Alpine Plants Engineer Their Climatic Environment

Why is there lush alpine vegetation but trees cannot grow.? Are alpine plants physiologically superior, able to cope with those low temperatures which otherwise are harming trees? There is good evidence that thermal constraints for growth, that is, building new tissue, are the same for alpine plants, cold-adapted trees, and winter crops (winter rape and winter wheat), all being completely halted when tissue temperatures drop below 5 °C, and growth is close to zero at 6-7 °C. In contrast, all these species reach 30-50% of maximum rates of photosynthesis at these same temperatures; thus the provision of raw material for growth (sugar) cannot be decisive. Neither are there critical differences in freezing resistance between alpine plants compared to trees. Hence, at tissue level, there is no physiological reason why alpine grasses, herbs, and shrubs should grow at a given low temperature and trees should not.

There are two reasons for alpine plant success above treeline:

1. By low stature and dense stand structure, alpine plants restrict aerodynamic exchange with the atmosphere, which causes heat to accumulate during periods with solar radiation and permit plants to operate at comparatively warm temperatures, much unlike those experienced by upright, ventilated trees. The life-form 'tree' does not permit any escape from the gradually declining ambient temperatures, whereas alpine plants engineer their microclimate and aircondition their meristems close to the ground so that they can build new tissue at otherwise cold air temperatures above the plant canopy (Figure 3).

2. By developmental flexibility and morphological adaptation, alpine plants are able to make use of short favorable weather conditions, they sprout rapidly, produce only a few, mostly short-lived leaves (c. 60 days), and have their meristems positioned very close to the ground, in the case of many grasses, sedges, or rosette plants, often 1-2 cm below ground, where the solar-heated soil provides a thermally buffered environment. In contrast trees operate at longer leaf duration (mostly >120 days, in evergreen treeline conifers 4-12 years) and leaves take longer to mature, and their aboveground meristems are fully exposed to the cold air temperatures.

The transition from trees to alpine vegetation is thus dictated by plant architecture and not by tissue-specific inferiority of trees compared to alpine plants. This close coupling of trees to atmospheric conditions also explains the surprisingly uniform leveling of treelines across mountain valleys which reminds one of the level of a water reservoir. In contrast, the climate in alpine vegetation varies with compactness and height ofthe leafcanopy

Figure 3 Trees are coupled to air temperature and thus, appear 'cool' on this infrared thermograph taken at 10 a.m. on a bright midsummer morning in the Swiss Alps near Arolla. Alpine grassland and shrub heath accumulate heat by decoupling from atmospheric conditions (low stature, dense structures). So the treeline can clearly be depicted as a thermal boundary driven by plant architecture.

Figure 3 Trees are coupled to air temperature and thus, appear 'cool' on this infrared thermograph taken at 10 a.m. on a bright midsummer morning in the Swiss Alps near Arolla. Alpine grassland and shrub heath accumulate heat by decoupling from atmospheric conditions (low stature, dense structures). So the treeline can clearly be depicted as a thermal boundary driven by plant architecture.

and exposure to the sun. A sun-exposed, sheltered microhabitat at 3000 m of altitude may be warmer than a shaded microhabitat at 1800 m. Altitude per se, or data from a conventional climate station, thus, tell us little about the climate actually experienced by alpine plants. It had long been known that mutual sheltering among alpine plants or leaves/tillers within a plant is very beneficial ('facilitation'), and removing this shelter effect by opening the plant canopy can be disastrous.

Alpine plants are small by design (genetic dwarfs); they are not forced into small stature by the alpine climate directly, though evolution had selected such morphotypes. What seems like a stressful environment is not really stressful for those well adapted. However, there is some additional modulative, direct effect on size by low temperature. Alpine plants that survive in low-altitude rock gardens indeed grow taller than their relatives in the wild. But plants grown in such rock gardens are commonly of montane origin, because most typical alpine plants fade at such high, low-altitude temperatures, possibly because of overshooting mitochondrial respiration.

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