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April 28, 2004
Habitat Typing
1. What is habitat typing?
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Habitat typing is a land classification system that was developed by the forest service in the
70ies as an alternative to characterizing site quality through site indices or existing
vegetation. It was seen as a more reliable way of classification, that is less dependent on
recent disturbance history and more accurately reflects the production potential of a site as a
function of local environmental factors, such as rainfall amount, temperature, soil fertility.

It is a classification system based on the distribution of climax communities over the
landscape. Based on Clementsian ideas of succession, it assumes that the average
environmental conditions of a site determine the species composition of the climax
community in a given biogeographical region. Furthermore, it is assumed that the climax
community can be deduced from the composition of earlier successional stages, again based
on the idea that successional trajectories are deterministic and unchanging.

A further assumption is that the climax community predicts better than any other successional
stage what the production potential of a site is, presumably because the climax community
makes the most complete use of the existing resources. This does not mean, however, that the
species of the climax community are those best suited for lumber production. Often species of
earlier successional stages are more resistant to disturbances, such as insect outbreaks and
wind throw.

If you accept all these assumptions, then habitat typing should be useful to delimit land areas
of relatively uniform production potential and management opportunity, and for any given
site, narrow the range of species and management regimes appropriate for the site.
2. How is it done?

Habitat typing is a hierarchical classification system reminiscent of species classification. The
highest level of classification is called “Series”. It is named after the most abundant tree
species of the climax community. The next level down is called “habitat type”. It is named
after the most dominant understory plant. A further level down takes us to the “phase” level,
named after the second most common understory species. The “phase” classification is not
often used, and used only to describe a floristic variant if it is common enough to be
noteworthy.

In the Western United States, habitat types most strongly correlate with elevational gradients
(thus temperature and precipitation), second with precipitation within an elevation, third with
aspect. For example, on top of Mount Lemon at 3000 m, we expect to see the Picea
engelmanii Series, perhaps with Erigeron eximius Habitat Type. Moving downward one
would expect to find the Abies lasiocarpa Series, the Picea pungens Series, the Abies
concolor Series, the Pseudotsuga ponderosa Series, The Pinus engelmannii Series, and
finally the Pinus leiophylla Series.

To classify habitat type, one must find clues as to what the climax community at a site would
be. If a close to climax community is there, one would look for the most shade tolerant of the
tree species. If a site has had a recent major disturbance, one would look for an adjacent site
at a later successional state. If there is no such site, one must try to deduce the climax
community from the species composition of the earlier successional state, based on known
successional trends.
April 28, 2004

Understory vegetation can give important information on site quality, through the abundance
of indicator species. Indicator species are those species with relatively narrow distribution
and strict environmental requirements, so that the presence of such a species indicates
properties of the physical environment. For example, there are species found only in very dry
sites, moderately dry or wet sites. Other species are found only in nutrient-poor, average or
nutrient-rich sites. A study from Canadian white spruce forests showed that a scoring system
based on the relative abundance of indicator species in the understory predicts site quality
(both in terms of soil moisture and soil nitrogen) accurately 50-60 % of the time. Most other
predictions deviated by only one site quality class.
3. How useful is habitat typing?

There are no research studies that assess the usefulness of habitat typing in management
situations. Although claims have been made that habitat type can predict productivity, and
even the water yield in forest catchments, to date no study has shown proof of this or
contrasted the habitat typing method with other methods used in forestry, such as site
indexing.

Habitat typing has also come under attack, because it is based on simplistic Clementsian ideas
of succession. Today we know that predictable succession trajectories are the exception, not
the rule. Chance events often play a big role in determining the future composition of
communities. For example, succession can depend on what species happen to be in the seed
bank at the time of major disturbance, or what the weather is like during the time of sapling
establishment. Therefore, succession can take communities to very different climax states on
physically identical sites. Often disturbance is frequent enough to prevent the establishment
of a climax community. Succession can also be arrested, for example, when the undergrowth
has grown so thick that tree species fail to push through. In that case, disturbance is required
to thin the undergrowth and allow tree establishment.

Finally, our knowledge of rapid climate change is a serious obstacle to the idea of predictable
succession. Climax communities take several hundred years to establish. On this time scale
we expect large shifts in average site temperature and precipitation. Thus, the site quality or
habitat type boundaries of today may not be in the same place as they were 100 years ago and
will again shift over the next 100 years. It is possible that today’s forests types are the “living
fossils” of an earlier climate regime, and conversely, that the “climax” community of a site
that is today in an early state of succession, is basically unpredictable.