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Biodisturbance and Colluvial Hollows

picture of beetle-killed tree
Beetle-killed tree in
Rocky Mountain
National Park, CO
(Photo by G. David)

The effect of insects and wind on colluvial hollows depends on the elevation and aspect, therefore the expected influence of biodisturbance on colluvial hollows is split by vegetation.

Upper Montane – The potential increase in runoff from decreased amount of interception and transpiration from trees is more likely to affect colluvial hollows in this zone of the basin. These basins are smaller and therefore it is more likely that 100% of the trees have been infected by beetles, causing a larger amount of water to enter into the colluvial hollows.

Subalpine – In the spruce/fir section of the subalpine zone the spruce beetle may be present. These beetles are more likely to attack trees in valley bottoms, therefore they are not likely to have a big impact on colluvial hollows unless a massive beetle outbreak occurs.

Alpine - No change because of lack of trees in this zone.

Fire and Colluvial Hollows

(Information compiled by G. David)

Picture of colluvial hollows
Colluvial Hollows -
Rocky Mountain National Park
(Photo by G. David)

The expected response of colluvial hollows to fire depends largely on vegetation zones, fire severity and rainfall intensity. Moody and Martin (2001) found that most of the sediment after a fire came from the headwaters of a drainage basin and was subsequently stored in the higher order parts of the basin in flood plain deposits, alluvial fans and channel fill. The expected influence of fire on colluvial hollows is therefore separated by vegetation zones:

Lower Ecotone and Lower Montane – Colluvial hollows are not a significant component of drainage basins in this vegetation zone. Nevertheless, in the smaller drainages where hollows exist there may be an increase in overland flow which leads to rilling and incision where the flow converges. An increase in erosion off the hillslopes is most likely to occur during a high severity fire. Although historically these zones most often have moderate to low severity fires, fire suppression practices may have increased tree density and fire severity in the lower ecotone and lower montane. The high severity fires are then combined with high intensity convective thunderstorms at this elevation, which leads to increased runoff and erosion off the hillslopes.

Upper Montane – The upper montane zone is composed mainly of lodgepole pine and ponderosa pine forests. Both forest types are dependent on periodic high severity fires. A high severity fire completely removes all the vegetation covering the ground and consumes the trees. The upper portion of a basin will have an increase in runoff from the decreased infiltration and interception following these fires. Colluvial hollows in this zone may be incised following a high severity fire.

Subalpine – This zone is characterized by less frequent high severity fires. A decrease in vegetative cover at this elevation could lead to increased snow avalanche activity on steep slopes (Shakesby and Doerr, 2006). Colluvial hollows may have an increased amount of runoff during the snowmelt period following a fire, but the effect is not predicted to be as large as in lower elevations.

Alpine - Fires are not expected in this zone because of lack of fuel and too much standing water.


Click a link below to learn about the influence of hillslopes on each process domain:

Colluvial Hollows Ephemeral Channels Glaciated Confined Glaciated Partially Confined Glaciated Unconfined Unglaciated Confined Unglaciated Partially Confined Unglaciated Unconfined
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