Holocene subalpine forest-parkland dynamics in Big Cottonwood Canyon, Wasatch Mountains, Utah, USA

Understanding long-term responses of subalpine forest-parklands to Holocene climate variability in local context is critical for better managing those ecosystems under future climate change. Available records suggest that western North American subalpine forest-parkland ecosystems responded to Holocene climate in various ways at different places. Here we present a Holocene record of upper montane forest-parkland dynamics from Silver Lake in Big Cottonwood Canyon, Wasatch Range, on the Great Basin’s eastern margin. Our results show that at the end of the Younger Dryas, Silver Lake was surrounded by open Pinus/Picea parkland mixed with Artemisia subalpine steppe. By ~9.0 cal ka BP Picea began to expand in response to early Holocene summer warming and enhanced winter moisture. Picea-dominated forests prevailed from ~8.0 to 5.0 cal ka BP, after which time a more open Picea parkland developed. By ~3.5 cal ka BP Pinus increased under cooling conditions, and Picea gradually rebounded to form the mixed conifer forest present today. The Silver Lake long-term record of moderate shifts within an upper montane ecosystem is consistent with other regional sites in the timing of major vegetation changes reflecting large-scale climatic forcings, but distinct in response to local factors including the importance of enhanced lake-effect snowpack and the absence of competing lower montane Pinus. These local factors may help account for the long-term stability of Picea-dominated forests in the area.

Frugivory by Coyotes Decreases the Time to Germination and Increases the Growth of Netleaf Hackberry (Celtis reticulata) Seedlings

Research Highlights: Frugivory by mammals is a common plant–animal interaction, but additional studies that examine the effects of frugivory on woody plants are needed. We show that ingestion of netleaf hackberry (Celtis reticulata Torr.) fruits by coyotes (Canis latrans Say) cuts the time to germination nearly in half and results in seedlings that are taller than the controls. Background and Objectives: Netleaf hackberry is a deciduous shrub to small tree that can be long-lived, but newly established stands are rare. The lack of juvenile hackberry in its native range of southwestern North America could be due to low percentages of germination and seedling survival. We hypothesized that passage through the digestive tract of a coyote would increase the germination and subsequent growth of netleaf hackberry. Materials and Methods: In the Wasatch Mountains of Utah, we collected coyote scats containing visible hackberry fruits and picked fresh fruits from nearby hackberry shrubs. All samples were cleaned and cold-stratified. We sowed 20 seeds from each of the 34 samples into containers in the greenhouse (a total of 680 seeds). We noted the date of emergence and final height of each seedling after 131 days. Results: The germination percentage of the coyote-treatment seeds did not differ from that of the controls. However, the coyote-ingested seeds took just over half as many days to germinate as did the undigested controls (35 days vs. 69 days, respectively; p < 0.001) and the resulting seedlings were 9.5% taller by the end of the growing season (6.4 vs. 5.8 cm, respectively; p < 0.001). Conclusions: Consumption by coyotes can benefit hackberries by enabling their seeds to germinate earlier in the year when conditions are wetter and cooler. The additional time for establishment and growth afforded by frugivory likely increases the fitness of netleaf hackberry seedlings that emerge into the unpredictable conditions of a semi-arid region.

Characterization and Dynamic Analysis of the Devils Castle Rock Avalanche, Alta, Utah

ABSTRACT Rock avalanches are large-magnitude mass movements with high mobility and fluid-like runout; however, because of their scarcity, little information is typically available to describe the hazard posed by these events. Geologic records thus provide key data regarding rock avalanche size, timing, and dynamics. Here we present a detailed case history analysis of the Devils Castle rock avalanche located near the town of Alta in the Wasatch Mountains of Utah. The deposit is ∼1.5 km in length with a Fahrboeschung angle of 14 degrees (height-to-length ratio = 0.25). Through topographic reconstruction, we calculated a deposit volume of 1.7 million m3 with a maximum thickness of 25 m and an average thickness of 7 m. Cosmogenic surface exposure dating of six deposit boulders indicates a failure age of 14.4 ± 1.0 ka. The Devils Castle headwall displays no obvious evidence indicating precise source location and geometry; therefore, we reconstructed two plausible source volumes and performed numerical runout simulations for each. Results agree well with mapped deposit boundaries for both source scenarios; however, the east source model better represents material and dynamic characteristics of the deposit observed in the field. While the region is seismically active, the Late Pleistocene age for the rock avalanche precludes ascribing direct correlation with any currently known surface-rupturing paleoearthquakes. We identified and describe five similar events in the region highlighting the extent of the potential hazard. Individual case history analyses such as this allow us to better understand the processes and controls of large-scale mass movements in the region.