Pit-and-mound topography

Pit and mounds are small, persistent microtopographical features that present themselves after a disturbance event occurs and uproots trees via windthrow. The uprooted tree falls, and a pit forms in the forest floor where the root mass and associated soil matrix used to be. Eventually after a period of time in which the roots decay, the associated soil matrix that was pulled out of the ground with the roots falls back to the ground, creating a corresponding mound.

A pit, as defined above, is formed when pressure is applied to the trunk and crown of the tree that is more powerful than the root and associated soil's ability to hold the tree upright and in place and knocks the tree down. This toppling of the tree can be caused by a multitude of different types of forest disturbance. Wind may blow the tree down; snow may accumulate and put excessive weight on the tree; the tree's roots may have decayed to a point where they are not strong enough to hold the tree upright. Soil conditions also play a role in the ability of the tree to remain upright. Wet soil can loosen the hold a tree's roots have on the soil, and dry soil can help hold the tree in place.

A mound, as defined above, is formed on average from five to ten years after the formation of a pit. The root mass must decay to an extent where the soil matrix that was suspended with it when it toppled over can slough off on to the ground near the corresponding pit. The necessity of the decay of the root mass is what causes this lag time. In more cold and wet climates, the rate of decay is slower and this time lag may be extended.

Pit and mounds always occur on a fine spatial scale, being the result of only one tree felling. Commonly they are a product of windthrow, but they can also be caused by other factors. Large amounts of snow accumulation on tree boughs or extensive root decay are other possible causes for tree uprooting. Pit and mounds have been analyzed on both on a small scale and larger scale forest systems. It has been observed that they can act as important soil disruptors and microsites for biodiversity and plant establishment.

It has been observed that pit and mounds on an individual basis generally have specific climatic and pedological characteristics. Mounds are generally observed to be warmer and drier than the forest floor or the corresponding pit; Similarly, the pit is generally found to be colder and have a higher soil moisture content than either the forest floor or the mound. One notable instance where there is an exception to the general observation is when a snow layer has fallen. The pit becomes insulated by the snow layer and then in some instances is observed to be warmer than the mound.

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