Juneau Forestry Sciences Laboratory
The Role of Microsites on Alpine Timberline Advance Associated with Climate Change part of Vignettes:Vignette Collection
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.
Natural tree death, forest harvest, changes in hillslope hydrology, and implications to slope stability part of Vignettes:Vignette Collection
Death of trees by either forest harvest or by natural forest decline may result in increases in soil saturation, flooding, and landslides. These geomorphic processes, potentially differing in frequency and magnitude for natural and harvested forests, may also differ spatially and temporally by forest type. Both naturally declining forests and healthy forests have steeper portions prone to landsliding (Fig. 1). Yellow-cedar (Chamaecyparis nootkatensis) decline, a slow death of trees associated with changes in soil temperature and/or soil saturation, occurs in conjunction with climate warming. In the Tongass National Forest of southeast Alaska, cedar decline occurs in about 200,000 ha of forest at elevations < 300 m primarily near bogs. In southwest British Columbia, cedar decline encompasses 47,000 ha at elevations close to 1000 m. Timber harvest, currently averaging 76 millions of board feet per year, is expected to cover a total of approximately 274,377 ha or approximately 10% of the Tongass National Forest in the next 100 years. Studies in the Pacific Northwest and Alaska detect a two- to ten-fold increase in landsliding following timber harvest as compared to a 3.8-fold increase in the frequency of landslides in areas with cedar decline. Tree death, either by forest harvest or by cedar decline, results in loss of soil cohesion from decreased root strength. Reduced shear strength, associated with increased saturation, results from decreased tree canopy interception and reduced transpiration. Here, site saturation and characteristics of landslide initiation, runout, and deposition are summarized for areas of forest harvest and yellow cedar decline in the Tongass National Forest. To determine associations between forest conditions and site saturation (water height/soil depth), wells and piezometers were installed in hollows of hillsides with various harvest intensities (Fig. 2) and within hollows of hillsides with and without cedar decline (Fig. 3). Sites were helicopter logged to reduce possible soil disturbance. Hollows, convergent areas on hillslopes that collect colluvium, woody debris, and water, are considered primary source areas for landslides. Relative influences on slope stability by changes in soil moisture and root strength were evaluated with the infinite slope stability model, appropriate for wide and shallow landslides of the area. The model estimates the factor of safety (FS) where theoretically, a critically stable slope would have a FS = 1 and a FS < 1 would be unstable (Fig. 4, 5 define equation and parameters used). Measurements made during several rainy seasons indicate that complete saturation of forest soils in steep regions of SE Alaska is common. Observations of peak water table heights in 56 ground-water monitoring wells showed that soil saturation levels on hillslopes differed significantly with all harvest intensities at only one of two study locations following 25%, 75%, and 100% harvest. Before the forest was cut 100%, the average rainfall needed for 50% saturation of the soil was 54 mm, but after clearcutting soils reached an equivalent saturation with 61% less rainfall (21 mm). Statistically significant changes in ground saturation associated with forest harvest were found only on study locations high on valley hillsides as opposed to those located at the base of hillslopes. At steep (> 30°) cedar decline and surrounding healthy forest sites, 120 piezometers indicated no statistical difference in maximum saturation, but the timing was different. Sites in decline had smaller hydrologic contributing areas and remained saturated longer than surrounding healthy forests. At most sites, soil depth was < 0.7 m, and the loss of root strength was estimated to have a greater influence on slope stability than observed changes in site saturation. In soils deeper than 1 m, changes in soil saturation have a proportionally larger influence on slope stability. To characterize initiation and runout characteristics of landslides, a random sample of 300 landslides was assessed in old-growth, second-growth, and in clearcuts on Prince of Wales Island. Slides initiated on slopes ranging from 28° to 54° with a mean slope of 36°. Over 50% of the landslides initiated on slopes < 33°. Blowdown and snags, associated with cedar decline and "normal" rates of mortality, were found adjacent to at least 75% of all failures regardless of land use. Natural death and forest decline clearly influence site stability in areas of forest harvest. Nearly 50% of the landslides within clearcuts occurred within one year following timber harvest; more than 70% of these sites had hydrophytic vegetation (wetland plants) directly above failures. In following the runout paths of failures, significantly more erosion per unit area occurred within clearcuts than in old-growth forests on slopes with gradients from 9° to 28°. Runout length, controlled by hillslope position within deglaciated U-shaped valleys, was typically longer in old-growth forests than in second growth and clearcuts (median values were 334, 201, and 153 m, respectively). Old-growth sites were often upslope of second-growth and clear cut sites due to ease of access, making some comparisons of landslides in different forest types irrelevant. Most landslides and debris flows deposited in first-and second-order channels (often used by resident fish populations) before reaching the main stem channels used by anadromous fish (typically, river-spawning salmon). Slide deposits in old-growth were estimated to contain five times more woody debris than slides occurring in clearcuts. Timing of landslides is different for cedar decline and harvested forests. A survey of root deterioration indicates that 70 to 90% of roots with diameters up to 30 mm are decayed when cedar decline reaches a mean age of 14 years and most of the root mass has decayed when decline reaches a mean age 80 years. Most landslides in cedar decline areas occur when the majority of cedar trees had been dead for about 50 years contrasting to sites where landslides generally occur < 5 years after forest harvest. In summary, although initiation site conditions for landslides are similar for harvested and naturally declining forests, the timing and consequences of landslides in these areas are different. A balance of sediment and large wood provide structure to streams, integral for creation of fish habitat. Large wood is useful in retaining sediment and creating pool habitat. Because the composition of landslides in harvested sites has a much lower proportion of large woody debris, the habitat created is likely of inferior quality.