Influences of Lower Treeline Migration on Eastern Idaho Mima-Mound Initiation

Mima-type mounds in eastern Idaho appear to initiate under influences of lower treeline migration.  Eastern Idaho mima mounds mark hundreds of hectares of Pleistocene landforms, and are regularly spaced, generally circular in plan form, 3-14 m diameter, and 0.5-2 m high.  They have developed in eolian silt overlying fan gravels, terrace gravels, and basalt flows.  The mounds typically host large morphotypes of big sagebrush (Artemisia tridentata), enhancing their prominence in the landscape.  Previous explanations for the initiation of eastern Idaho mounds include frost heaving and rodent burrowing.

Mima mound initiation in the Big Lost River valley can be linked with climate-driven migration of pioneering tree species across terrace and fan surfaces.  Specifically, mound initiation appears linked to the presence of limber pine (Pinus flexilis), the regional lower-treeline pioneer, and its effects on eolian silt deposition.  As the lower treeline has retreated across mid-elevation terraces and fan surfaces, probably as a result of post-Little Ice Age warming, living limber pine individuals have been isolated below that treeline, and isolated dead limber pines lie at slightly lower elevations.  Limber pines provide shelter for a variety of shrubs and grasses and occupy low (0.3-1 m), roughly circular mounds of fine silt on the coarse gravel terraces, which are otherwise largely devoid of silt and shrub cover.  I propose a novel mechanism for mound initiation: 1) lower treeline retreat isolates limber pines; 2) isolated limber pines serve as sheltering hosts for shrubs and grasses; 3) the isolated limber pines and associated shrubs capture wind-blown silt common in the area, forming crude mounds; 4) over 10³ to 10⁴ years, bioturbation and soil diffusion processes modify the crude mounds into the forms observed on lower-elevation surfaces.  Ubiquitous mounds on lower-elevation surfaces probably initiated through similar influences of lower treeline migration, during late Pleistocene time.