terminology

[Jim Martin]

What is the difference between cohort and stand?

[Kevin Mathers]

Jim,

These terms can be confusing, and complicated depending on what factors you are placing the most value on (silviculture vs. ecology). Stands may or may not be made up of mostly one cohort, depending on the history of the stand.  Below are definitions from The Dictionary of Forestry, John Helms, Editor, Society of American Foresters (I added the terms in parentheses).

Cohort - a group of trees developing after a single disturbance (harvest, storm, fire, etc.) - trees are commonly of about the same age.  Stand - a contiguous group of trees sufficiently uniform in age-class distribution, composition (tree species), structure growing a on site with sufficiently uniform quality (growing conditions - such as soil type, moisture, exposure), to be a distinguishable unit.

Hope this helps.

[Jim Martin]

Kevin,
Thanks for the help with the two terms.
So, a cohort could consist of trees of uneven age but with many species, and be on a site with vastly un-uniform quality; the main characteristic is that it developed from a single disturbance. Right? It follows that a cohort could consist of one or many stands.
Whereas a stand´s age is irrelevant, but its age-class distribution, composition, and uniform quality make it a distinguishable unit, presumable for silvicultural treatment. Right? And a stand could consist of many cohorts?
I am still puzzled by what the distinction is good for.

[Kevin Mathers]

The distinction may be valuable for foresters or others who are viewing the forest from a technical perspective.  For most landowners the distinction is probably not that important.  Landowners can focus on managing the stands on their property that a knowledge forester has identified.

[Peter Smallidge]

Hi Jim:

Kevin has it right.  A cohort is going to have high similarity of trees of the same age, though perhaps of different species.  They may not be geographically connected.  For example, you might think of the disturbance created by the 1998 ice storm in northern NY (and beyond) which created numerous canopy openings.  To the extent that disturbance allowed for a pulse of tree regeneration, those trees would be of the same cohort.  If this was a big enough event you could hypothetically monitor the cohort through decades and anticipate pulses in wood products, etc.  On a smaller scale, you might clearcut a stand that results in a pulse of regeneration and all those trees in that stand are part of the same cohort.

A stand is a contiguous land area that has homogeneity in age class structure, species composition, disturbance history and site conditions.  A stand might be even-aged or uneven-aged. 

The differences are (1) cohort has age similarity while stand may or may not (2) stand is contiguous land area while cohort might be geographically dispersed.  The meaningful distinction is that management actions are applied at the level of the stand, that is the management unit.  The things done in one stand are not typically appropriate for an adjacent stand because it has different soils, disturbance history, age class structure, species composition.  From a management perspective (not a technical definition perspective) the forest stand is equivalent to the farmer's field...you would take certain management/cultural actions in a corn field that would differ from an adjacent hay field.

It is infrequent that I think about or hear about cohorts in the context of woodlot management.  Not rare, but close.

Peter

[Jim Martin]

OK, Thanks.  I get it.

Jim