World-Wide and Regional Examination of Substrates Facilitating Timberline Expansion

Upward advance of timberlines, associated with climate warming, is occurring in the Pacific Northwest (PNW) as well as many other mountainous regions of the world. Examination of seedling establishment and survival of sensitive seedlings, rather than examination of older resilient trees, may give a clearer understanding of current climatic factors affecting potential expansion of timberline. Our investigation of seedling establishment along timberline edges in the PNW indicates that trees often germinate on small landforms known as microsites. Microsites include small convexities or concavities on the soil surface having a scale of centimeters to meters, but also include associations with slope, aspect, rocks or plants, or substrates dominated by mineral soil or wood. Growing on favorable microsites helps seedlings cope with some of the stresses that exist at high elevation sites including wind, cold temperatures, high radiation, drought, animal predation, and infestation by fungal pathogens found in snow and soil. Microsites, by providing warmer substrates, adequate moisture, and shelter, allow plants to function more affectively in mountain environments. Our summary of microsite type and associated timberline advance in a world-wide context indicates that factors such as snow accumulation, summer rainfall, and availability of microsites, will control timberline advance. In windswept timberline locations, rocks and plants provide shelter from wind and reduce the likelihood of night frost. In arid climates, concave microsites aid in snow deposition providing needed moisture to seedlings during periods of drought. In contrast, convex microsites and wood substrates, typical sites of regeneration in the PNW where precipitation typically exceeds 150 cm per year, facilitate early snow melt, thereby increasing growing season. Large trees at the edge of timberline fall into alpine meadows, decay, and provide sites for seedling establishment. These sites commonly called nurse logs, much better known as key microsites in lower elevation forests, have been found to be conspicuous sites of timberline forest regeneration extending from the forest edge into alpine meadows. Nurse logs appear to be particularly important sites of regeneration in wetter alpine areas of the world such as the North Cascade Mountains of Washington in the PNW. Depending upon aspect and slope, one tree can potentially advance timberline close to 20 meters, a typical length of a tree growing at timberline. Nurse log temperature during the growing season is significantly greater than the adjacent soil, particularly in areas with reduced overstory canopy. Increased substrate temperature, associated with increased root growth, has been found to facilitate growth of seedlings. Further, the water holding capacity of rotten logs, which often surpasses that of soils, aids in seedling growth during summer droughts.