Possible mechanisms explaining the thermal tree limit

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?

1. Temperature limitation of carbon acquisition - photosynthesis.
2. Temperature limitation of carbon investment - structural growth.
3. 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.