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The Development and Movement of Tree Islands Near the Upper Limit of Yree Growth in the Southern Rocky Mountains

Marr, John W.
Ecology 1977 v.58 no.5 pp. 1159-1164
Abies lasiocarpa, Picea engelmannii, branches, ecology, islands, photographs, roots, seedlings, stem elongation, stumps, tree growth, trees, tundra, wood, Colorado, Rocky Mountain region
Dwarfed and deformed plants of tree species occur as 'tree islands' in a matrix of tundra vegetation near the upper limit of tree growth in the Southern Rocky Mountains. Many of these islands are asymmetrical, having a surface layer of dead branches to windward and some live branches to leeward. I made repeated observations of these islands in all seasons from 1956 to 1967 along a 1.75—km belt ?20 m wide in Niwot ridge in the Front Range was of Denver, Colorado. The location of islands was mapped on a transparent overlay on an aerial photograph, annual growth of stem tips was measured, and an abundance of notes and photographs was taken. Some tree islands of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) are conspicuous because they grow in microsites in which no tree seedlings have been found, and which general observations suggest would be outside their environmental amplitude. These islands often have live branches connected to windward with dead branches, some of which have roots. These observations led to the hypothesis that some tree islands move along the ground climbing out of more moderate microsites in which a seed germinated and moving into microsites for which they are not well adapted. In this movement process, wind—exposed organs are killed, while leeward stems grow to leeward often taking root and forming an upright stem system in the process of 'stem tip layering.' Islands with interconnected live and dead stems to windward, illustrating movements of 5 m, are common; isolated dead stumps and wood fragments in a line extending to windward indicate that some islands may have moved at least 15 m. The origin of tree islands, support for the movement hypothesis, and other features of tree island ecology and discussed.