High Altitude Treeline: An Evidence of Climate Warming!

High altitude treeline ecotone is an area of transition high on mountain where closed canopy subalpine forest gives way to the open alpine shrub land or tundra and rocky expanses above (Fagre, 2009). This zone is one of the most conspicuous ecosystem boundaries on the earth (Moen et al., 2004) where the vegetation pattern changes markedly along the altitudinal gradient. This is the area where limits of the tree adaptations to various abiotic constraints such as the harsher climate, rocky substrates etc.; interactions with different biological processes such as competition, facilitation of seedling establishment; anthropogenic activities etc. produce there a complex mosaics of surviving trees (Fagre, 2009) i.e. abiotic, biotic and anthropozoogenic influences on flora in the ecotone are interrelated in a complex fashion (Holtmeier, 2003).

The treeline is a result of many factors that are unfavorable for tree regeneration and growth. With approaching the limits/ thresholds with increasing altitude, the tree stature decreases gradually and the tree individuals are increasingly deformed by the increasing adversities (Holtmeier & Broll, 2005). Individuals are dwarf, stunted, prostrate, crooked, flagged, abraded, twisted and clumped behind shelters in the ecotone. There is a strong thermal advantage of being short, which is why such stunted growth forms succeed at high elevation whilst normal trees do not (Grace et al., 2002).

Treelines may be climatic, anthropogenic and orographic. The orographic treeline is set by orographic hindrance whereas anthropogenic treeline is controlled by human landuse and activities. And, mainly the treeline of the concern is climatic treeline which occurs to its full potential with respect to climatic limits, while, the orographic and anthropogenic treelines may not be in full climatic potential.

In climate determined treeline, if climatic conditions become less intense (global warming), this is where remarkable change can be seen. The sensitivity of the treeline to environmental change implies a certain state of readiness of the trees to respond to changing conditions and the responses can be the changes in growth, growth forms, regeneration, treeline structure (Holtmeier & Broll, 2005), in composition and position of the treeline (Grace et al., 2002).

Treeline is determined mainly by heat deficiency. Migration of treeline species to higher position, increase in growth rate and regeneration, and rise in treeline position in responses to recent climate change (over the last 20th century) have been documented in several treelines in the world and the trends have been particularly visible in the last few decades (Szeicz &MacDonald, 1995; Llyod & Fastie, 2002). The trend and changes in the treeline structure and position are generally attributed to recent warming.

To some extent, archived climatic information on various timescales can be gathered from fluctuation of treeline (Kullman, 1996) and regeneration dynamics also is a better indicator of sensitivity of the treeline ecotone to environmental changes (Holtmeier & Broll, 2005). Recruitment of seedlings above the treeline apparently reflects improved climatic conditions. Hence, treeline ecotone is ideally suited for climate change monitoring (Becker et. al., 2007) and the treeline is considered as a sensitive biomonitor and key to understanding ecosystem responses to ongoing climate change.

So, if climatic constraints release, this is where abrupt ecological transition in response to the global change is visible javascript:void(0)and such conspicuousness of this zone with clear links to climatic change makes the treeline a reliable and promising proxy for studying the ecosystem responses to ongoing climate change and variability.