What might be the mechanisms behind the clear temperature association of the upper limit of tree growth and the lower limit of the alpine belt?
- Temperature limitation of carbon acquisition - photosynthesis.
- Temperature limitation of carbon investment - structural growth.
- Temperature limitation of nutrient uptake.
Existing evidence argues strongly against (1). All measurements of photosynthesis at treeline revealed excellent performance, with 25-30 % of the maximum rates often reached already at 0 °C. There is also no data suggesting that nutrient addition could cause an elevational advancement of the treeline (3). In fact, it is a common afforestation experience that young trees fail if fertilised. Hence (2) is the most likely cause. Treeline trees fix a surplus of carbon and have difficulties in transforming photoassimilates into tissue when it is too cold.
Evidence from N-Sweden, the Alps and Mexico underpins this hypothesis: As one approaches the upper limit of tree growth, the tissue concentrations of starch and lipids increase. In other words: there is no indication of carbon limitation at the tree limit.
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