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Engelmann Spruce (Picea Engelmannii Engel.) at Its Upper Limits on the Front Range, Colorado
Author(s) -
Wardle Peter
Publication year - 1968
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1934115
Subject(s) - ecotone , tundra , picea engelmannii , shoot , tree line , altitude (triangle) , environmental science , physical geography , vegetation (pathology) , biometeorology , ecology , horticulture , geography , biology , montane ecology , ecosystem , climate change , canopy , medicine , geometry , mathematics , shrub , pathology
Engelmann spruce is the dominant tree at timberline in the Front Range at approximately 3,350 m elevation; it occurs as krummholz in the forest—tundra ecotone up to about 3,500 m, and occasional individuals are found in the tundra up to 3,730 m. Temperatures decrease with increasing altitude above timberline, whereas wind velocity increases, especially during winter. Winter snow is deeper and persists longer in the forest than in the krummholz above, its depth in the latter tending to remain constant once the lower portions of the plants are packed. Soil temperatures fluctuate widely beneath tundra vegetation in the neighborhood of krummholz plants, whereas under forest variations are small and there is a prolonged period in spring when they remain within 0.6°C (1°F) of freezing point. Krummholz growth forms of spruce arise through death of needles and shoots exposed to the prevailing westerly winds. Even in the summer young exposed needles tend to be somewhat chlorotic, and many show lesions. In winter windward needles dry out, become bleached, and are eventually shed. Certain needles, instead of becoming bleached during winter, turn brown and dry out in early spring. In krummholz, even within a single shoot, sharply contrasting differences develop in winter between leeward needles that show only small seasonal decreases in water content, and windward needles that dry out and die. Desiccation is usually confined to krummholz, mainly affected small needles on stunted shoots and the distal needles of long, robust shoots, but in the winter of 1961—62, following a cold, wet September, it extended to leading shoots of saplings below timberline. Replenishment of water in needles during winter is probably from water stored in sapwood above the snow pack, since sapwood beneath the snow pack remains frozen in both forest and krummholz. Late—lying snow delays the spring growth of seedlings below timberline, but they occur even where snow persists until late in June. Above timberline, spruce seems to be less tolerant of late—lying snow. It is concluded that though the position of timberline is correlated with summer temperatures, dry winter winds are the immediate, though probably not the ultimate, cause of the krummholz growth forms in the forest—tundra ecotone.