uniper haircap moss occurs in coniferous forest, grassland, alpine, and herbaceous subalpine communities. In British Columbia it occurs in subalpine fir-Engelmann spruce (Abies lasiocarpa-Picea engelmannii) forests [82]. Juniper haircap moss is "occasional to frequent" in white spruce (P. glauca) stands of central Alberta [92]. Juniper haircap moss occurs in the coast Douglas-fir (Pseudotsuga menziesii var. menziesii)- mountain grassland transition zone in western Washington [77] and in Rocky Mountain Douglas-fir/mountain big sagebrush (P. m. var. glauca/Artemisia tridentata subsp. vaseyana) communities of Hell's Canyon National Recreation Area and Medicine Lodge Peak, Idaho [46].

Juniper haircap moss may form cryptogamic soil cover on palouse prairies and mountain grasslands. Tveten and Fonda [141] found juniper haircap moss was a dominant moss on the Wier Prairie of Fort Lewis, Washington; juniper haircap moss and hairy rhacomitrium moss (Rhacomitrium canescens) had approximately 23% cover. Idaho fescue (Festuca idahoensis) dominated the palouse prairie site; 3- to 5-year-rotation prescribed fires and livestock exclusion are used to maintain historical palouse prairie vegetation [141]. Juniper haircap moss formed part of the cryptogamic soil crust in a bluebunch wheatgrass (Pseudoroegneria spicata)-Idaho fescue community of Hell's Canyon National Recreation Area, Oregon [15].

Juniper haircap moss occurs in alpine and subalpine herbaceous as well as coniferous forest communities in the Cascade Range. In British Columbia and Washington, juniper haircap moss was noted in woolly pussytoes (Antennaria lanata) alpine fellfield, sedge (Carex spp. and Kobresia myosuroides) meadow, timber oatgrass (Danthonia intermedia) meadow, heath, and willow (Salix nivalis and S. cascadensis) communities. It was most important in capitate sedge (C. capitata) meadows [29]. Juniper haircap moss was a component of an alpine fellfield community on Monument Peak in Oregon. Common woolly sunflower (Eriophyllum lanatum) and woolly rhacomitrium moss (Rhacomitrium languinosum) dominated the community [3].

Juniper haircap moss was the most abundant species on subalpine sites in Wyoming where snowfields remained through mid-July. The authors suggested that on sites covered with snow for most of the year, the short growing season limits establishment of vascular species. Rocky Mountain lodgepole pine (Pinus contorta var. latifolia) and subalpine fir-Engelmann spruce forests surround the snowfields [72].

Juniper haircap moss is occasional in cold western hardwood communities such as aspen (Populus spp.). In central Alberta, juniper haircap moss was more frequent in balsam poplar (Populus balsamifera ssp. balsamifera) stands (14%) than in balsam poplar-quaking aspen (P. tremuloides) (8%) and quaking aspen (3%) stands [92].

Juniper haircap moss is often associated with fireweed (Chamerion angustifolium) across broad geographic areas [42,65,118,135,145]. Juniper haircap moss also associates with other haircap mosses [42,57,68,78,130,135], fire moss [65], reindeer lichens (Cladonia spp.) [50,130], and the liverwort Marchantia polymorpha [130,151] on many sites across juniper haircap moss's distributional range. Common hair cap moss generally grows taller than juniper haircap moss and may successionally replace juniper haircap moss on mesic sites [42,57,68,68]. See Successional Status and Northern ecosystems for further information on the ecology of these associations.