Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 357

atures stop tree growth is still unknown. Boysen-Jensen (1949) suggested that temper-
ature plays an important role in the allocation of energy needed to maintain normal
metabolic processes versus the production of new wood. This theory would explain why
timberline trees minimize the development of nonproductive tissue and develop a small
stature; the slow growth of trees at or near timberline; and their longevity (ca. 4,800
years for bristlecone pine) (Ferguson 1970; LaMarche and Mooney 1972). Slow growth
rates may be a positive adaptation to a short growing season because they allow the
shoot tissues to mature (Wardle 1974).
Wardle (1971, 1974) hypothesized that the inability of shoots to mature (ripen) is
the principal cause of tree-growth cessation. This can occur when the shoots freeze or
become desiccated during periods when transpiration requirements exceed moisture
availability. Ripened shoots of timberline trees possess well-developed cuticles that can
withstand low temperatures and desiccation, and their cells are not damaged by the
growth of ice crystals (Wardle 1971). Upper timber-line, according to this theory, rep-
resents the highest altitude at which woody shoots can grow and mature under the en-
vironmental conditions at the height of the tree canopy. The critical attribute of ripen-
ing is the protection it provides against winter desiccation. Temperature thus controls
the time available for needle maturation, cuticle thickness (protective covering), and
drought resistance (Tranquillini 1979). Following needle maturation, trees can with-
stand severe winter conditions, allowing them to reach higher elevations in extreme
continental areas where winter conditions are more severe than in milder marine areas
(e.g., Becwar et al. 1981). The capacity to withstand severe conditions is also influenced
by the inherited tolerances of different species to various climatic extremes (Clausen
1963).
Körner (1998) hypothesized that tree growth is limited by a minimum soil temper-
ature threshold that precludes root growth (cellular and tissue formation), thereby in-
hibiting shoot and canopy development. Recent evidence from root growth experiments
by Alvarez-Uria and Körner (2007) appears to support this hypothesis, finding a critical
temperature of ∼6°C for significant root growth in six tree species. This temperature
threshold closely approximates the worldwide mean soil temperature (6.7°C) at climatic
treelines (Körner and Paulsen 2004).
DISTURBANCE
Both natural and anthropogenic disturbances, including fire, grazing, logging, mining
activities, avalanches, and mass wasting, play an important role determining the el-
evation and pattern of local and regional treelines (Jentsch and Beierkuhnlein 2003;
Körner 2007; Young and Leon 2007). Treelines and ecotones are often lowered and
lengthened following disturbances, creating more gradual treeline-to-alpine transitions
(e.g., Daniels and Veblen 2003) and a patchy landscape characterized by individual
trees and regeneration clusters (e.g., Cullen et al. 2001; Batllori et al. 2009; Coop and
Schoettle 2009). Disturbance can also temporarily stabilize treeline where local factors,
such as canopy cover, are a more important determinant of regeneration success than
climate change (Cullen et al. 2001). Conversely, treeline sensitivity and change may be
greatly increased following the cessation of human activities (Holtmeier and Broil 2005)
and a decrease in disturbance frequency (e.g., Bolli et al. 2007).