The Role of Microsites on Alpine Timberline Advance Associated with Climate Change

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. Timberline advance has gained recognition worldwide because regeneration of trees mitigates global increase in CO₂. Loss of alpine meadows by tree invasion results in loss of foraging habitat for species including black bear, mountain goat, and wolverine. Timberline, demarking the upper boundary of subalpine forest with trees of large stature, is adjacent to a zone consisting of either treeless alpine slopes or clusters of stunted trees extending upward in elevation to tree limit or treeline. Uneven timberlines in the PNW reflect a history of forest growth in combination with orographic-related disturbances including glaciation, snow avalanching, rock fall, and landslides (Fig. 1). A closer look at seedling establishment along timberline edges indicates that trees often germinate on small landforms known as microsites. Microsites may include small convexities or concavities in the soil surface on the 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. These stresses include wind, cold temperatures, high radiation, drought, animal predation, and infestation by fungal pathogens found in snow and soil. Wind, snow cover, cold temperatures, and drought limit photosynthesis, the mechanism by which plants use sun energy to derive CO₂ from the atmosphere and convert it to starch. Respiration, including plant maintenance and growth, is fueled by the starch produced in photosynthesis, and is limited by cold temperatures, wind, and lack of water. Thus, microsites, by providing warmer substrates, adequate moisture, and shelter, allow plants to function more affectively in mountain environments.

Factors such as snow accumulation, summer rainfall, and availability of microsites, will control timberline advance. A specific microsite type may be superior for seedling survival given a particular topographic position and climatic setting. 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 areas such as the PNW where snow accumulation is high, seedling growth is facilitated by convex sites and wood substrates, microsites that facilitate both melting of snow and increase in growing season (Fig. 2).

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 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. Like convex soil microsites, nurse logs at timberline facilitate early melting of snow. Snow melts because the dark surfaces of wood having low albedo warm the log surfaces greater than the adjacent snow. In addition, nurse logs lessen the destructive influence of snow movement known as snow glide, reduce species competition, often have increased mycorrhizal populations, and have fewer pathogens than the adjacent soil. The water holding capacity of rotten logs often also surpasses that of soils, aiding seedlings during summer droughts. At lower elevation pristine forests, rows of old-growth trees are indicative of nurse log establishment (Fig. 3). Potentially, given time, the thriving seedlings found on logs at alpine locations will have the same presence as those found at lower elevations (Fig. 4).

Timberline advance, outside the influence of localized orographic disturbances, initiates on microsites by one of two processes: 1) seed germination or 2) asexual cloning of trees by sprouting. Examination of seedling establishment and survival of sensitive seedlings, rather than examination of older resilient trees, gives a clearer understanding of current climatic factors affecting potential expansion of timberline. For example, trees such as bristlecone pine having lived over 3000 years, have endured many climate changes whereas, a newly germinated subalpine fir will survive only if site conditions meet its present requirements for survival (Fig. 5).

Formation of microsites is associated with both physical and biologic processes. Scouring and deposition of the earth's surface by glacial, fluvial, and periglacial activity make convex and concave sites favorable to seedling establishment. Nurse log microsites are dependent on introduction of large woody debris by tree breakage, wind throw, or wood transport by snow avalanches. Wood must then be highly decayed in order to facilitate seed germination. Future work aimed at predicting timberline forest expansion will be enhanced by collaboration of interdisciplinary teams composed of climatologists, plant physiologists, geomorphologists, and others.

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