What is the standard timeframe for soil climate? Is it the most recent 30 year normal, which shifts forward every ten years? Is it a longer period? Is it a fixed period of time or a moving average? I ask because soil temperature and moisture regimes are subject to change over long periods. If it were simply cyclical changes, then a long enough running average should suffice. But if climate change is directional over a period of a century or more, then either the classification of the mapped soils should eventually change with it. Or alternatively, we should be treating climate change as a dynamic soil property that has changed from a reference condition.
We would like to improve on the boundary between mesic and frigid soil temperature regimes in northern Michigan. As time progresses, we get improved temperature maps from PRISM which better account for lake effect and elevation. But each new PRISM also includes a half degree or so of temperature increase, which also affects the line placement by several miles. I should note that we do not attempt to follow the temperature line exactly, but attempt to follow major landform breaks whenever possible.
As an ecological site specialist I am in favor of standardizing the temperature boundaries to the 20th Century average, since it is our task to describe a reference condition from which to compare changes to plant communities. The past hundred years or so is the approximate time frame which today's forests have matured and for plant populations to have reached their present positions. Subsequent changes in vegetation attributable to various other factors such as herbivory, fire, and land use should also be placed into context with the fact that reference climate conditions have also changed. Restoration of reference conditions may or may not be possible on the basis of climate alone, but we could reference sites at slightly warmer locations for clues on how a plant community may respond.
[For Michigan weather stations, 1981-2010 was 0.4 C higher than 1971-2000, and 0.6 C higher than the 20th Century average. Annual and decadal variability obscures the long term trends.]
