Changes in sub-alpine tree distribution in western North America: a review of climatic and other causal factors

Abstract

Changes in the distribution of sub-alpine tree species in western North America have been attributed to climatic change and other environmental stresses. These changes include tree-line fluctuations throughout the Holocene and recent invasion of sub-alpine meadows by forest. Most palaeoecological studies suggest that the tree-line was higher during a period of warmer climate approximately 9000 to 5000 BP and lower during the last 5000 years, with short periods of local tree-line advance. Recent advances in sub-alpine tree distribution can be compared with weather records, allowing an examination of relationships between tree advance and climate at a finer resolution. In general, recent sub-alpine forest advances in western North America, based on studies representing three climatic zones (maritime, Mediterranean and continental), have been associated with climatic periods favouring tree germination and growth, although factors such as fire and grazing by domestic livestock have had an impact in some areas. Limitations to tree establishment (e.g., winter snow accumulation, summer drought) vary in relative importance within each climate zone, as do predicted consequences of anthropogenic climatic change. Recent increases in establishment of sub-alpine trees may continue if climatic change alleviates the limitations to tree establishment important in each climatic zone. However, factors such as topography and disturbance may modify tree establishment on a local scale.

The Role of Wood Microsites at Timberline-Alpine Meadow Borders for Conifer Regeneration

“…Lack of substrate moisture, typically limiting seedling growth (Cui andSmith 1991, Smith andJohnson 2007), is ameliorated by wood substrates. Wood microsites are known to serve as reservoirs during periods of drought (Harmon and Franklin 1989, Amaranthus et al 1989, Cornett et al 2000, common during summers in the PNW (Rochefort et al 1994, Peterson et al 2002 and expected to increase with climate warming (Markoff and Cullen 2008). Enhanced temperature of wood substrates as compared to adjacent soil substrates (covered by vegetation) can be attributed, at least in part, by the lower albedo of dark wood surfaces.…”

“…Examination of seedling establishment sites for sensitive young seedlings in the ATE, rather than examination of older resilient trees, gives a clearer understanding of current climatic factors affecting potential expansion of timberline (Rochefort et al 1994;Smith et al 2009). For example, distorted krummolz trees at treeline ( Fig.…”

“…The importance of climate warming on forests is recognized worldwide and has increased attention on the significance of both timberline advance and alpine meadow invasion by forests (Franklin et al 1971, Rochefort et al 1994, Körner 2003, Wather 2003, Holtmeier and Broll 2005. Timberline, one of three areas included in the alpine tree ecotone (ATE), represents a line between continuous forest below and area of treeline and krummholtz above (Wardle 1973, Tranquillini 1979, Holtmeier 2003.…”

“…Seed dispersal, germination, and seedling establishment must occur well in advance of any permanent changes in the spatial distribution pattern of mature forests (Noble 1993, Zens and Peart 2003. Increase in seed-generated trees and viable seed populations in the ATE is associated with climate warming (Franklin et al 1971, Rochefort et al 1994, Korner 1998, Kullman 2006, but successful seedling regeneration in alpine meadows may depend on availability of suitable substrates for seed establishment (Woodward et al 1995, Peterson 1998, Germino et al 2002.…”

“…ATE microsites include convexities, concavities, objects that provide shelter from sun and wind, and substrates composed of wood, mineral soil, or rock (Holtmeier and Broll 1992, Rochefort et al 1994, Moir et al 1999, Castro et al 2002, Maher et al 2005). Microsites range in size from millimeters to meters in diameter.…”