they do not give adequate attention
to multiple stable states. (Laycock,
1991)
Plant Community Changes in the Pacific
Northwest
Plant communities can be changed and manipulated by natural
and anthropogenic forces. In recent history, human use of western
rangelands has had extensive impacts on plant community structure.
Many of these newly established plant communities may be stable-state
communities but are often less desirable from both a livestock
production standpoint and an ecological standpoint. Re/establishing
more desirable plant communities will require an understanding
of the history that led to the current plant community and a
comprehensive management plan for obtaining the desired plant
community.
Juniper-grassland
Climate change has affected the range of juniper for thousands
of years but the expansion of juniper in the last 120 years
due to anthropogenic factors is unprecedented. Juniper continues
to expand, since American-European settlement, in range and
density throughout the west. In prehistory, wetter periods led
to the expansion of juniper. Juniper has increased since about
A.D. 1500. Prior to Euro-American settlement Juniper were sparsely
placed throughout savannah-like environs, restricted mostly
to rocky outcrops and rocky soils. Juniper stands have expanded
and become more dense, even invading riparian areas, in recent
history due to human caused factors such as decreased fire frequency
(heavy grazing reduced fuel for fires and early Euro-American
settlers suppressed fires), seed dissemination by livestock,
and increased atmospheric carbon dioxide (which juniper respond
positively to). Increased juniper leads to less grasses and
forbs as forage for wildlife and livestock and causes decreased
plant species richness. Juniper extract nutrients from inter-tree
spaces and deposit them beneath the trees’ canopies. Juniper
intercept 15 to 20% of precipitation, leaving less for forage
plants. Juniper themselves do provide some forage for some wildlife
species and shelter for wildlife. (Miller
and Wigand, 1994)
