Factors influencing the establishment and growth of tree seedlings at Subarctic alpine treelines

Abstract

Citation: Kambo, D., and R. K. Danby. 2018. Factors influencing the establishment and growth of tree seedlings at Subarctic alpine treelines. Ecosphere 9(4):e02176. 10.1002/ecs2.2176Abstract. Significant treeline advance can only occur with successful establishment, growth, and survival of new seedlings. Several studies have examined microsite factors at single locations to explain the presence/ absence of seedlings at treeline. We conducted a much larger observational study and included multiple factors to determine (1) which variable(s) was/were most important, (2) whether their importance differed between aspects, and (3) whether the same variables explained variation in seedling growth and damage. We analyzed five biophysical and six shrub variables along four forest-tundra ecotones in southwest Yukon at 640 points. The model that best explained seedling occurrence was similar between north-and southfacing slopes. Of all variables, seedling occurrence was best explained by the proximity, height, and upslope orientation of shrubs (relative to the seedling). The data indicate an optimal range of shrub cover, which differed with aspect. On north-facing slopes, seedlings occurred most often when shrub cover exceeded 13%, while on south-facing slopes seedlings occurred most when shrub cover was between 9% and 72%. We also found that with the exception of shrub-related factors, very few biophysical variables explained size and growth characteristics and damage of tree seedlings, suggesting that the relative strength and importance of variables change depending on the life stage and size of the tree seedling. Collectively, our results demonstrate a non-random distribution of seedlings in the forest-tundra ecotone, suggesting that as shrub distributions change with climate change, colonization sites for seedlings will also be influenced.

A Model-Dependent Method for Monitoring Subtle Changes in Vegetation Height in the Boreal–Alpine Ecotone Using Bi-Temporal, Three Dimensional Point Data from Airborne Laser Scanning

“…Second (Case B), despite the very limited geographical extent of the study area (200 m × 600 m), it displayed variation in wind exposure, temperature, soil depth, and moisture due to its location on and along a small ridge with variations in aspect. As discussed by Kambo and Danby [63], these are all factors influencing seedling establishment, growth, and mortality in tree line environments, and aspect-related differences have been established in the literature in multiple forest, tree line, and tundra sites [63]. Thus, we divided the area into four sub-regions (see Figure 6A) according to aspect, slope, elevation, and soil depth (occurrence of bedrock outcrops).…”

Limited overall impacts of ectomycorrhizal inoculation on recruitment of boreal trees into Arctic tundra following wildfire belie species-specific responses

“…We observed apical death and in some cases resprouting of many seedlings at the treeline site. Wind damage and desiccation is an important determinant of tree seedling success at other treeline locations [64,65].…”

Evidence of vegetation greening at alpine treeline ecotones: three decades of Landsat spectral trends informed by lidar-derived vertical structure

“…Ultimately having multi-temporal acquisitions of ALS data at treeline ecotones could allow for a more informed assessment of the vertical forest structure (e.g., Nelson 2007, Hauglin et al 2018). Kambo and Danby (2018), working in sub-arctic treelines in the Yukon, explored the drivers of tree seedling establishment at alpine treelines. The authors found that the occurrence of tree seedlings across 640 sites was best explained by the proximity, height, and upslope orientation of shrubs.…”