Geoecological significance and feeding habits of the mountain pocket gopher (Thomomys monticola) on Lassen Peak, an active volcano in the southern Cascade Mountains (California, USA)

Abstract

This study focuses on the overall ecosystem characteristics of a pocket gopher (Thomomys monticola) colony inhabiting high-elevation (~2700-3100 m) talus slopes on Lassen Peak, California, an active volcano in the southern Cascade Mountains of northern California. This is the first study assessing the ecological relationships of pocket gophers in an alpine talus-slope environment. I examine the geographic, geomorphic, sedimentological, pedological, and vegetation elements of this system. Talus substrates are very steep (≤32°), extremely unstable, and affected by a diversity of geomorphic processes. Sediments include a large proportion (55-64%) of debris ≤5 cm in diameter. The soil fraction (≤2 mm) consists mainly of fine-grained sandy and silty loams, which facilitate excavation by gophers. Soil organic matter content varies widely (~1-50%) but increases sharply on basal slopes, loosening the soils and allowing substantial water storage (10.8-13.3%) during dry summer months. Thus, gopher activity is concentrated on lower talus areas. Lassen receives significant snowfall (15.2-17.8 m/year) and extensive snowfields persist until late-summer, allowing rodents to dig though the talus snowpack, and providing an ample source of moisture for talus substrates. Substrate instability produces a characteristic root system deformation, with roots oriented upslope from the plant’s base. This causes roots to remain within a shallow soil layer, used by gophers during burrowing. Vegetation presumably utilized by gophers includes 38 vascular plant species (19 herbs, 6 suffrutices, 9 shrubs, and 4 graminoids). Here, I analyze the most important resources provided by these plants. The results obtained underscore (i) the significant role of substrate—both talus sediments and soils—on the digging activities of pocket gophers; (ii) the role of different plant species on their feeding habits; and (iii) the importance of an extensive snowpack during winter burrowing.