Lateral growth and vegetative reproduction in krummholz mats and tree islands in high elevation spruce-fir forests of southeastern Wyoming, USA

Background/Question/Methods

Alpine ecosystems offer a unique opportunity to study tree species at the edges of their distributions and physiological limits. Clonal growth has shown to be an advantageous trait for woody species in alpine zones because it provides protection from high winds, freezing temperatures, and ice abrasion. The two dominant tree species in the Medicine Bow National Forest of southeastern Wyoming, Englemman spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), possess the ability to reproduce through the vegetative process of layering. Layering occurs when low-lying branches produce adventitious roots and begin to grow laterally, but maintain the ability to produce apical shoots when conditions are favorable. Krummholz mats are stunted clonal growth forms found at the very upper edge of the treeline, while tree islands are larger groups of clumped trees occurring in the middle and lower elevations of the treeline ecotone. The current study investigated the frequency of vertical stems produced by layering versus seed germination within the krummholz and tree islands found across the alpine treeline ecotone. Observational data on vertical stem origin, size, and cone production were collected for fifty krummholz mats and forty tree islands within the Libby Flats area.

Results/Conclusions

Krummholz mats sampled contained 2 to 18 vertical stems per mat with a mean density of 0.5±0.4stems/m². Tree islands contained from 9 to over 120 vertical stems per island with a mean density of 1.8±1.2stems/m². Vertical woody stems originated mainly from layering in both krummholz mats (84.8±24.4%) and tree islands (78.9±12.8%) with only small percentages of stems originating from seed (krummholz: 3.6 ± 9.8% and tree islands: 5.3 ±8.4%). There was no significant difference between the mean number of vertical stems produced by seed across the elevational gradient from highest occurring krummholz mats to lowest tree islands (p=0.1096). Cone production was greater in tree islands, 95% of all tree islands produced cones compared with only 32.7% of all krummholz mats. The results provide preliminary data and insight into the extent of layering and vegetative reproduction throughout different zones of the treeline ecotone. With recent global increases in temperature, as well as pathogen epidemics in the Rocky Mountains threatening these sensitive areas, it is important to understand the reproductive potential of these species.