Of all the native Selaginellas of California, S. watsonii is perhaps the most
wide ranging species of them all. It can be found in all the states from the Rocky Mountains to the Pacific Ocean except in Washington. It appears to occur only in high montane areas south of the Canadian border. In California it is found in the Sierras from Central California southward into the Transverse Ranges towards the Salton Sea.
Map but is incomplete leaving out southwest Montana, Idaho, and Oregon populations
The attached images by Jean Pawek were taken in Mono County and provide an overview of a typical habitat for the species as well as a close up of a colony of the species and a more intimite look at the species with its obvious strobili. Some morphological traits can be detected, such as microphyll (leaf) and sporophyll (the leaf of the strobili which covers the sporangia at its base) and the bristles at the apex of both the microphylls and the sporophylls. The bristles are generally shorter than those encountered with other species and may be lacking altogether on the sporophylls.
The species appears to be localized to outcrops probably because it occurs in high mountainous regions and is subject to snow cover. The areas of its occurrence appear to have earlier snow melt off than neighboring habitats.
The following image brings home the role that the species and the members of the subgenus play in their environment: as pioneering plants that by their size and mat like growth trap debris and help in developing a shallow substrate for angiospermous plants and ferns alike to take hold in an otherwise inhospitable environment.
A habitat builder